Question 1

What size wire do I need for a 30 amp circuit?

Accepted Answer

For a 30 amp circuit, you need a minimum of 10 AWG copper wire or 8 AWG aluminum wire per NEC Table 310.16 (at 75°C). However, for longer wire runs, you may need to upsize to account for voltage drop. Use the calculator above to find the exact size for your specific distance and voltage.

Question 2

What is the NEC 3% voltage drop rule?

Accepted Answer

NEC 210.19 Informational Note No. 4 recommends a maximum 3% voltage drop on branch circuits and 5% total (feeder + branch) to the farthest outlet. While this is a recommendation rather than a requirement, following it ensures optimal equipment performance and energy efficiency.

Question 3

When should I use aluminum wire instead of copper?

Accepted Answer

Aluminum wire is commonly used for larger feeders and service entrances (4 AWG and larger) where cost savings are significant. It requires one to two gauge sizes larger than copper for the same ampacity. Always use AL-rated connectors and apply anti-oxidant compound per NEC 110.14.

Question 4

Does wire length affect the size needed?

Accepted Answer

Yes. Longer wire runs experience more voltage drop, which may require upsizing beyond the minimum ampacity rating. A 100-foot run at 30 amps may need 10 AWG copper, but a 200-foot run at the same load might need 6 AWG to keep voltage drop under 3%.

Question 5

What does AWG stand for?

Accepted Answer

AWG stands for American Wire Gauge, the standard system for measuring wire diameter in the United States. Lower AWG numbers indicate thicker wire — for example, 4 AWG is thicker than 12 AWG and can carry more current.

Question 6

How do I size wire for a 240V circuit vs 120V?

Accepted Answer

Higher voltage circuits experience less voltage drop for the same power level, so you can often use smaller wire. A 240V circuit carrying 20 amps has half the voltage drop of a 120V circuit at the same current. However, the minimum ampacity requirement remains based on the current, not voltage.

AWG	Copper (A)	Aluminum (A)	Circular Mils
14	15	—	4,110
12	20	15	6,530
10	30	25	10,380
8	40	30	16,510
6	55	40	26,240
4	70	55	41,740
3	85	65	52,620
2	95	75	66,360
1	110	85	83,690
1/0	125	100	105,600
2/0	145	115	133,100
3/0	165	130	167,800
4/0	195	150	211,600
250	215	170	250,000
300	240	190	300,000
350	260	210	350,000
500	320	260	500,000

Wire Size Calculator — Free Online Calculator

How to Use This Calculator

The Formula Explained

NEC Wire Ampacity Table (75°C Insulation, NEC 310.16)

Frequently Asked Questions

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