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donf · Oct 3, 2007, 02:42 PM

Earlier you recommended a 2 cooper or 1 alum. Cable for a 150 ft. run to a pole barn.

From previous posts you suggested a 4/2 cable from post lugs out to the sub panel.

What I do not understand is the following:

If you need to determine what size cable you first define the needed amperage +10%.

So if the actual draw on the line is 110 Amps, how do calculate an 80% usage?

tkrussell · Oct 3, 2007, 02:56 PM

First, I know that a residential 100 amp feeder is allowed to be #4 copper (85 amps) or #2 al (90 amps).

A circuit is assumed to run continuous, 3 hours or more, and will be derated by 80%.

Using the 75 Deg C column of Table 310.16 of the NEC,

110 amps time .8 = 88 amps, so a feeder branch will be assumed to carry the max of 88 amps. Next size wire is #3 copper.

A residential 110 amp feeder is allowed to be #3.

Doing voltage drop calcs, assuming the same 130 feet, #2 is needed to keep the Vd at or below 2%, at 1.8%.

Recommended feeder can be 2%, plus 3% at the branch circuit level,or vice versa, depending on the length of either.

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