RMU vs. MV Switchgear for a 10 MW Data Center: Design, Redundancy & Reliability

Part A — RMU vs. MV Switchgear for a 10 MW Data Center at a Glance

Every data center owner eventually faces the same medium-voltage (MV) question: should the 13.8 kV distribution be built from compact ring main units, from conventional metal-clad switchgear, or from a combination of the two? The answer shapes reliability, maintainability, arc-flash exposure, expansion cost and footprint for the life of the facility.

This guide works through that decision for a representative US facility: 10 MW of IT load, served at 13.8 kV, 60 Hz, with transformation to 480Y/277 V. It translates the European "ring main unit" concept into US equipment categories and standards, sizes the transformers and switchgear properly, checks the redundancy claims that are often made too casually, and shows when each option is the right engineering answer.

The facility in this guide

What Keentel's analysis shows

Figure 1. Keentel analysis for the 10 MW facility. Left: installed transformer capacity and the capacity remaining after the worst single transformer failure, against the 12.63 MVA peak demand and a 25% growth allowance. Right: illustrative unavailability of the MV path to a 480 V bus for four topologies, using failure-rate and repair-time values in the range of published industry survey data; generators and UPS are excluded so that the MV architecture can be compared on its own.

Five lessons

Compact ring switchgear and metal-clad switchgear are not competitors; they do different jobs. Metal-clad drawout breaker switchgear is the right tool for the main 13.8 kV bus, where incomers, ties, generator paralleling and relay-based protection meet. Compact load-interrupter or fault-interrupter switchgear is the right tool at the transformer, where it saves space and cost.

Count redundancy after the failure, not before it. Six 2.5 MVA transformers carrying 12.63 MVA cannot lose one. A design is N+1 only if the remaining capacity, after the worst single failure, still carries peak load — and only if the load can actually be transferred to it.

A ring is only redundant if it has two sources. An open loop fed from one utility circuit protects against cable faults, but not against loss of that circuit. Two "independent" utility feeders from the same substation bus share a common-mode failure.

Design to US practice. In the US, the secondary is 480Y/277 V, the equipment standards are IEEE C37 and UL, and the installation rules are the NEC and NFPA 70E — not IEC 62271 and 400 V.

Transformer repair time dominates. In the reliability model, a failed 2.5 MVA transformer that takes weeks to replace contributes more downtime than every switch and breaker combined. Secondary-selective arrangements, 2N paths or an on-site spare matter more than the MV switch type.

Engineering review of common statements about RMUs and MV switchgear

Part B — Ring Main Units vs. Metal-Clad MV Switchgear for a 10 MW Data Center: A US Engineering Guide

1. Why the MV architecture decision matters

A 10 MW IT facility is large enough that its MV system is a real power system: two utility services, several unit substations, generators that may be paralleled at 13.8 kV or at 480 V, and protective relaying that must coordinate with the utility. It is also small enough that owners are tempted to treat MV equipment as a commodity and choose on first cost and footprint.

That temptation is where problems start. The MV architecture fixes:

how many single points of failure stand between the utility and each UPS input;

whether equipment can be maintained without shutting down critical load (concurrent maintainability);

the arc-flash incident energy on every MV and 480 V switchboard;

how fast a failed transformer or cable can be isolated and its load restored;

how easily the site grows from 10 MW to 15 or 20 MW.

Compact ring main units and conventional metal-clad switchgear answer these questions differently. The right answer is usually a combination, but only after the load, redundancy target and utility service are understood.

2. Equipment categories in US terms

The term "ring main unit" comes from European and IEC practice, where compact gas-insulated switching assemblies (typically two ring switches plus one transformer tee) are standard in urban distribution. In the US, the same functions are delivered by several equipment categories, each with its own IEEE standard.

In this guide, "RMU" therefore means compact metal-enclosed or pad-mounted switchgear with ring (loop) switches and a transformer tee, whether the tee is a fused switch, a vacuum fault interrupter or a compact breaker. "Conventional MV switchgear" means metal-clad, drawout vacuum breaker switchgear with relays.

2.1 What actually differs

The 600–630 A ring-bus rating of most compact units is the first clue about where they belong. At 13.8 kV, 630 A carries about 15 MVA. That is enough for a transformer loop, but not much margin for the main bus of a growing 12.6 MVA facility.

