Commercial electrical service sizes, amps, kVA and transformer

Service size chart, 100 to 4000 amps

kVA is the full capacity of the service, amps times volts times 1.732 divided by 1000. The transformer column is the smallest standard three phase pad mounted rating that can carry the full service rating. Above 2500 kVA a site usually moves to a unit substation or to several services.

Free to cite with a link to this page. kVA is computed. The equipment column follows the panelboard and switchboard ranges published by Tech Energy America. Panelboards top out at 1200 A.

What each transformer size can feed

Read the other way, a transformer rating sets the largest service it can carry at full load. A 500 kVA transformer delivers 601 A at 480 V, which matches a 600 A service, and 1,388 A at 208 V.

Free to cite with a link to this page. Full load amps equal kVA times 1000 divided by volts times 1.732.

208 or 480 volts

The same amps carry 2.3 times as much power at 480 V as at 208 V. A 1200 A service is 432 kVA at 208Y/120 and 998 kVA at 480Y/277. 208Y/120 suits a building whose load is mostly 120 V lighting and receptacles. 480Y/277 suits large motor and HVAC loads, with lighting at 277 V and dry type transformers inside the building stepping down to 208Y/120 for receptacles.

Going to 480 V shrinks the service gear and the feeders for the same load. A 2000 A load at 208 V becomes about 867 A at 480 V.

How many amps per square foot

The 2020 NEC cut the general lighting load for most occupancies. Office buildings dropped from 3.5 to 1.3 VA per square foot. The 2023 NEC renumbered the table as 220.42(A). Lighting is only part of a commercial load, and receptacles, HVAC, kitchen equipment and motors are added on top, so the totals below are a floor for the lighting part, not a service size.

EC&M worked a 300,000 square foot office building on 480Y/277 V. At the old 3.5 VA its lighting needed about 1,260 A of service. At 1.3 VA it needs about 469 A.

Lighting load only, computed from the 2020 NEC Table 220.12 unit loads as published by EC&M and exam prep references. Check the edition your jurisdiction enforces.

Ground fault protection at 1000 amps and up

NEC 230.95 requires ground fault protection of equipment on a solidly grounded wye service of more than 150 V to ground where the service disconnect is rated 1000 A or more. In practice that means every 480Y/277 V service of 1000 A or more. A 208Y/120 V service is not covered because it is only 120 V to ground.

The maximum setting is 1200 A, the maximum time delay is one second at 3000 A of ground fault current, and the system has to be performance tested when it is first installed. Budget for the test and the relay when you price a 1200 A or larger 480 V service.

Panelboard, switchboard or switchgear

Lead times differ by an order of magnitude. Tech Energy America lists a 400 A Eaton PRL panelboard at 1 to 2 weeks from its stock and 5 to 7 weeks from the factory, an 800 to 1600 A switchboard at 10 to 14 weeks, and 1600 to 4000 A switchgear at 16 to 26 weeks.

• Panelboards (UL 67) are wall mounted, front access and carry up to 1200 A. Most tenant services from 100 to 400 A are panelboards with a main breaker
• Switchboards (UL 891) are free standing and typically run from 800 to 4000 A. Most service entrances of 800 A and up are switchboards
• Switchgear (UL 1558) is used where the fault current or the size calls for it. Tech Energy America lists low voltage switchgear from 1600 to 4000 A

Who owns the transformer

On many services the utility owns and sizes the pad mounted transformer and the customer owns everything from the meter on. Primary metered customers buy their own transformer, and then the service size, the transformer rating and the lead time are all the customer's problem. If that is you, the chart above gives the starting rating and the pad mounted transformer pages give amps, dimensions and weights for each one.

What are the standard commercial electrical service sizes

100, 200, 400, 600, 800, 1000, 1200, 1600, 2000, 2500, 3000 and 4000 amps, the bus and main breaker ratings that switchboard makers build to. Most small commercial buildings have 200 to 800 A, larger buildings 1200 to 4000 A.

What size transformer do I need for an 800 amp service

At 480Y/277 V an 800 A service is 665 kVA, so a 750 kVA transformer carries it at full rating. At 208Y/120 V it is 288 kVA, so a 300 kVA transformer. The utility may install a smaller one sized on the calculated load.

What size transformer for a 1200 amp service

1000 kVA at 480Y/277 V, since 1200 A is 998 kVA. At 208Y/120 V it is 432 kVA, which takes a 500 kVA transformer.

How many kVA is a 400 amp 3 phase service

144 kVA at 208Y/120 V and 333 kVA at 480Y/277 V.

When is ground fault protection required on a service

Under NEC 230.95, on a solidly grounded wye service of more than 150 V to ground with a service disconnect rated 1000 A or more. That covers 480Y/277 V services of 1000 A and up and excludes 208Y/120 V.

How many amps per square foot for a commercial building

For the lighting part, the 2020 NEC uses 1.3 VA per square foot for offices, 1.9 for retail and 0.25 for warehouses. At 480Y/277 V, 10,000 square feet of office lighting is about 16 A. Receptacles, HVAC and equipment are added on top and usually dominate.

Can a panelboard be a 1200 amp service

Yes, panelboards are made up to 1200 A, but above about 800 A most service entrances are switchboards, which take larger mains and more metering and distribution sections.