The calculator is set to a whole-house service at 100 amps and 240 volts. Enter the distance from the meter to the panel and choose the metal to see the size, and whether a long run calls for more.
How to use this calculator
- Leave the rating at 100 amps, the main breaker of the house.
- Choose copper or aluminum. Most new services use aluminum SE or USE cable.
- Enter the one-way distance from the meter or the service disconnect to the main panel.
- For a subpanel or a detached building, switch to a circuit or a subpanel: the service allowance does not apply there.
Worked examples
A meter on the outside wall, panel just inside
The wire feeds: A whole house (service) · Load current: 100 A · Circuit voltage: 240 V · One-way distance: 15 ft · Conductor: Aluminum
- Wire size
- 2 AWG aluminum
- Current the service wire must carry
- 83.0 A
- Rated for
- 90 A
- Voltage drop
- 0.40%
- Longest run within 3%
- 113 ft
A meter on a pole at the edge of the lot
The wire feeds: A whole house (service) · Load current: 100 A · Circuit voltage: 240 V · One-way distance: 140 ft · Conductor: Aluminum
- Wire size
- 1 AWG aluminum
- Current the service wire must carry
- 83.0 A
- Rated for
- 100 A
- Voltage drop
- 2.95%
- Longest run within 3%
- 142 ft
A copper service in an older house
The wire feeds: A whole house (service) · Load current: 100 A · Circuit voltage: 240 V · One-way distance: 30 ft
- Wire size
- 4 AWG copper
- Current the service wire must carry
- 83.0 A
- Rated for
- 85 A
- Voltage drop
- 0.77%
- Longest run within 3%
- 117 ft
Service conductor size for a 100 amp dwelling by distance
| One-way distance | Copper | Aluminum |
|---|---|---|
| 25 ft | 4 AWG | 2 AWG |
| 50 ft | 4 AWG | 2 AWG |
| 100 ft | 4 AWG | 2 AWG |
| 150 ft | 2 AWG | 1/0 AWG |
| 200 ft | 1 AWG | 2/0 AWG |
Each cell: the smallest conductor under the dwelling service rule that also keeps the drop at full rated current within the branch-circuit limit.
The dwelling service rule
NEC Table 310.12 lists the service sizes for one-family homes, and the rule behind it is simple: the conductors must carry 83 percent of the rating. For a service of 100 amps the table gives 4 AWG copper, rated 85 amps, or 2 AWG aluminum, rated 90 amps. It applies to the service entrance conductors and to the main feeder that carries the whole house's load, single-phase, without heat or bundling corrections.
When the allowance does not apply
A feeder to a subpanel, a detached garage or one wing of the house is sized from the regular ampacity table at the full rating: see the 100 amp subpanel page, where the answer is 3 AWG copper or 1 AWG aluminum. A duplex served by one set of conductors, a three-phase service and a hot attic run all fall outside the rule too.
Upgrading from a 100 amp service
Many older homes have exactly this service. A heat pump, a car charger or an electric range can push the load past it, and the usual step up is a 200 amp service with new service conductors, meter base and panel, arranged with the utility. A load calculation by an electrician tells you whether the upgrade is needed; the amps calculator helps total up the big appliances first.
Common questions
What size wire for a 100 amp service?
4 AWG copper or 2 AWG aluminum for the service of a single-family house, under NEC Table 310.12, which sizes the conductors for 83 percent of the rating.
Is 2 AWG aluminum good for 100 amp service?
Yes, for a whole-house service: it is rated 90 amps, above the share of the rating the dwelling rule asks for. For a 100 amp subpanel it is too small; that needs 1 AWG aluminum.
Can I use 4 AWG copper for 100 amps?
For the service of a single-family house, yes. For any other 100 amp feeder, no: 4 AWG copper is rated 85 amps and the feeder must carry the full hundred.
Does the distance to the meter change the size?
On a long run, yes. With a meter on a pole far from the house, voltage drop can call for a larger size; enter the distance above to check.