3. Understanding the electrical load properly

3.1 From IT load to facility demand

The facility load starts with the IT load and the power usage effectiveness:

With 10 MW of IT and an annualized PUE of 1.20, the facility average is 12 MW. That figure is correct for energy, but not for sizing. Annualized PUE averages cool and hot days. On the design day, chillers or dry coolers work hardest, and peak PUE is often 1.30–1.45 even in an efficient facility. Keentel uses 1.35 in this example:

UPS losses and battery recharge add load on the input side of the UPS. A double-conversion UPS at 96% efficiency draws about 10.4 MW to deliver 10 MW, and recharge after a discharge can add 5–10% of UPS rating for a period. Some of this is captured in PUE; check that it is not double counted, but do not ignore it.

3.2 Apparent power and current

Transformers and switchgear are rated in kVA and amperes, so convert real power with the power factor:

Worked example 1 — facility current.

The common estimate of 529 A is arithmetically right. It is the starting point, not the rating. Modern UPS rectifiers and IT power supplies often operate near unity power factor, while mechanical loads with variable-frequency drives are also near unity at the fundamental; 0.95 is a reasonable planning value, but harmonic currents still flow and must be considered in transformer and cable heating (Section 6.3).

3.3 Sizing the main bus, incomers and tie

In a main-tie-main arrangement, each main breaker must carry the entire facility when the other source is out, and the tie must carry the load of the dead side:

where g is the growth allowance. With 661 A including 25% growth, a 1,200 A main bus and 1,200 A mains and tie are the minimum. If the owner expects to add a second 10 MW phase on the same service, 2,000 A is the sensible choice: the incremental cost in metal-clad gear is modest, and replacing a bus later is a major outage.

A compact ring bus rated 630 A cannot be the main bus for this facility once growth is considered. That alone pushes the main distribution toward metal-clad switchgear.

4. The utility service and short-circuit duty

4.1 Service options

For 10 MW, dual 13.8 kV feeders are the common service. Owners should ask the utility exactly where the two feeders separate: at different substation transformers, different buses, or merely different feeder breakers on the same bus. That answer decides whether the second feeder protects against a substation event.

4.2 Short-circuit duty at 13.8 kV

Worked example 2 — utility fault duty. The utility quotes 500 MVA three-phase available at the service point:

Metal-clad breakers rated 25 kA symmetrical would be adequate today, but the X/R ratio, motor contribution from large chiller motors, possible future utility system reinforcement and generator paralleling at 13.8 kV can all raise the duty. Many owners specify 40 kA for the main lineup to avoid a future rating problem. Compact ring units are commonly rated 16–25 kA; at 20.9 kA plus margin, their rating must be verified, not assumed.

Worked example 3 — owner substation. If the facility instead takes 138 kV service with its own transformer, the 13.8 kV fault level depends mainly on the transformer impedance. With 40 kA available at 138 kV:

The larger, lower-impedance transformer pushes the 13.8 kV bus to 24.5 kA — beyond a 25 kA rating once motor contribution is added. Transformer impedance, switchgear rating and future growth must be selected together.

4.3 X/R ratio and asymmetry

Breaker ratings under IEEE C37.04/C37.06 assume a standard DC time constant (45 ms, roughly X/R = 17 at 60 Hz). Where the system X/R is higher — close to large transformers or generators — the asymmetrical current at contact parting rises, and IEEE C37.010 methods must be used to check the breaker. Compact switchgear and fault interrupters have their own test-duty assumptions under C37.60 and C37.74; confirm the X/R basis in the rating.

5. Topologies for a 10 MW facility

5.1 Radial

One source, one path to each transformer. Simple and cheap, but every component is a single point of failure. Not acceptable as the only path for a critical 10 MW facility, although radial feeders downstream of a redundant main bus are normal.

5.2 Loop (ring), open point

Transformers are connected in a loop with an open switch somewhere in it. A cable fault is isolated by opening switches on either side and closing the open point; all transformers are then re-energized. This is where ring switchgear excels.

But a loop fed from one source has no protection against loss of that source. To be redundant at the source, the loop must be fed from two sources — two utility feeders, or two bus sections of the main switchgear — with the normally open point between them.

5.3 Primary-selective

Each transformer has two MV supplies (A and B) with a selector switch or two interlocked switches. Loss of one MV feeder is covered by transferring to the other. This can be built from compact switchgear with motorized switches or from metal-clad breakers.

