Electrical Service Load Calc: NEC 220

An electrical service load calculation determines the minimum service size needed to safely supply a building's loads while applying the NEC's demand-factor rules — the code's recognition that not everything runs at full load at once. But the number is only right if the load calc, the panel schedules, the one-line, and the utility's requirements all agree, which is where this actually gets interesting.

An electrical service size is one of the few numbers on a set that two separate authorities check independently. The building department reviews it against the code. The utility reviews it against what they'll actually feed you. When your load calculation, your panel schedules, and your one-line all tell the same story, both say yes. When they don't, you get comments from one, a redesign request from the other, and a service you may have to re-coordinate after the drawings are "done."

Here's how we approach a commercial service calc so it holds up on both desks — grounded in NFPA 70 (the National Electrical Code), Article 220, which governs branch-circuit, feeder, and service load calculations.

Short answer: Under NEC Article 220, a service load calculation totals the connected loads (lighting, receptacles, HVAC, motors, and special loads), applies the code's demand factors so the service isn't sized for an impossible simultaneous peak, adds the 125% adder for continuous loads, and yields the calculated load that sizes the service, main, and feeders. The result must match the panel schedules and one-line.

Demand Factors: Connected Load vs. Calculated Load

The whole reason NEC Article 220 exists is that connected load (the sum of every nameplate) is not calculated load (what the service actually has to carry). Demand factors — sometimes called diversity factors — bridge the two, built on a simple reality: in a real building, not every receptacle, lamp, and motor draws its maximum at the same instant.

The code applies demand factors selectively: some load types get a straight percentage reduction above a threshold (receptacle loads, certain feeders), some get a graduated schedule, and some — the ones that genuinely run at full load — get no reduction at all. Sizing the service on connected load "to be safe" isn't conservative engineering; it's an oversized service, a bigger main, and a utility transformer larger than the building will ever use. The calculated load, done per the article, is the honest number — and the one a reviewer expects to see traced.

Continuous Loads and the 125% Rule

There's a rule that pulls the other direction from demand factors. A continuous load — one expected to run for three hours or more (most lighting, many HVAC loads, EV charging) — must be sized at 125% of its value for the conductor (Article 310) and overcurrent device (Article 240); a noncontinuous load carries no such adder. It's the code's thermal margin: gear carrying current for hours runs hotter than gear that cycles.

So a real service calc does both at once: it reduces diversified loads by demand factor and increases continuous loads by 125%. Getting the two straight is where a lot of calcs go wrong — applying the 125% to the wrong loads, or forgetting it on continuous lighting or an EV bank. The two rules aren't contradictory; they answer different questions (how much runs at once, vs. how long it runs), and the service has to satisfy both.

Service, Feeder, Transformer — and the Utility's Side

"Load calc" spans a few related sizings governed by different NEC articles, and it's worth keeping the distinction straight:

• Service (Article 230) — the service-entrance conductors and main, sized from the whole-building calculated load and typically landing in a switchboard or switchgear lineup.
• Feeders (Article 215) — each feeder to a downstream panelboard or MCC, sized from that panel's calculated load (same demand-factor logic, smaller scope).
• Load calculation itself (Article 220) — the method that feeds both.
• Transformer sizing — where the service is fed from a transformer (dedicated or utility), the transformer is sized to the calculated load with appropriate margin.

Then there's the boundary CoreX designs to but doesn't own: the utility's side. The design has to match the utility's requirements — available fault current (which sets equipment ratings and feeds directly into the short-circuit and coordination studies), metering form (self-contained vs. CT metering and a CT cabinet), the service voltage offered, and where the utility's transformer and service point land. CoreX sizes the service and coordinates to those requirements; the utility owns its transformer and its rules. Getting that coordination settled early avoids a service the utility won't energize.

Optional Methods, EV Loads, and Generator Interaction

Optional calculation methods. The standard method isn't the only path. NEC 220.87 lets you size from measured demand on an existing building (useful for renovations and additions), and 220.88 is an optional method for qualifying restaurants.

EV charging. EV loads are continuous (125% applies) and increasingly large; the design decision is how much diversity to assume across many chargers and how much future capacity to build in — under-provision and the next phase needs a service upgrade.

Generator interaction. Where emergency or standby power exists — the kind of coordination that's non-negotiable in healthcare facilities running on generator backup — the load calc feeds the generator sizing and the transfer scheme, the same classification and coordination work covered in our emergency and standby power guide. The service calc doesn't stop at the utility side; it hands off to the backup-power design.

A Quick Worked Example — and Why It Has to Agree With Everything Else

Take an office floor (illustrative): total the general lighting (VA per square foot), the receptacle load, the HVAC and motor loads, and any special or kitchen loads. Apply the demand factors where the article allows, add 125% on the continuous portions, and you get the calculated load — which sizes the service, the main breaker, and the feeders.

Here's the part that often gets skipped: that number can't live alone. The panel schedules have to sum to it, the one-line has to show a service and main that match, and the utility has to confirm it can serve it at the available fault current and metering form. The common failures are all coordination failures: using connected load directly, forgetting the 125% on continuous loads, a panel schedule that doesn't reconcile with the service calc, or a one-line that shows a different main. A calc that's internally right but disagrees with the panel schedule is exactly what bounces at plan check.

The Reviewer's Real Test: Does Everything Agree?

This is the part that bounces sets. The load calculation says the service is X. The panel schedules add up to Y. The one-line shows Z. If those three disagree, a reviewer can't tell which is the real design — so all of them get flagged, and now you're reconciling under a clock.

Treat the load calc, the panel schedules, and the one-line as three views of the same system. They have to agree before the set goes out. Every large load on a schedule should be traceable in the calc; the calc's total should match what the one-line distributes. That's MEP coordination in miniature — one system, three documents, reconciled before anything is issued.

Headroom Without Hand-Waving

Owners add load. Tenants get built out. EV counts climb. Good electrical design leaves documented spare capacity — but *documented* is the operative word. Spare capacity that's shown as a deliberate provision (labeled spare breakers, reserved feeder capacity, a stated growth allowance) reads as engineering. A service padded 40% "to be safe" with no basis reads as guessing, costs the owner real money in gear and utility charges, and invites the reviewer to ask why. Locking the right amount of headroom is really a design-phase decision — cheap in schematic design, expensive once construction documents are underway.

Coordinate With the Utility Early

The city checks code; the utility checks *their* system. Service entrance location, metering, CT cabinets, transformer pad, available fault current at the service point — these are utility questions that can reshape your design if they surface late. Getting the load and the service configuration in front of the utility early keeps their requirements from becoming a redesign after the drawings are set.

Related reading: Power-System Studies (Short-Circuit / Coordination / Arc-Flash) · Emergency vs. Standby Power (NEC 700/701/702 & NFPA 110) · MEP Coordination Best Practices

We run the standard method by default, reach for an optional method only when the project genuinely qualifies, and reconcile the load calc against the panel schedules and one-line before anything is issued. See our electrical services, or schedule a scope call.

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