Before you can size a residential service, you need to calculate the total expected load. The NEC provides two methods: the standard method under Article 220 Part III, and the optional method under 220.82. Both are code-compliant and both can result in different service sizes for the same house — the optional method frequently allows a smaller service because it uses more realistic demand factors.
Why the load calculation matters
An undersized service will trip the utility disconnect under normal load conditions, and an oversized one wastes money on infrastructure. More importantly, the load calculation is the foundation of the design — it determines the service ampacity, which drives the main breaker rating, the meter base, and the service entrance conductor size. Getting this wrong at the design stage creates problems that are expensive to fix later.
Standard method (NEC Article 220 Part III)
The standard method adds up loads category by category, applying demand factors where the NEC allows. The steps for a single-family dwelling are:
Step 1 — General lighting and receptacle load
NEC 220.12 requires a general lighting load of 3 VA per square foot of floor area (or 33 VA/m²). This covers general-purpose lighting and receptacle circuits. For a 2,000 sq ft house: 2,000 × 3 = 6,000 VA.
Apply the demand factors from Table 220.42:
- First 3,000 VA at 100%: 3,000 VA
- Next 3,001–120,000 VA at 35%: 3,000 × 35% = 1,050 VA
- Total after demand: 4,050 VA
Step 2 — Small appliance and laundry circuits
NEC 220.52 requires a minimum of two 20 A small-appliance branch circuits for kitchen areas (1,500 VA each) and one 20 A laundry circuit (1,500 VA). Total before demand: 3 × 1,500 = 4,500 VA. Apply the Table 220.42 demand factors to this amount added to the general lighting load.
In practice, most designers add 4,500 VA to the 6,000 VA general load before applying Table 220.42 demand factors to the combined total.
Step 3 — Large appliances at nameplate
Fixed appliances with motors or heating elements are added at 100% of nameplate rating (or calculated load). Common items include:
- Dishwasher: 1,200 VA (typical)
- Garbage disposal: 900 VA
- Water heater (electric, 240 V): 4,500 VA
- Clothes dryer: 5,000 VA or nameplate, whichever is larger
If there are four or more fixed appliances (excluding electric ranges, clothes dryers, space heating, and A/C), NEC 220.53 allows a 75% demand factor.
Step 4 — Heating and cooling (larger load only)
NEC 220.60 states that where a building has both heating and cooling, only the larger of the two is included in the service calculation — they will not run simultaneously at full load. A 5-ton A/C unit at 240 V drawing 20 A contributes 4,800 VA; if the electric furnace is smaller, only the A/C figure is used.
Step 5 — Electric range
Ranges are derated under Table 220.55. For a single household range rated up to 12 kW, the demand load is 8,000 VA (8 kW). For ranges rated over 12 kW, the 8 kW base is increased by 5% for each kW above 12.
Step 6 — Convert to amperes
Total the VA from all steps. For a 240 V single-phase service:
Amperes = Total VA ÷ 240
Select the next standard service size that equals or exceeds the calculated value. Standard residential service sizes are 100, 150, 200, 320, and 400 A.
Optional method (NEC 220.82)
The optional method is simpler and generally produces a lower calculated load, which is why many designers prefer it. It is only available for existing or new single-family dwellings where the total service load can be calculated.
Step 1: Add 100% of the first 10 kVA of the total load to 40% of the remainder. The "total load" includes the general load at 3 VA/sq ft (no demand factors), all small appliance and laundry circuits at 1,500 VA each, and all nameplate appliance loads.
For heating and cooling, you use the larger of:
- 100% of the nameplate of A/C equipment
- 65% of the nameplate of electric space heating (central systems)
Worked example
A 1,800 sq ft house with the following equipment: electric range (10 kW), electric water heater (4.5 kW), clothes dryer (5.5 kW), dishwasher (1.2 kW), central A/C (4.8 kVA), and two small appliance circuits.
Using the standard method:
- General load: 1,800 × 3 = 5,400 VA
- Small appliance + laundry: 3 × 1,500 = 4,500 VA
- After Table 220.42 demand on 9,900 VA: 3,000 + (6,900 × 0.35) = 5,415 VA
- Range (Table 220.55): 8,000 VA
- Water heater: 4,500 VA
- Dryer: 5,500 VA
- Dishwasher: 1,200 VA (4 appliances >3, apply 75%: 1,200 × 0.75 = 900 VA)
- A/C (larger of heating/cooling): 4,800 VA
- Total: 5,415 + 8,000 + 4,500 + 5,500 + 900 + 4,800 = 29,115 VA
- Service amps: 29,115 ÷ 240 = 121 A → 150 A service
EV charger additions
NEC 220.57 (added in 2023 NEC) addresses dwelling unit EV charging. A level 2 EVSE (240 V, 48 A rated circuit) adds 11,520 VA at 100% if it is the only EV charger. With multiple chargers, demand factors may apply. This is one of the main reasons newer homes are moving from 200 A to 320 A or even 400 A services — the EV charger and electric vehicle charging while the HVAC and range are running is a genuinely high combined load.
Calculate panel load instantly
Use the free Voltix electrical load calculator to work through a NEC Article 220 calculation for any panel or service. For sizing the conductors that feed the service, the wire size calculator gives you the correct AWG for any ampacity under NEC Table 310.16.