5.4 Secondary-selective (double-ended unit substations)

Two transformers, each fed from a different MV bus, supply two 480 V buses joined by a normally open tie. If one transformer or its MV source fails, the 480 V tie closes. This is the backbone of many data center designs because it protects against the longest outage — transformer failure — and allows transformer maintenance without load loss.

5.5 Main-tie-main at 13.8 kV

Two utility incomers, two bus sections and a normally open tie, with automatic transfer. This is the conventional switchgear arrangement and the natural point of connection for 13.8 kV generators.

5.6 Data center redundancy architectures

Keentel's related guide on data center tier classification explains how these architectures map to concurrent maintainability and fault tolerance. The MV system must support the chosen architecture: an elegant 2N UPS design fed from a single-source ring is not 2N at the facility level.

6. Transformer sizing and loading

6.1 Why six 2.5 MVA units are not N+1

Worked example 4 — counting redundancy. The facility peak apparent power is 12.63 MVA (annualized basis). Test each option after the worst single transformer failure:

Two caveats apply. First, N+1 only works if the load of a failed transformer can be transferred — through a 480 V tie, a reserve bus or dual-corded IT. Adding a seventh transformer that cannot pick up anyone's load adds nothing. Second, the check above uses annualized load; at design-day peak (14.21 MVA), the six-transformer case is short by 1.7 MVA after a single failure, and the 4N/3 case keeps only about 0.7 MVA of margin.

6.2 Transformer loading guidance

Liquid-filled transformers are designed to IEEE C57.12.00 and loaded under IEEE C57.91; dry-type transformers to C57.12.01 and C57.96. Both permit operation above nameplate for limited periods at the cost of insulation life. In data centers, Keentel typically:

sizes so that normal loading is at or below about 80% of the base (self-cooled) rating, leaving the fan-cooled rating for failure contingencies;

checks the contingency case (one transformer out) against the fan-cooled rating at the design ambient;

uses temperature-rise ratings (55/65 °C liquid; 80, 115 or 150 °C dry) deliberately — a lower-rise dry transformer has more overload capacity.

6.3 Harmonics and K-factor

UPS rectifiers with active front ends draw low-distortion current, but legacy six-pulse rectifiers, VFDs and IT power supplies still produce harmonics. Harmonic current increases eddy-current losses in proportion to the square of harmonic order. The K-factor quantifies the effect:

where I(h,pu) is the harmonic current at order h divided by the rms load current. A transformer feeding modern active-front-end UPS modules may need only K-4; one feeding older six-pulse equipment or many VFDs may need K-13 or a derating calculated per IEEE C57.110. IEEE 519 sets the limits at the point of common coupling with the utility.

6.4 Transformer currents and secondary fault duty

Worked example 5 — a 2,500 kVA, 13.8 kV – 480Y/277 V unit substation.

On an infinite source with 5.75% impedance:

Including the 500 MVA utility source (0.5% on the 2.5 MVA base):

Reflected to the 13.8 kV side, that secondary fault is only about 1,670 A — roughly 16 times primary full-load current. This matters for protection: a primary fuse or relay must see this through-fault and clear it in time to protect the transformer, yet ride through magnetizing inrush that can be 8–12 times full-load current for about 0.1 s.

7. Transformer protection: fuse, fault interrupter or breaker

The protection device at each transformer is the single most important difference between "RMU" variants, and between compact switchgear and metal-clad switchgear.

7.1 NEC limits for transformers over 1,000 V

NEC 450.3(A) and Table 450.3(A) set the maximum primary and secondary overcurrent protection for transformers over 1,000 V. For a transformer with impedance not more than 6%, in any location, the primary may be protected by a circuit breaker set at up to 600% of rated primary current or a fuse rated up to 300%. Where conditions of maintenance and supervision ensure only qualified persons service the installation, the table permits a different set of limits (for example 300% breaker or 250% fuse on the primary when secondary protection is also provided). Always confirm against the edition of the NEC adopted locally.

Worked example 6 — NEC limits for the 2,500 kVA unit. Primary full-load current is 104.6 A.

A 13.8 kV power fuse sized around 150E–200E satisfies the NEC maximum and typically rides through inrush. The question is whether it protects the transformer against through-faults and coordinates with the 480 V main breaker.

