Is 4 AWG enough for 100 amps?

Yes for a qualifying 100 A dwelling service: the 83% calculation requires 83 A, and 4 AWG copper is rated 85 A in the verified 75 °C table. Otherwise, check the actual method in the Ampacity Check tool.

What wire size should I use for a 100 amp sub-panel?

Use the 100-amp sub-panel wire size guide. It owns the feeder distance tables and checks that job under the ordinary feeder rules.

Should I use copper or aluminum for a 100 amp service?

Both can work when the equipment and installation are listed for the conductor. The dwelling-service derivation gives 4 AWG copper and 2 AWG aluminum; aluminum is often chosen to reduce weight and material cost.

Does the 83% rule apply to Romex?

No. NM/Romex is treated as a 60 °C assembly, while this dwelling-service allowance requires conductors rated 75 °C or better.

🏠 What Size Wire for a 100 Amp Service?

For a qualifying dwelling service, the verified result is 4 AWG copper or 2 AWG aluminum. 🇨🇦 Canadian version

The short answer

A 100 A dwelling service that meets NEC 310.12 can use 4 AWG copper or 2 AWG aluminum. That is a service answer, not a general 100 A feeder answer.

Service, whole-dwelling feeder, or sub-panel?

A feeder carrying the entire dwelling load may use the same allowance when every condition below is met. Feeding a 100 A sub-panel that does not carry the whole dwelling is a different job; its ordinary feeder sizing and distance tables are covered here: 100-amp sub-panel wire size guide.

All four conditions for the 83% allowance

100 A × 83% = 83 A → 4 AWG copper (85 A at 75 °C); 2 AWG aluminum (90 A at 75 °C).

Dwelling service or a feeder carrying the entire dwelling load only — never a branch circuit or a non-dwelling installation.
The service or feeder rating must be from 100 A through 400 A.
No ampacity adjustment or correction factors can be required.
Conductors must be rated 75 °C or better. NM/Romex is a 60 °C assembly, so it is out.

The ordinary 310.16 answer

Without the dwelling-service allowance, use the ordinary ampacity table and the temperature method the installation permits. This includes non-dwelling work and any installation where derating applies. At 75 °C, the verified table selects 3 AWG copper or 1 AWG aluminum. A 60 °C method needs its own lookup and cannot borrow the 75 °C result.

Plain 75 °C table: 100 A → 3 AWG copper (100 A) or 1 AWG aluminum (100 A).

Long service laterals and voltage drop

This table starts with the NEC 310.12 dwelling-service minimum, then raises the conductor when needed to keep calculated drop at or under the 3% planning target. It uses 240 V, the full 100 A rating, and one-way distance.

100 A dwelling service at 240 V

Material	50 ft	100 ft	150 ft	200 ft	250 ft	300 ft
Copper	4	4	2	1	1/0	2/0
Aluminum	2	2	1/0	2/0	3/0	4/0

Wire Size calculator · Ampacity Check tool

Copper versus aluminum

Aluminum is common at service sizes because it reduces conductor weight and material cost. Use conductors, terminals, and equipment listed for the material, and have the complete service design checked locally.

100-amp sub-panel wire size guide · 200-amp service wire size guide · Ampacity Check tool

100-amp service wire size FAQ

Is 4 AWG enough for 100 amps?: Yes for a qualifying 100 A dwelling service: the 83% calculation requires 83 A, and 4 AWG copper is rated 85 A in the verified 75 °C table. Otherwise, check the actual method in the Ampacity Check tool.
What wire size should I use for a 100 amp sub-panel?: Use the 100-amp sub-panel wire size guide. It owns the feeder distance tables and checks that job under the ordinary feeder rules.
Should I use copper or aluminum for a 100 amp service?: Both can work when the equipment and installation are listed for the conductor. The dwelling-service derivation gives 4 AWG copper and 2 AWG aluminum; aluminum is often chosen to reduce weight and material cost.
Does the 83% rule apply to Romex?: No. NM/Romex is treated as a 60 °C assembly, while this dwelling-service allowance requires conductors rated 75 °C or better.