12 AWG voltage drop at 100 feet
You already have 12 AWG in mind. This page only answers whether 100 ft still fits your percent.
How to run this check
- Open Voltage Drop and lock the size at 12 AWG.
- Enter the real voltage, load current, and one-way feet.
- If it is over target, read the suggested larger size or switch to Wire Size.
Why 120 V looks worse than 240 V
The volt drop is the same for the same current, size, and feet. The percent is volts dropped divided by system voltage, so 120 V doubles the percent compared with 240 V.
That is why a 100 ft 12 AWG run can be a problem on a 120 V branch and still look acceptable on 240 V (3.3% in this example).
Do not use a rule of thumb in place of the percent
“100 ft needs one size up” is a memory aid, not a check. Motor loads, 3-phase, and a lighter actual current all change the number.
If you already know the AWG, stay on the voltage-drop tool. Use Wire Size only when you want the next size that clears both checks.
12 AWG copper, 20 A, 100 ft one-way, single-phase
| Voltage | Drop | Percent | 3% target |
|---|---|---|---|
| 120 V | 7.9 V | 6.6% | Over target |
| 240 V | 7.9 V | 3.3% | Over target |
Related
FAQ
Is 100 ft of 12 AWG always too long?
No. It depends on voltage and current. 20 A at 120 V is over 3% in this example. The same feet at 240 V is close to the line.
Do I enter one-way feet or round trip?
Enter one-way feet. The single-phase math already uses the 2-wire path.
Planning reference only. Verify against the current NEC, manufacturer data, and the authority having jurisdiction. This is not engineering advice or a license substitute.