7.2 Through-fault protection

IEEE C57.109 (liquid-immersed) and C57.12.59 (dry-type) define through-fault capability curves. A 2,500 kVA unit falls in Category III, where the damage curve has a frequent-fault portion that depends on impedance. The primary device must lie to the left of (faster than) the damage curve, adjusted for the delta-wye connection: a single-line-to-ground fault on the 480 V wye secondary appears on the delta primary as only 58% of the per-unit current in the faulted phases. Keentel shifts the damage curve by this factor before checking protection.

7.3 Fuse limitations

7.4 Vacuum fault interrupters and breakers

A vacuum fault interrupter (VFI) in pad-mounted or compact switchgear trips all three phases from an electronic control with selectable time-current curves and ground-fault settings. It removes the single-phasing risk and allows transformer alarms to trip. A metal-clad breaker with a relay goes further: 50/51 and 50N/51N, transformer differential (87T) on larger units, breaker failure, communications and event records.

For a 10 MW data center, Keentel generally recommends VFIs or breakers at transformer primaries. The cost difference over fused switches is small against the consequence of single-phasing a UPS input.

7.5 Coordination

Coordination studies should be performed with the full 13.8 kV and 480 V system: utility relay, 13.8 kV main and feeder relays, transformer primary device, 480 V main and feeder breakers, UPS input breakers and generator protection. IEEE 242 (Buff Book) gives coordinating time intervals; with modern relays and vacuum breakers, 0.2–0.3 s between relay curves is typical. NEC 700.32 and 701.32 require selective coordination for emergency and legally required standby systems, and NEC 645 and 708 add requirements in some facilities. Selective coordination of the data center's critical (non-emergency) load is usually an owner requirement rather than a code requirement, but it should be specified explicitly.

8. Arc-flash and worker safety

8.1 Incident energy at MV and 480 V

IEEE 1584-2018 calculates arc-flash incident energy for systems from 208 V to 15 kV. At 13.8 kV, incident energy is driven mainly by clearing time; at 480 V downstream of a 2,500 kVA transformer, by arcing current and the clearing time of the transformer primary device for faults on the transformer secondary terminals and 480 V main.

The weak point in most unit substations is the 480 V main breaker line side: a fault there is cleared only by the transformer primary protection. A fuse sized for inrush may take a second or more at the arcing current, producing tens of cal/cm².

Worked example 7 — clearing-time sensitivity. Incident energy is roughly proportional to arcing time. If a 480 V arc between the transformer and the 480 V main produces 5 cal/cm² per 0.1 s at 18 in., then:

Values are illustrative; a full IEEE 1584 study with electrode configuration, enclosure size and gap is required.

8.2 Arc energy reduction

NEC 240.87 requires arc energy reduction for 480 V circuit breakers rated or adjustable to 1,200 A or more: zone-selective interlocking, differential relaying, an energy-reducing maintenance switch, an active arc-flash mitigation system, an instantaneous trip setting below the available arcing current, or another approved method. NEC 240.67 has a similar requirement for fuses rated 1,200 A or more. The 2,500 kVA transformer's 3,000 A secondary main is squarely in scope. The MV choice influences how easily this is achieved: a relay-controlled primary breaker can be tripped by an arc-flash light sensor or differential scheme, while a fuse cannot.

8.3 Arc-resistant construction

IEEE C37.20.7 defines arc-resistant switchgear testing. Type 2B accessibility protects personnel at the front, back and sides even with low-voltage compartment doors open. Arc-resistant metal-clad switchgear is now common in data centers, but it requires arc-exhaust plenums or a room designed for the pressure and gases. Compact gas-insulated switchgear also offers internal-arc classifications; check that the test conditions (current, duration, accessibility) match the installation.

8.4 NFPA 70E and 70B

NFPA 70E sets work practices: energized work permits, approach boundaries, PPE and the electrically safe work condition. NFPA 70B, issued in 2023 as a standard rather than a recommended practice, requires an electrical maintenance program with condition assessments and maintenance intervals. Drawout breakers, test positions and remote racking make 70B maintenance far easier to perform without exposure; sealed compact units need less maintenance but are harder to diagnose and repair.

9. Automatic transfer and source management

9.1 Main-tie-main logic

In the usual 13.8 kV main-tie-main arrangement, both mains are closed and the tie is open. On loss of voltage on one incomer (27 undervoltage with a time delay to ride through utility reclosing), the scheme opens that main and closes the tie, after checking that the dead bus has decayed (or that the sources are synchronized). When the utility returns and is stable for a set time, the scheme retransfers.

