How to Do a Commercial Load Calculation

1. Establish areas and occupancies first

Measure each occupancy's floor area the way Article 220 directs — from the outside dimensions. A mixed building gets split: the 4,000 ft² of office and the 2,000 ft² of storage each take their own row in the stack with their own unit lighting load. Guessing areas is how calcs drift 20% before a single table is opened.

While you're at the plans, list everything that will actually connect: mechanical schedule, kitchen schedule, sign, site lighting, elevator, water heating. Article 220 is only as complete as the inventory it starts from — the calc that misses the well pump misses it forever.

2. General lighting: Table 220.12, times 125% where continuous

Multiply each occupancy's area by the unit load in Table 220.12. Check the table in the adopted edition rather than quoting an old number from memory — the 2023 NEC rebuilt these values around modern LED energy budgets, and they dropped hard: office space fell from the 3.5 VA/ft² older editions carried to 1.3 VA/ft². A 6,000 ft² office is now 7,800 VA of general lighting, not 21,000.

Commercial lighting is a continuous load — expected to run three hours or more — so the service and feeder calculation carries it at 125% per 210.20(A) and 215.3. Track lighting, show windows, and outside lighting get their own adders under 220.14; don't bury them in the square-footage number.

3. Receptacles: 180 VA per yoke, then the 220.44 haircut

Count every receptacle yoke on the plans at 180 VA per 220.14(I) — a duplex is one yoke, a quad is two. Office buildings and banks get an alternative under 220.14(K): figure the receptacle load at 1 VA/ft² where the actual count is unknown, or the larger of the two when it is. On a furniture-heavy office, the counted number nearly always governs.

Then apply Table 220.44: the first 10 kVA of nondwelling receptacle load counts at 100%, and everything beyond at 50%. Seventy-four yokes is 13,320 VA connected, but only 10,000 + 3,320 × 0.5 = 11,660 VA lands in the service calc. This is the code acknowledging that nobody plugs in everything at once — take the haircut, it's yours.

4. HVAC: the bigger of heating or cooling — never both

220.60 lets you drop the smaller of two loads that can't run at the same time, and heat versus cool is the everyday case: a heat pump's compressor load and its supplemental heat can coincide (read the sequence of operation), but gas heat versus DX cooling never do. Take the larger, drop the smaller, and document which one won.

Use real equipment data, not tonnage folklore: minimum circuit ampacity and nameplate ratings off the mechanical schedule, converted to VA at the equipment voltage. Then add 25% of the largest motor in the whole building per 430.24 — one motor, once, usually the biggest compressor. Every motor at 125% is a classic over-count; the premium belongs to the largest only.

5. Kitchens, signs, and fixed appliances

Commercial kitchen equipment gets Table 220.56: three units at 90%, four at 80%, five at 70%, six or more at 65% of the summed nameplates — with the floor that the demand figure can never be less than the sum of the two largest appliances. A deli line of six pieces totaling 30 kVA calculates at 19.5 kVA, and that's real money off the transformer size.

Buildings with grade-level public entry owe a sign outlet at a minimum 1,200 VA per 220.14(F) and 600.5(A) — continuous, so 1,500 VA in the stack. Water heaters, elevators, air compressors, and similar fixed equipment ride at nameplate; nothing in Part III of Article 220 discounts them for ordinary commercial occupancies.

6. Sum the stack and convert to amps

The worked example, all blocks together on a 6,000 ft² office, 208Y/120V three-phase: lighting 7,800 × 1.25 = 9,750. Receptacles 13,320 → 11,660 after Table 220.44. Sign 1,500. Cooling 16,000 beats the 12,000 of heat, plus 1,700 for 25% of the largest motor. Water heater 4,500. Total: 45,110 VA.

Amps = VA ÷ (volts × √3) for three-phase: 45,110 ÷ (208 × 1.732) = 45,110 ÷ 360.3 ≈ 125 A. A 200 A service carries it at 63% — which is the answer we'd actually build, because the next tenant improvement always brings load, and the delta between 150 A and 200 A gear is small next to a future service change.

7. Sanity-check against reality

A calc that produces 45 kVA for a building whose twin next door bills 18 kVA peak is telling you about the code's margins, not the building's future bill — Article 220 is deliberately conservative. That's fine for sizing gear; it's a problem only when someone reads the calc as an energy forecast or sizes a generator from it without a measured profile.

Cross-check three things before the calc ships: the service conductors and OCPD rating against 230.42 and 230.90, the transformer kVA if one feeds the service, and the panel schedule's phase balance — 125 calculated amps spread badly across three phases is still a hot leg. Our panel schedule tool flags the imbalance for free.

8. Existing buildings: measure instead of guessing — 220.87

When the question is 'can this service take the new load,' 220.87 beats re-deriving a paper calc for a building full of unknowns: take the maximum demand over the most recent 12 months — utility demand data qualifies — multiply by 125%, add the new load, and compare against the service rating. The alternative where records don't exist is a recording meter for 30 days under the conditions the section spells out.

This is the method that gets EV chargers, RTU swaps, and tenant build-outs approved without a service upgrade — a 400 A service peaking at 210 A has 400 − 262.5 = 137.5 A of code-defensible headroom. Bring the utility data to the plan reviewer and the conversation is short. It's the exact study we run for owners weekly, and it routinely saves the cost of a new service.

What changed in the 2023 NEC lighting loads?

Table 220.12 was overhauled to reflect LED-era energy codes, and most occupancy values dropped sharply — office space went from 3.5 VA/ft² in older editions to 1.3 VA/ft². Always read the adopted edition's table; a memorized pre-2023 value can double the lighting block of a calc.

How many amps is a commercial load calculation total?

Divide total VA by the system voltage times the square root of three for three-phase: 45,110 VA on a 208Y/120V service is 45,110 ÷ (208 × 1.732) ≈ 125 A. For single-phase, divide by the line-to-line voltage alone. Then pick the next standard service size with the growth margin the owner can live with.

Do I count heating and cooling both?

No — NEC 220.60 permits omitting the smaller of two loads that are unlikely to operate simultaneously, and heat versus cool is the textbook case. Use the larger, and document it. The caveat is heat pumps with supplemental electric heat, where compressor and strip heat can run together and both belong in the number.

What is the 180 VA receptacle rule?

NEC 220.14(I) assigns 180 VA to each receptacle yoke — a duplex counts once, a quad twice — for nondwelling calculations. Table 220.44 then applies demand: 100% of the first 10 kVA and 50% of the rest. Office buildings and banks can alternatively use 1 VA/ft² under 220.14(K) where counts are unknown, taking the larger of the two when both exist.

How do I prove an existing service can handle new load without a full recalculation?

NEC 220.87: take the maximum demand from the most recent 12 months of utility data, multiply by 125%, add the proposed load, and compare to the service rating. No 12-month record means a 30-day recording meter under the section's conditions. It's the standard path for adding EV charging or replacement RTUs without a service upgrade.

Does spare capacity mean I should downsize the service?

Almost never. The cost difference between 150 A and 200 A gear at construction time is small; a service upgrade later is excavation, utility coordination, shutdown, and permit work. We size to the calculation, then round up to the standard size that leaves honest tenant-improvement headroom — most owners thank the margin within five years.

Are commercial EV chargers part of the load calculation?

Yes — EVSE is a continuous load, carried at 125% of its rating, and a bank of chargers adds up fast. Article 625 permits an energy management system to cap the calculated load at the EMS setpoint per 625.42, which is the practical way to put six chargers on a service that has headroom for three. See our commercial EV charger guide for the full treatment.