Is a #4 copper feeder large enough for a 100 amp sub-panel 70 ft from the main panel?

Question

100 amp breaker

not in conduit

Answer 1

I would say no.  usually #2 copper is used for a feeder to a 100 amp panel as #4 is only rated at 85 amp for romex .  If you were using conduit, and thhn wire you could possibly get away with it as long as you sized the conduit big enough and the run was not too long otherwise be safe and use #2.

Answer 2

Some confusion here.

#4 Cu. is rated 85 A. in cable or conduit which is what

you should be using for interior wiring.

The higher ratings quoted are for 'cable in free air'

which means overhead lines.

For a 100A. feeder use #2 Cu. minimum.

If by chance you are going overhead, be sure to use

'outdoor grade` insulated cables.

At 70' your voltage drop situation with #4s might be 'iffy'.

Answer 3

I'm not sure what you mean by "not in conduit", but in most instances it would probably be only rated for 85 amps.

Answer 4

yes  - in free air - #4 can carry 105Amps

Answer 5

The ampacity of single-insulated conductors for #4 copper range from 105(type TW) to 140. So you are marginal to OK, depending on the type your using.