Voltage drop with amperage

[joshfromoshoto]

I'm in a dispute with my co worker. Basicly we have a 144a load in a panel fed from a 150a breaker and we're trying to calculate the necessary wire size. We're trying to pull 4/0 copper roughly 300'. I did the voltage drop calculation and ended up with 201 volts at the panel, then took 201÷ the resistance of 4/0 at 300' and ended up with roughly 5 amps. So in theory at the panel with full load wr should pull no more than 149a worst case scenario in my opinion. My co workers thinks we don't need to add the extra amperage that the resistance of the wire generates.

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[James L]

What exactly is your dispute?

Wire size?
Wire size would be 4/0 and 5 amps wouldn't change that.

Actual voltage under load?
202-203v presuming you start with 208
I highly doubt it's actually 208v to start, and that will affect the drop % as well as realized voltage under load

Whether or not to add amperage from wire resistance? No

 

[winnie]

The voltage drop in the wire doesn't add any current directly.

The changed voltage at the load will change the current drawn by the load. But different loads will change in different ways.

If you have resistance loads (heaters and the like) then reducing the voltage will _reduce_ the current drawn.

If you have constant power loads (motors, switching power supplies, other regulated loads) then reducing voltage will _increase_ current drawn.

Usually with normal voltage drop you simply assume nominal current at nominal voltage.

I hope that 144A load on a 150A breaker isn't continuous.

Jon