Compact ring switchgear with motorized switches can perform loop restoration (fault location, isolation and service restoration), but it is generally slower and less flexible than a relay-based main-tie-main scheme. If the transfer must happen within seconds and coordinate with generators, metal-clad switchgear is the natural platform.

9.2 Generators at 13.8 kV or at 480 V

For a 10 MW facility, paralleled 13.8 kV generators (for example six 2.5 MW units with one redundant) are attractive. That decision effectively mandates metal-clad switchgear for the main bus.

9.3 UPS placement

UPS systems are almost always at 480 V in a 10 MW facility, downstream of the unit substations. MV UPS systems exist, but are typically justified at larger scales. The MV system's job is to deliver two independent, maintainable sources to the UPS inputs. If the IT is dual-corded, the UPS architecture (2N, distributed redundant or block redundant) sets the minimum MV requirement.

10. Insulation media and environmental rules

Many compact switchgear designs use SF6 gas. SF6 is an excellent insulating and interrupting medium, but it has a global warming potential about 23,500 times that of CO2 (100-year basis) and an atmospheric life of thousands of years.

The US EPA Greenhouse Gas Reporting Program (40 CFR Part 98, Subpart DD) requires reporting of SF6 emissions from electric power systems above a threshold of nameplate capacity.

California's regulation on gas-insulated equipment (California Code of Regulations, Title 17) phases out the purchase of new SF6 equipment, starting with lower voltage classes such as distribution-class equipment, and sets annual emission limits. Several other states, including Massachusetts, New York and Washington, have SF6 or high-GWP gas rules.

Owners should confirm the current rules for their state before specifying SF6 equipment. SF6-free alternatives — vacuum interrupters with dry air, solid dielectric or alternative gas mixtures — are increasingly available in compact 15 kV equipment. Metal-clad switchgear uses vacuum interrupters in air and avoids the issue entirely.

11. Reliability: putting numbers on the architecture

11.1 Method

IEEE 493 (the Gold Book) gives a practical method for comparing configurations: represent each component by a failure rate λ (failures per year) and a mean repair or restoration time r (hours), then combine them.

For components in series (all must work):

For two redundant paths in parallel (either is enough), with r in hours:

Unavailability U is in hours per year; availability is:

11.2 Illustrative component data

The values below are illustrative and lie in the range of published survey data; a real study uses the owner's and utility's data and current editions of the survey tables.

11.3 Results

Worked example 8 — four MV architectures to one 480 V bus.

Four conclusions follow:

The single-source ring is no better than radial. Its benefit — fast restoration after a cable fault — is swamped by the exposure to the one utility feeder. Rings need two sources.

Main-tie-main cuts outage frequency by a factor of six, because utility outages are covered by transfer. But the transformer and its radial feeder now dominate unavailability: 0.006 failures per year × 300 h = 1.8 h/yr of the 2.2 h/yr total.

2N removes the transformer from the critical path. What remains is the common-mode loss of both utility circuits, which is why source independence (Section 4.1) matters.

UPS and generators change the absolute numbers, not the ranking. Utility outages are carried by UPS batteries and generators; the MV path still determines how often the facility runs on its backup systems and how exposed it is when a transformer fails.

Availability expressed in "nines" can mislead. A design with 0.99998 availability and one long outage every decade is very different from one with frequent short events; use frequency and duration together.

12. Footprint, cost and schedule

The figures below are indicative ranges for planning in 2026 US dollars and vary with region, ratings and supply-chain conditions.

Two cautions: lead times for MV switchgear and transformers have been long since 2022 and drive project schedules; and the cheapest switching device is not cheapest if it forces extra transformers, generators or space elsewhere.

13. The recommended architecture for this 10 MW facility

Keentel's typical recommendation for a 10 MW IT facility with dual 13.8 kV service is a hybrid:

This keeps the drawout breakers, relaying and transfer logic where the system's flexibility is needed — at the main bus — and uses compact equipment where footprint and cost matter and functions are simple.

