# Voltage drop calculator

Solar Expert Posts: 1,973 ✭✭✭
At this site http://www.nooutage.com/vdrop.htm there is a voltage drop calculator, but what I am seeing there is confusing me.

For a three phase 480VAC run of 470 feet and a current of 163A, it tells me that for a double run of 4/0 Cu in steel conduit, I get a 1% Vd, but for a single run I can't get down to 1% even with the maximum conductor size of 750. Does this make sense? From Table 9, 4/0 Cu has a resistance of 0.207 ohms/1000'. To get to half that resistance I only have to go to 500.

Am I missing something?

## Comments

• Banned Posts: 17,615 ✭✭
Re: Voltage drop calculator

Try a different calculator: http://www.powerstream.com/Wire_Size.htm

750mil on those specs has a V-drop of 0.39% according to them.
You can get below 3% with 1/0.

I've no idea why the difference. Looks like the calc you used is out of whack.
• Solar Expert Posts: 1,973 ✭✭✭
Re: Voltage drop calculator
Try a different calculator: http://www.powerstream.com/Wire_Size.htm

750mil on those specs has a V-drop of 0.39% according to them.
You can get below 3% with 1/0.

I've no idea why the difference. Looks like the calc you used is out of whack.
To confuse things even further, this one http://www.electriciancalculators.com/calculators/vdpfcalculator_initial.html says I need to go to 400 to get to around 1%, while the one you reference says I can get there with 300. If any two of them agreed I'd know what to do.
• Solar Expert Posts: 1,973 ✭✭✭
Re: Voltage drop calculator
ggunn wrote: »
To confuse things even further, this one http://www.electriciancalculators.com/calculators/vdpfcalculator_initial.html says I need to go to 400 to get to around 1%, while the one you reference says I can get there with 300. If any two of them agreed I'd know what to do.
It's going to be expensive no matter what, and I want to get it right.
• Banned Posts: 17,615 ✭✭
Re: Voltage drop calculator

If any two of them agreed it'd probably be a sign of the Apocalypse.

The difference may be in conduit (400) or not in conduit (300) as the heat dissipation plays a major role.
They all tend to have disclaimers about checking with locale AHJ to make sure the plan meets code. Good idea. I'd suggest submitting it with 400 spec'd in conduit.

Yeah, it's going to be très expensive!
• Super Moderators, Administrators Posts: 31,690 admin
Re: Voltage drop calculator

Different calculators will make different assumptions with temperature of copper wire. Hot wire, higher resistance.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 280 ✭✭
Re: Voltage drop calculator

I like http://www.electriciancalculators.com/ , especially the VB app, though at higher temperatures you are stuck with a javascript version as the VB version starts going in steps of 10 and rounds everything pessimistic (the code book keeps going in steps of 5).

http://www.electriciancalculators.com/ sticks to the code book as much is possible. Where it deviates, it rounds pessimistically (bigger wire)
• Solar Expert Posts: 9,276 ✭✭✭✭✭
Re: Voltage drop calculator

Or, if you have not installed the conduit yet, go up 2 sizes, and use aluminum wire, much easier to pull, (but don't forget the anti-ox goop) .
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