Quote Originally Posted by kenwhite View Post
Geezer you must derate the ampacity of a cable if the wires are bundled togethter.

Also, insulation type determines how much current a particular sized cable can carry. For example, 3 wires bundled together operating at 30*C (86*F) have the following ratings:

6 AWG THHN -> 75A rating when at 30*C and can be operated all the way up to 90*C (194*F) with proper derating.

6 AWG THHW -> 65A rating when at 30*C and can be operated all the way up to 75*C (167*F) with proper derating.

6 AWG UF -> 55A rating when at 30*C and can be operated all the way up to 60*C (140*F) with proper derating.

If there are more wires in the bundle than 3, the ampacity goes down. If the ambient around the wires is higher than 30*C, the ampacity goes down.

So there is really more to correct power distribution design than just wire size and length...
Interestings facts to be sure if the welder is being installed in a submarine or some other critical area with all the cables bundled, However my purpose is to simplify it for a fellow trying to make a single run from his panel box to the end of his shop/garage. Throwing in fractional calculations although accurate do not do any thing except add to his confussion. Rather than argue amongst ourselves about who is exactly right, let us step back and solve the problem for him in a form he may understand.

The simplist answer I can give anybody is: when in doubt use a big thick wire