Question 1

What size wire do I need for a 20 amp circuit?

Accepted Answer

12 AWG copper, as a minimum. That comes from the 60°C ampacity column, which NEC 334.80 requires for NM-B cable in a dwelling. But that is only the minimum — on a long run the voltage drop may push you to 10 or even 8 AWG, which is exactly what this calculator checks.

Question 2

What is voltage drop and why does it matter?

Accepted Answer

Resistance in the wire means the voltage arriving at the far end is lower than the voltage leaving the panel. Motors run hot and lights dim. NEC 210.19(A) recommends holding a branch circuit under 3%. It is a recommendation rather than a rule, but it is why a long run wants heavier wire than the breaker alone suggests.

Question 3

If I use bigger wire, can I use a bigger breaker?

Accepted Answer

No — and this is the mistake that starts fires. The breaker protects the wire, and it is sized to the load and the smallest conductor in the circuit. Upsizing wire for voltage drop is fine and common. Upsizing the breaker to match is not.

Question 4

Can I use aluminium wire?

Accepted Answer

For feeders and large circuits, commonly yes. For small branch circuits, many jurisdictions do not allow it, and where it is allowed it needs terminations rated AL/CU plus anti-oxidant compound. Aluminium also needs to be roughly one size larger than copper for the same load.

Question 5

Does this account for conduit, heat, or bundled cables?

Accepted Answer

No. It uses the 60°C column that applies to NM-B cable in a dwelling. Conduit fills, ambient temperature above 30°C, and more than three current-carrying conductors in a raceway all require derating that this does not do.

Question 6

Can I do this work myself?

Accepted Answer

In many places a homeowner may do work on their own home, but it still needs a permit and an inspection. Local amendments override the national code, and the inspector is the one who decides. Treat this as planning, not as permission.

Question 7

Is my data sent anywhere?

Accepted Answer

Everything is worked out in your browser as you type. Nothing you enter is sent anywhere, stored, or attached to you.

Wire size and voltage drop calculator