Which comes first, the derating of a wire or the voltage drop calculations? I have an EV charger that is 500' from the panel, it requires single phase 208 volt 32 amps and 120 volt 16 amps. when I do the voltage drop calculations, I get #4s are required for each circuit. If I'm installing in a single underground conduit, this would be 4 current carrying conductors which the code requires to adjust to 80%. How do I calculate for the adjustment factor?
Voltage Drop & derating
- Thread starter bgs1362
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- Tennessee NEC:2017
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- Semi-Retired Electrician
The code does not mandate calculating for voltage drop except in a few cases, which doesn't fit your installation, they only suggest it in an "informational note" or "FPN"=fine print note. That being said, you should do your derating, then do your voltage drop calculations.
When you have to increase conductor size due to voltage drop, I doubt you'll run into any issues with derating. To me, the two issues are independent (don't see how derating affects voltage drop) -- using just the ampacity tables and derating for bundling/ambients will force a minimum wire size. Doing the voltage drop calculation will force another minimum size. Do the two calculations and pick the larger of the two minimums.
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- Lockport, IL
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- Retired Electrical Engineer
This is actually quite simple. You have selected #4 on the basis of VD. A #4 has an ampacity (with 75C insulation) of 85 amps. Putting 4 CCCs in the same conduit gives you a derating factor of 80%. Taking 80% of 85 amps leaves you with an ampacity of 68 amps. That is more than you need for your two circuits. Therefore, a #4 is acceptable for this application.
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