NEC 310.16 Feeder Sizing
Sub-Panel Wire Size Calculator
The feeder conductor, equipment ground, breaker, conduit, and voltage drop for any residential sub-panel, in copper or aluminum, with the 4-wire feeder and neutral-isolation rules that pass inspection.
Sub-Panel Wire Size Chart
| Sub-panel | Copper feeder | Cu ground | Aluminum feeder | Al ground | Breaker |
|---|---|---|---|---|---|
| 60 A sub-panel | 6 Cu | 10 | 4 Al | 8 | 60 A |
| 100 A sub-panel | 3 Cu | 8 | 1 Al | 6 | 100 A |
| 125 A sub-panel | 1 Cu | 6 | 2/0 Al | 4 | 125 A |
| 150 A sub-panel | 1/0 Cu | 6 | 3/0 Al | 4 | 150 A |
| 200 A sub-panel | 3/0 Cu | 6 | 250 Al | 4 | 200 A |
How to Wire a Sub-Panel: Isolate the Neutral
A sub-panel feeder carries four conductors: two ungrounded (hot) legs, a grounded (neutral) conductor, and a separate equipment grounding conductor. The neutral and the ground are separate conductors all the way to the sub-panel - never a shared 3-wire feed.
In the sub-panel the grounded (neutral) bus and the equipment-ground bus must be kept separate. Remove the main bonding jumper - the green bonding screw or strap that ties the neutral bar to the metal can. Bonding neutral to ground here would put normal neutral current onto the ground path and every metal enclosure downstream.
The one place the neutral and ground are bonded is the service disconnect (the main panel), via the main bonding jumper. The grounded conductor cannot be reconnected to ground on the load side of that service disconnect, which is exactly what a sub-panel is.
A sub-panel in a separate building or structure still gets a 4-wire feeder with the neutral isolated, plus its own grounding electrode (a ground rod or rods) connected to the sub-panel's ground bar. New feeders to a separate structure must include an equipment grounding conductor - the old 3-wire feed is allowed only on existing installations.
Frequently Asked Questions
What size wire do I need for a sub-panel?
It depends on the sub-panel rating. A 60 A sub-panel takes #6 copper, 100 A takes #3 copper, 125 A takes #1 copper, and 200 A takes 3/0 copper - all from NEC Table 310.16 at the 75°C column, with an equipment grounding conductor from Table 250.122. Aluminum is one to two sizes larger. Pick your rating in the planner above for the exact feeder, ground, breaker, conduit, and voltage drop.
Do you separate the neutral and ground in a sub-panel?
Yes. In any sub-panel the neutral (grounded) bar and the ground (equipment grounding) bar are kept separate. Remove the main bonding jumper and run a 4-wire feeder. The neutral-to-ground bond is made only at the service disconnect (NEC 250.24, 408.40).
Why is #4 copper wrong for a 100 amp sub-panel?
The #4 copper figure comes from NEC 310.12, the reduced dwelling-feeder table, which applies only when the feeder carries the entire dwelling load. A sub-panel that feeds only part of the home is sized from Table 310.16 instead, which calls for #3 copper at 100 A.
Adding a sub-panel? Check the main first.
A new sub-panel adds load to the service. Confirm the main panel has the headroom before you tap it.