voltage drop

[Ryan Currin]

I have an installation requiring a new sub panel. The sub panel will be located 350 feet away from the meter. The panel will be fed with 100 amp conductors. The breakers in this panel will be 8 20 amp single poles and 1 50 amp 2 pole. The load in this panel will not all be continuous and I cant imagine the load ever exceeding 50 amps continuous at any given time. When sizing the conductors to feed this panel, do I need to calculate the actual load being served or base it on 100 amps? If I base it on 100 amps then the feeders are 300mcm which I don't want to run. Any suggestions?

 

[jumper]

The feeder conductors can be sized to the load being served, however the OCPD protecting the feeders must be sized accordingly.

 

[Ryan Currin]

Ok, when I do the calculation do I use 120 volts or 240 volts?

 

[Mouser]

240 Volts.

 

[luckylerado]

240 Volts.

I don't think so. I believe you need to calculate the design load on the heaviest leg and use 120V.

I will also share something that I just learned while studying for a LEED exam. If your are trying to meet ASHRAE 90.1, you no longer need to limit the feeder to 2%, rather you can use the 5% applied to the whole length. So if your branch circuits are very short and limited to 1%, you now have 4% to play with on the feeder side of the equation which can have a significant impact on the conductor size..

 

[Ryan Currin]

I did some math on this service. The total lighting load will be 24 amps when fully on and have a garage door opener with OCP at 50 amps with a load of probably 34-40 amps when used. But will be used while the lighting is on and there will be a few general use outlets of load as well. That's a total load of 64 amps which I'll round up to 70 amps I'm guessing. At 350 feet single phase 240 volts 70 amps I'm coming up with 1/0 awg. If I calculate it at 120 volts I get 4/0 awg. That's a big jump. Do you think 1/0 will suffice?

 

[luckylerado]

... a garage door opener with OCP at 50 amps with a load of probably 34-40 amps ...

What is it a draw bridge??

What is the length and load of the heaviest branch circuit?

 

[ActionDave]

I did some math on this service. The total lighting load will be 24 amps when fully on and have a garage door opener with OCP at 50 amps with a load of probably 34-40 amps when used. But will be used while the lighting is on and there will be a few general use outlets of load as well. That's a total load of 64 amps which I'll round up to 70 amps I'm guessing. At 350 feet single phase 240 volts 70 amps I'm coming up with 1/0 awg. If I calculate it at 120 volts I get 4/0 awg. That's a big jump. Do you think 1/0 will suffice?

Given what you have described I would use 1/0 AL and be quite content.

 

[luckylerado]

Given what you have described I would use 1/0 AL and be quite content.

That is about 8.1% just on the feeder.

I am coming up with much bigger numbers using 70 amps.

500 MCM al for 2%

 

[ActionDave]

That is about 8.1% just on the feeder.

I am coming up with much bigger numbers using 70 amps.

500 MCM al for 2%

I'm not using any calc, just straight up cowboy engineering. Based on the op I think 70A is overkill.

 

[luckylerado]

...Based on the op I think 70A is overkill.

I totally agree. Even just not rounding up from 64 to 70 would save a wire size.

Its all voodoo anyhow.

 

[luckylerado]

I totally agree. Even just not rounding up from 64 to 70 would save a wire size.

Its all voodoo anyhow.

I see now that 70 was total load on both phases:slaphead:. When I spread it across A and B I am getting 250MCM AL