Wire Size for a 100 Amp Service: Copper, Aluminum and Distance
The exact conductors for a 100A residential service: 4 AWG copper or 2 AWG aluminum under NEC 310.12, plus grounding sizes and voltage-drop checks for long runs.
Run the Numbers
For NEC 310.15(B)(1) correction. 30°C is the table baseline.
Ampacity follows the NEC procedure: Table 310.16 ampacity corrected for ambient temperature and conductor count, capped by the termination rating and the 240.4(D) small-conductor limits, then checked against your voltage-drop target. For continuous loads, enter the load current multiplied by 1.25.
Recommended Conductor Gauge
Nominal Area: 5.26 mm² (drop alone would need 3.99 mm²)
Table 310.16 ampacity × 1 ambient × 1 conductor-count, capped by termination rating
Sizing Scope Notice
Step-by-Step Engineering Calculation
- 1Load: at , one-way length , COPPER conductors.
- 2Ampacity derating (NEC 310.15): ambient 30°C correction (90°C column) × conductor-count adjustment (3 current-carrying) = combined factor .
- 3Termination limit (NEC 110.14(C)): allowable ampacity is capped at the 75°C column of Table 310.16, and at the NEC 240.4(D) small-conductor limits where they apply.
- 4Voltage drop limit: , checked with NEC Chapter 9 Table 8 resistance at 75°C. (Drop alone would need at least 3.99 mm².)
- 5Selected conductor: 10 AWG (5.26 mm²). Allowable ampacity after derating and caps: ✓. Actual voltage drop: (2.28%) ✓.
- 6Voltage at load:
Step-by-Step Procedure
Confirm the NEC 310.12 Dwelling Rule Applies
For a single-family dwelling service (or a feeder carrying the entire dwelling load) rated 100A to 400A, NEC Table 310.12 lets you size conductors at 83% of the service rating instead of using the general Table 310.16 values. 100A × 0.83 = 83A minimum conductor ampacity.
Pick the Conductor: 4 AWG Copper or 2 AWG Aluminum
From Table 310.12: copper 4 AWG (85A at 75°C) or aluminum 2 AWG (90A at 75°C) both meet the 83A floor. Aluminum SER cable is the common budget choice for service laterals; copper costs more but terminates in smaller lugs and needs no anti-oxidant at listed lugs.
Check Voltage Drop for the Actual Distance
Table 310.12 guarantees ampacity, not voltage drop. At a full 100A over 100 ft, 4 AWG copper drops about 6.2V (2.6%), and past roughly 120 ft it crosses the 3% guideline. Real dwelling load is rarely a sustained 100A, but for long runs, or a detached building, upsize one step (3 AWG copper or 1/0 aluminum) and verify with the wire size calculator embedded above.
Size the Grounding Electrode Conductor (GEC)
Per NEC Table 250.66, a service with 2 AWG or smaller copper (or 1/0 or smaller aluminum) service conductors takes an 8 AWG copper / 6 AWG aluminum GEC to the ground rods. A GEC to a ground rod alone never needs to be larger than 6 AWG copper per 250.66(A).
Size the Equipment Grounding Conductor (Subpanel Feeds)
If the 100A run feeds a subpanel, the equipment grounding conductor follows Table 250.122 from the 100A breaker: 8 AWG copper or 6 AWG aluminum. A detached building also needs its own grounding electrode system and a disconnect, per 225.31 and 250.32.
Neutral and Inspection Notes
The neutral is sized from the calculated neutral load (220.61) and may be smaller than the ungrounded conductors, but for a 100A dwelling service 310.12 sizing is normally applied to the whole cable assembly. Service work almost always needs a permit and inspection, and the utility controls the meter and drop side.
Worked Practical Example
100A Service Upgrade, 90 ft Lateral
A home upgrades from 60A to a 100A, 120/240V service. The new panel sits 90 ft from the meter base. The owner wants aluminum SER for cost.
- •310.12 minimum ampacity: 100A × 0.83 = 83A
- •Aluminum 2 AWG ampacity (75°C) = 90A ≥ 83A ✓ (Table 310.12 lists 2 AWG Al for 100A services)
- •Voltage drop check at a realistic 80A coincident load, using the calculator above: about 4.5V, roughly 1.9% of 240V ✓
- •GEC: 8 AWG copper to two ground rods (250.66). Main breaker: 100A.
- •Result: 2-2-2-4 aluminum SER (the 4 AWG is the reduced neutral/ground assembly size commonly supplied) or 2 AWG THHN/XHHW-2 conductors in conduit.
Common Mistakes to Avoid
- ✕Using the 83% dwelling rule for a partial-load subpanel or a commercial service, where full Table 310.16 ampacity is required.
- ✕Picking 4 AWG aluminum for 100A. Aluminum needs 2 AWG; 4 AWG aluminum is a 65A-class conductor at 75°C and is a classic failed-inspection error.
- ✕Ignoring voltage drop on long rural laterals because the ampacity table 'says it fits'.
- ✕Bonding neutral and ground together in a subpanel. They are separated everywhere downstream of the service disconnect.
Frequently Asked Questions
Can I use 4 AWG copper for a 100 amp subpanel?
Only if that subpanel carries the entire load of a dwelling unit (310.12). For a normal partial-load subpanel, 4 AWG copper is 85A at 75°C, short of 100A, so you need 3 AWG copper (100A) or the panel must be fed at 90A or less.
Is aluminum wire safe for a 100A service?
Yes. AA-8000 series aluminum alloy feeders are standard practice, listed for the use, and installed by utilities everywhere. The terminations must be AL-rated, torqued to spec, and the cable protected from physical damage.
What size wire for 100 amps at 150 feet?
Ampacity still allows 4 AWG copper or 2 AWG aluminum, but voltage drop at heavy load reaches roughly 3 to 4%. Upsizing to 3 AWG copper or 1/0 aluminum keeps drop near 2% at an 80A working load. Run the exact numbers in the calculator above.