Worked example 9 — a 2N layout with 2,500 kVA units. Each of the A and B sides has six 2,500 kVA transformers (15 MVA per side). Each side alone carries 12.63 MVA at 84%, and under normal 2N operation each transformer runs at about 42%. The 13.8 kV feeders: each of the twelve transformers draws 104.6 A at full rating, so a single 600 A feeder breaker could supply up to five units — but for maintainability, Keentel normally uses one feeder per one or two transformers. The main bus carries the full facility, 528–661 A, well within a 1,200 A rating.

If the owner prefers block-redundant (N+1) UPS to reduce cost, seven 2,500 kVA transformers with a 480 V reserve bus meet the requirement at 72% normal loading — provided the reserve can actually pick up any one block's load.

14. Expansion and phasing

Data centers rarely stop at the first phase. Practical steps:

Size the main bus for the ultimate service (2,000 A at 13.8 kV covers about 47 MVA thermally, but the utility service and short-circuit rating will limit first).

Install spare breaker cells or bus-extension provisions on both ends of the lineup.

Plan a second lineup and utility service rather than overloading the first; at 20–30 MW, an owner 69, 115 or 138 kV substation often becomes economic.

Keep spare duct-bank capacity to every future unit substation location.

Re-run short-circuit, coordination and arc-flash studies with each phase; fault duty grows as transformers and generators are added.

15. Testing and commissioning

Whatever equipment is chosen, the acceptance test program should follow the NETA Acceptance Testing Specifications and the manufacturer's instructions, and include:

insulation resistance and, for metal-clad equipment, power-frequency or VLF withstand of buses and cables;

breaker timing, contact resistance and vacuum integrity;

relay secondary injection against approved settings, and end-to-end trip testing to each breaker;

CT ratio, polarity and excitation; VT ratio and polarity;

transfer-scheme functional testing under every source-loss scenario, including generator integration;

fuse and VFI control verification on compact units, and gas-pressure checks where SF6 or other gas is used;

integrated systems testing (pull-the-plug) of the complete facility at load banks before IT load arrives.

16. Common misconceptions corrected

17. Key takeaways

Use metal-clad drawout switchgear for the 13.8 kV main distribution of a 10 MW facility, especially with MV generators or growth plans.

Use compact ring, pad-mounted or metal-enclosed switchgear with VFIs or breakers at the transformers, where it saves space and cost.

Check redundancy after the failure, against design-day peak, and confirm the load can actually transfer.

Make sure every ring and every "dual" service has truly independent sources.

Translate to US practice: 480Y/277 V, IEEE C37 and C57, UL, NEC and NFPA 70E/70B.

Treat transformer replacement time as the dominant reliability risk and design around it.

18. How Keentel Engineering helps

Keentel Engineering designs and studies MV distribution for data centers and other critical facilities, from 13.8 kV and 34.5 kV services to owner substations at 69, 115, 138 and 230 kV.

Why clients choose Keentel Engineering

Power system depth applied to data centers: utility-grade protection, short-circuit and reliability engineering, not just building distribution.

Technology-neutral recommendations: we specify the equipment that fits the requirement, not a product line.

Numbers behind every decision: load, fault duty, coordination, arc-flash and availability are calculated, not assumed.

US standards fluency: IEEE, NEC, NFPA, UL and utility requirements applied correctly.

Licensed professional engineering: a Florida-registered firm with offices in Tampa, Austin, Sacramento and Baltimore, supporting projects nationwide.

Planning a data center MV system, or checking whether an existing design really delivers N+1 or 2N? Talk to Keentel Engineering at (813) 389-7871, contact@keentelengineering.com, or book a 15-minute scoping call at calendly.com/keentel-engineering/15min.

Part C — Case Studies

The following are illustrative composite scenarios based on conditions typical of US data center projects. They show how Keentel Engineering approaches MV architecture decisions; site details are generalized and figures are rounded.

Case Study 1 — The "N+1" that wasn't

Situation. A developer in the Southeast was finalizing design for a 10 MW IT colocation facility with dual 13.8 kV utility feeders. The concept single-line showed a compact gas-insulated ring of six units, each with a fused tee to a 2,500 kVA, 13.8 kV – 480Y/277 V transformer, and was described in marketing material as "N+1 at every level." The ring was fed from one utility feeder; the second feeder connected only to the far end of the ring through a normally open switch.

