I'm not sure why my brain is not connecting on this...
I have a sub panel located a long distance from the main. Calculations show a 30% voltage drop by the time it gets there. No problem... a wire sized for 100A will only deliver 70A.
What size is the breaker in the main? 70A or 100A?
Volts drop, amps don't.
In reply to matthewmcl (Forum Supporter) :
Correct. Wire was sized for voltage drop.
Are you saying 100A breaker?
Would the breaker size not be dependent on what the intended use of the circuit is?
That said, if you want a 100amp circuit it would be counter intuitive to use a 70amp breaker
You can under rate the feeder (70 A breaker with wire rated for 100 A connected to it) as long as the wire will fit into the lugs on the breaker. We do this (more correctly my electrical subs do this) occasionally when we’re replacing older HVAC equipment and the replacement equipment maximum over current (breaker / fuse) rating is less than the existing breaker but we’re allowed to leave the feeders in place.
That said a 30% voltage drop (assuming 240 V single phase at the main panel) is a huge drop, like 168 on the other end. That’s 84 V to neutral, electrical stuff is not going to be happy with that. How far is the run? Is it possible there’s something else going on here?
Edited to add, as voltage drops amperage increases as mentioned above. A 30% voltage drop would result in a significant rise in amperage. My code books are not nearby, otherwise I’d dig into this one because now curious.
Edit to edit as my brain just engaged somewhat: I’m guessing the 100 amp feeder (larger wire size) is used to counteract the voltage drop due to feed length to the sub panel and keep the resulting voltage at the sub panel within acceptable limits.
Got it. Thanks.
In a situation like that, if you have 100A at 240V at the start and you lose 40V in the feeder, you'll have 200V and 100A at the sub panel. The amperage increase with reduced voltage is if you want to maintain the same power (P = VA), so you need to increase amperage to make up for the reduced voltage. I don't know of any loads off the top of my head that would do that, but that is what a transformer does.
Also, in that situation, you're not going to lose 30% at any load, that'll only be at full load and anything in between will have a lower loss. For instance, you'll only be losing 3% at 10A.
With a 30% loss though, I'd assume that the feed is way undersized to code, as it'll likely get quite hot, as the cable is absorbing that power and releasing it as heat, possibly causing some bad things to happen.
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