(20A x .8 =16 x 120v = 1,920va.)

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deee

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A single 20A circuit is good for 1,920va. (20A x .8 =16 x 120v = 1,920va.) Next take the total va and divide by the load of a single lamp: 1,920va/100watt = 19 fixtures allowed.
I have seen this equqtion before. I understand it, but the .8 or 80% comes from where in the NEC?
(20A x .8 =16 x 120v = 1,920va.)
 
.8 or 80%

.8 or 80%

The .8 or 80% rule is also known as the 125% rule found in 210.19.

It states you must size the conductors to 125% the FLC for a continuous load (over 3 hours), with the inverse of 125% being 80% you can also solve with this equation.

20 / 1.25 = 16
 
I see now what's happening. I wasn't even thinking of the inverse. I know about the 125% per 210.19(A)(1)(a). I really appreciate the replies.
 
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