Analysis. Keentel developed the load from first principles: 10 MW IT at UPS output, UPS losses at 96% efficiency, a design-day peak PUE of 1.36 and 0.95 power factor. Peak demand was 14.3 MVA. Six 2,500 kVA transformers provided 15 MVA — 95% loaded at peak, and only 12.5 MVA after one transformer failure. The 630 A ring bus would carry 598 A at peak with no growth margin. Both utility feeders originated from the same substation bus. The fused tees meant a single blown fuse would single-phase a UPS input, and the transformer temperature and pressure alarms had no way to trip. The arc-flash study showed 38 cal/cm² at the 480 V main line side, because only the primary fuse cleared that zone.

Engineered solution. Keentel redesigned the system as a 15 kV, 1,200 A, 40 kA metal-clad main-tie-main lineup with two bus sections, each feeding a separate set of transformers. Eight 2,500 kVA double-ended unit substations (four pairs with 480 V ties) replaced the six single-ended units, giving secondary-selective operation. Compact vacuum fault interrupter switchgear was retained at each transformer primary for footprint, with transformer alarms wired to trip. The developer asked the utility to supply the second feeder from a different substation transformer, which the utility agreed to with a modest line extension. Arc energy reduction used a light-sensing relay on each unit substation tripping the upstream VFI.

Outcome. After any single transformer or feeder failure, the remaining capacity exceeds design-day peak. 480 V arc-flash energy at the main dropped from 38 to under 8 cal/cm². The design was accepted by the facility's prospective anchor tenant, whose reliability consultant had flagged the original ring as non-compliant with its fault-tolerance requirement.

Case Study 2 — Retrofitting a fused loop at an enterprise campus

Situation. A financial-services enterprise in the Mid-Atlantic operated a 6 MW IT data hall fed from a 13.8 kV campus loop of pad-mounted fused switches installed in the 1990s. The owner planned a 4 MW expansion and experienced two events in three years in which a single primary fuse operated, leaving a transformer energized on two phases. In one event, a chiller motor tripped on unbalance and a UPS transferred to battery.

Analysis. Keentel performed short-circuit, coordination and arc-flash studies and inspected the loop. Utility fault duty had risen to 23 kA after a substation upgrade, exceeding the 16 kA rating of several pad-mounted units. The fuses were sized correctly under NEC 450.3, but their curves did not coordinate with the 480 V main breakers for low-level secondary faults. An IEEE 493 model showed that the single campus utility feeder, not the loop, accounted for most expected outage hours.

Engineered solution. The pad-mounted fused switches were replaced with 25 kA vacuum fault interrupter pad-mounted switchgear with three-phase tripping and ground-fault elements. A second 13.8 kV utility service from another substation was added, landing on a new metal-clad main-tie-main lineup that now feeds both ends of the loop through breakers. The loop operates normally open at the midpoint, with motorized switches and automatic restoration logic. The 4 MW expansion was supplied from new double-ended unit substations on the A and B buses.

Outcome. Single-phasing events were eliminated, short-circuit ratings were brought into compliance, and the expansion was energized without an outage to the existing hall. The modeled MV-path unavailability fell by about 60%.

Case Study 3 — Choosing the voltage and switchgear for a growing AI campus

Situation. A developer in the Southwest planned a campus starting at 10 MW IT and growing to 60 MW over five years. The utility could provide two 13.8 kV feeders for phase 1 but indicated that later phases would need 138 kV service. The initial design proposed compact ring switchgear throughout, to minimize first cost.

Analysis. Keentel compared three paths: compact switchgear now and replacement later; metal-clad 13.8 kV switchgear now, transferred to an owner substation later; and an owner 138 kV substation from day one. Future short-circuit duty at 13.8 kV with two 50 MVA, 8% transformers in parallel would reach about 46 kA — beyond any compact equipment rating — so the design was changed to operate those transformers split, each on its own bus section (24.5 kA each). The study also showed that 13.8 kV generator paralleling would cut the number of generators by a third compared with 480 V units.

Engineered solution. Phase 1 used 15 kV, 2,000 A, 40 kA metal-clad switchgear arranged as a main-tie-main lineup, with provisions for two future 138 kV transformer incomers and a 13.8 kV generator paralleling section. Compact VFI switchgear served transformer primaries in each data hall block. The owner reserved land and right-of-way for the 138/13.8 kV substation, which Keentel designed for phase 2.

Outcome. Phase 1 was energized on dual 13.8 kV feeders. When the 138 kV substation came online, the utility feeders were re-landed as alternate sources without replacing any switchgear, and the campus scaled without rebuilding its MV backbone.