winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
This is why I said that using the breaker rating is a reasonable starting point and almost certainly sufficient...but the other side of the coin is that even if you size the system for acceptable voltage drop at the breaker trip rating, you might still have too much voltage drop.
The reason is that some loads draw large starting current, and this starting current almost certainly exceeds the trip rating of breakers permitted to supply the load. This short duration starting current doesn't trip the breaker, but does cause voltage drop.
Imagine a feeder to a workshop that has a large compressor as one of the tools. The feeder is sized for 3% voltage drop at full load, and the compressor represents 50% of the full load. Every time that compressor starts it draws 6x rated current to get moving from a standstill. For a moment the feeder is seeing 300% rated load and 9% voltage drop. The compressor works just fine, the lights flicker, and computers have problems.
Which all goes back to what post #2 and others said: you really need to know the load.
Still, the setup described is probably just fine with 100A conductors protected by a 100A breaker.
Jon
The reason is that some loads draw large starting current, and this starting current almost certainly exceeds the trip rating of breakers permitted to supply the load. This short duration starting current doesn't trip the breaker, but does cause voltage drop.
Imagine a feeder to a workshop that has a large compressor as one of the tools. The feeder is sized for 3% voltage drop at full load, and the compressor represents 50% of the full load. Every time that compressor starts it draws 6x rated current to get moving from a standstill. For a moment the feeder is seeing 300% rated load and 9% voltage drop. The compressor works just fine, the lights flicker, and computers have problems.
Which all goes back to what post #2 and others said: you really need to know the load.
Still, the setup described is probably just fine with 100A conductors protected by a 100A breaker.
Jon