Service Size Calculator
Enter a known service load in amps or VA and get the minimum service disconnect rating (NEC 230.79), the service-entrance conductor sized to 230.42 on the Table 310.16 75 C column, and the grounding electrode conductor from Table 250.66, with the 250.66(A) and 250.66(B) electrode caps.
How to use this calculator
- Enter the calculated load in amperes, or switch to VA and add the voltage and phase. Get the load from one of the Article 220 load calculators first if you have not run it yet.
- Enter the continuous portion (loads running 3 hours or more) so the 230.42 125 percent factor is applied only where it belongs.
- Pick the service type for the 230.79 minimum, then the service conductor material and the grounding electrode conductor material.
- Choose the electrode connection to apply the 250.66(A) rod, pipe, or plate cap or the 250.66(B) concrete-encased cap. Read the service rating, conductor size, and grounding electrode conductor, with the notes explaining any caps and the 310.12 dwelling allowance.
NEC reference
Sizing follows NEC 2020. Service-entrance conductor ampacity is 230.42(A)(1): the noncontinuous load plus 125 percent of the continuous load, sized on the Table 310.16 75 C column per 110.14(C). The service disconnect rating is the 230.79 minimum for the occupancy (100 A one-family dwelling, 60 A all others, 30 A two-circuit, 15 A one-circuit), rounded up to a standard 240.6(A) rating. The grounding electrode conductor comes from Table 250.66, keyed to the largest ungrounded service conductor, with the 250.66(A) and 250.66(B) electrode caps. These tables and rules are unchanged across NEC 2017, 2020, and 2023.
Results are for reference only. Verify against the applicable adopted edition of the NEC and consult a licensed electrician for code compliance.
The three things "service size" means
- The disconnect rating (230.79). The ampere rating on the main breaker or fused disconnect. It is the number people mean by "200 A service." It cannot be smaller than the calculated load or the article minimum for the occupancy.
- The service-entrance conductors (230.42). Sized for the load ampacity, 125 percent of continuous plus 100 percent of noncontinuous, on the 75 C column of Table 310.16.
- The grounding electrode conductor (Table 250.66). Sized from the service conductor, not from the load directly, and often reduced by the 250.66(A) or 250.66(B) electrode caps.
Table 250.66 read-across (copper service conductor)
| Largest service conductor (copper) | GEC copper | GEC aluminum |
|---|---|---|
| 2 AWG or smaller | 8 AWG | 6 AWG |
| 1 or 1/0 AWG | 6 AWG | 4 AWG |
| 2/0 or 3/0 AWG | 4 AWG | 2 AWG |
| Over 3/0 through 350 kcmil | 2 AWG | 1/0 AWG |
| Over 350 through 600 kcmil | 1/0 AWG | 3/0 AWG |
| Over 600 through 1100 kcmil | 2/0 AWG | 4/0 AWG |
| Over 1100 kcmil | 3/0 AWG | 250 kcmil |
Aluminum service conductors shift the thresholds up one column (for example 4/0 aluminum through 250 kcmil takes the same 4 AWG copper grounding electrode conductor as 2/0 or 3/0 copper). Exceptions 250.66(A) rod, pipe, or plate cap at 6 AWG copper and 250.66(B) concrete-encased cap at 4 AWG copper apply to that portion of the run.
Worked example: 200 A dwelling, aluminum service
A one-family dwelling with a 200 A calculated load, aluminum service conductors, and a copper grounding electrode conductor. The 230.79(C) minimum is 100 A, but the 200 A load governs, so the service rounds to a 200 A disconnect (240.6(A)). The service-entrance conductor is the smallest 75 C aluminum that carries 200 A, which is 250 kcmil (205 A). From Table 250.66, a 250 kcmil aluminum service conductor takes a 4 AWG copper grounding electrode conductor. If the only electrode were a driven ground rod, 250.66(A) would allow that conductor to drop to 6 AWG copper. Note that NEC 310.12 would permit 4/0 aluminum for this dwelling service, one size below the general result here.
Frequently asked questions
What is the minimum service size for a house?
NEC 230.79(C) requires a 100 A minimum service disconnect for a one-family dwelling. Most modern homes calculate to 150 A or 200 A once ranges, dryers, HVAC, and EV charging are included.
What grounding electrode conductor for a 200 A service?
4 AWG copper from Table 250.66 for a 2/0 or 3/0 copper (or 4/0 aluminum through 250 kcmil) service conductor. A ground rod only run can drop to 6 AWG copper under 250.66(A).
Why is my service conductor smaller than this tool says?
For 100 A through 400 A single-family dwelling services, NEC 310.12 permits conductors sized at 83 percent of the rating, so real dwelling installs run one size smaller than the general Table 310.16 result. This calculator reports the general full-ampacity conductor and flags the 310.12 allowance, which is a documented v1 exclusion.
Does this size the neutral or the equipment grounding conductor?
No. The grounded (neutral) service conductor is sized per 250.24(C) and 220.61, and equipment grounding conductors per Table 250.122. This tool covers the ungrounded service conductor, the disconnect rating, and the grounding electrode conductor only.
Related calculators
- Electrical Load Calculator - dwelling load by the NEC 220.82 optional method.
- Residential Electrical Load Calculator - the Article 220 Part III standard method.
- Commercial Load Calculator - non-dwelling load to a service size.
- Wire Size Calculator - feeder and branch conductor sizing with voltage drop.
- Breaker Size Calculator - overcurrent device sizing per 240.6(A).
Studying for the licensing exam? Our own NEC Code Quickstart works all twelve exam calculations start to finish, each with the code reference. Also in paperback.
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