Voltage Drop

[wrobotronic]

This is a very odd question for me to ask.

I will be setting a 100A panel to a new garage. From the meter ped to the new panel is 420 ft.

When calculating the voltage drop if I use 100A, the wire size is absurdly big. As a general rule of thumb I have upsized the wire one size for every 200 ft of distance. From the 75 degree table Im looking at perhaps a 1/0?

Does anyone have a method or direction on making this calc? The loads from the panel have yet to be determined and thusly aren't really practical to use for my VD.

TIA!!!

 

[JFletcher]

From southwire:

1 conductors per phase utilizing a #1 Copper conductor will limit the voltage drop to 5.20% or less when supplying 100.0 amps for 420 feet on a 240 volt system.

http://www.southwire.com/support/voltage-drop-calculator.htm

 

[ActionDave]

This is a very odd question for me to ask.

I will be setting a 100A panel to a new garage. From the meter ped to the new panel is 420 ft.

When calculating the voltage drop if I use 100A, the wire size is absurdly big. As a general rule of thumb I have upsized the wire one size for every 200 ft of distance. From the 75 degree table Im looking at perhaps a 1/0?

Does anyone have a method or direction on making this calc? The loads from the panel have yet to be determined and thusly aren't really practical to use for my VD.

TIA!!!

You need some kind of idea what the loads are going to be to do a load calc. Think about this though, how many custom homes out there are being built with ten to twenty guys running around with power tools and compressors on a temp panel with a 50A main? What's the max the garage will see and for how long?

 

[JFletcher]

You need some kind of idea what the loads are going to be to do a load calc. Think about this though, how many custom homes out there are being built with ten to twenty guys running around with power tools and compressors on a temp panel with a 50A main? What's the max the garage will see and for how long?

Agree, tho those 10-20 guys will trip breakers far more often than the HO: framers' air compressor is running, they start a chopsaw, electrician runs a nail biter bit into the framer's bed of nails, kerosene forced air blower and site lights are on, all on a 3-1 cord or two to a 20A breaker... trip.

100A for the garage may be massive overkill, or marginal if they are going to feed a spa, large air compressor, and sump/well pump from that circuit.

If the HO isnt sure, upsell the larger wire. Much cheaper to do it once than have to come back and retrench or pull 400+' of feeder out of conduit to install larger, or Cu (for that length AL would be MUCH cheaper).

 

[Julius Right]

Thanks JFletcher for the link.
In my opinion, this program use the resistance[corrected for 60oC] and reactance from the table 9 of NEC, instead of recommended Z on the same table and calculate the voltage drop following "exact" formula from IEEE STD 141:
Vd=V+I*R*COSFI+I*X*SINFI-SQRT(V^2-(I*X*COSFI-I*R*SINFI)^2)
Where Vd is voltage drop in V and V=240/SQRT(3)-for instance
I would comply with Z recommended corrected for 60 oC as the result being 4.61% instead of 4.51%.