Electrical Load Calculator — Residential Service Sizing

How a residential load calculation works

When to use this: deciding whether a house needs a 100 A, 150 A or 200 A service; checking whether an existing panel can take an EV charger or a heat pump; or sanity-checking a figure an electrician has quoted you.

The obvious approach — add up every appliance nameplate — gives a wildly wrong answer. A modest house has perhaps 40 kW of connected equipment, which at 240 V would suggest a 170 A service before you have plugged in a single lamp. But you never run all of it at once, and the NEC codifies that reality as demand factors.

The standard calculation, in the order the code specifies:

1. General lighting at 3 VA per square foot of habitable area
2. Small-appliance circuits at 1,500 VA each, minimum two
3. Laundry circuit at 1,500 VA
4. Demand factors applied to that group only — first 3,000 VA at 100%, the rest at 35%
5. Fixed appliances at nameplate, reduced to 75% if there are four or more
6. Range from Table 220.55 — not nameplate
7. Dryer at nameplate or 5,000 VA, whichever is larger
8. Heating or cooling, whichever is larger — never both
9. 25% of the largest motor

The order matters because the demand factors apply to one specific group. Applying them to the whole total, or to the appliances, is the error that makes two people's hand calculations disagree.

The demand factor that does the heavy lifting

Only the first 3,000 VA of the lighting group counts in full:

For a 1,500 ft² house: general lighting is 4,500 VA, plus 3,000 VA of small-appliance circuits and 1,500 VA of laundry, giving 9,000 VA connected. After demand factors that becomes 3,000 + (6,000 × 0.35) = 5,100 VA — a 43% reduction, and the single biggest saving in the whole calculation.

Why a 12 kW range counts as 8 kW

Table 220.55 sets the demand for one household range at 8 kW for any nameplate up to 12 kW. Above that, add 5% for each additional kilowatt.

The logic is that a range never draws nameplate for any length of time. Elements thermostatically cycle, and you rarely run four burners and the oven simultaneously at full power. Using nameplate instead of the table is the most common error in this calculation and it oversizes the service — expensively, since a service upgrade is largely labour.

Worked example — the textbook 1,500 ft² house

12 kW range, 5.5 kW dryer, 6 kW air conditioning, gas water heating.

24,600 / 240 = 102.5 A → the smallest standard service that carries it is 125 A.

Note the gap: 32,500 VA connected, 24,600 VA demand. Without the code's demand factors this house would appear to need a 150 A service instead of 125 A.

What size service do houses actually need?

The trend is firmly upward. Electrification — heat pumps replacing gas furnaces, induction replacing gas ranges, EV charging — is pushing houses that were comfortable on 100 A towards 200 A.

Adding an EV charger

This is now the most common reason to run a load calculation on an existing house. A charger is a continuous load, so it is counted at 125% of its rating:

A 48 A charger adds 14,400 VA — 60 A at 240 V — to the calculated load. That is enough to push many 150 A services past their limit on its own.

If it does not fit, the alternatives before a full service upgrade are: set the charger to a lower current (most are configurable, and 32 A still adds roughly 40 km of range per hour), or fit an energy management system that sheds the charger when the rest of the house draws heavily. The code explicitly permits load management as an alternative to upsizing.

The optional calculation

Article 220 also contains an optional method for dwellings, which usually produces a smaller result. It takes the first 10 kVA of general load at 100% and everything above at 40%, with separate rules for heating and cooling.

For an existing house that fails the standard calculation, running the optional method is the first thing to try — it frequently shows the service is adequate after all. This calculator implements the standard method, which is the conservative one.

Where this calculation stops

• It sizes the service, not the branch circuits. Each circuit has its own requirements for conductor size, overcurrent protection and GFCI/AFCI.
• It assumes a single-family dwelling. Multi-family and commercial calculations use different articles and different demand factors entirely.
• It does not check the panel. A 200 A service still needs enough physical breaker spaces, and an old panel may have no room or be a type no longer supported.
• It does not check the utility supply. The service drop, meter base and transformer all have to support the upgrade, and that is the utility's decision.