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Joined: Feb 2006
Posts: 78
L
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I need to feed a 200 amp 3ph 120/240 panel from the power co. 450' will be (al) areial and 110' will be (cu)underground over to the panel. How do you size the wire for voltage drop

2017 / 2014 NEC & Related Books and Study Guides
Joined: Mar 2005
Posts: 1,213
S
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Compute the two separately, and add up the cumulative voltage drop. You can use the tables in the back of NEC, or one of the calculators online like this one: http://www.csgnetwork.com/voltagedropcalc.html

Do you mean 3-phase 120/208, or 1-phase 120/240?

FYI, the "half-length" the calculations ask for, in your case, would be 450' and 110', as the electricity has to take a return trip down back the same wire, so the actual electrical length required for calculating voltage drop would be twice that.

Joined: Jan 2004
Posts: 1,507
G
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We need more information to calculate the wire size. Namely what is the expected load on the panel. It would not be practical to size the wire for a 200a. load and try to maintain the 240/120v. status. Once we know the "load" we need to know what voltage drop you can tolerate. Then there are formulae for giving you the wire size.


George Little
Joined: Oct 2000
Posts: 5,389
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don't most poco's do the sizing inclusive in their doctrine for us?

~S~

Joined: Feb 2006
Posts: 78
L
Member
The service is 120/240 3ph hi leg and the connected load is 150 amp. motor load.

Joined: Mar 2005
Posts: 1,213
S
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Oh wow, there's going to be some SERIOUS kick-down when that motor starts. What's the locked-rotor current for that motor?

This is a textbook case where you're probably better off stepping up to a higher voltage and stepping back down. Any chance of getting your service at 480V and using a 480V motor? This will eliminate the need for two 72kVA transformers. You can throw on a small transformer for the 120V loads, and come out WAY ahead in copper.

Joined: Feb 2006
Posts: 78
L
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I have three 7-1/2 hp one 5 hp thats 108 amps with the 125%, the 42 amps is lighting. I should have listed it earlier in the post it looks like there is 150 amps in motor load but there is not.

Last edited by lite bulb; 05/20/09 06:30 PM.
Joined: Jan 2004
Posts: 1,507
G
Member
I agree with Steve. Consider also getting a separate drop from the utility company. Getting that much power over that distance is extremely expensive. Get an engineer to figure your wire size if you can't get the utility to give you a separate drop. I could figure it out but I think you'd end up with xformers as the best route if the utility can't accommodate you with a drop.


George Little
Joined: Apr 2002
Posts: 7,283
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Litebulb:

Is your question in this thread a 'school project'?



John
Joined: Jan 2004
Posts: 1,507
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Ya know John, I think you are right, I seem to remember that same problem showing up before and it was off some test or school project someplace. I'm sticking with my suggestion and see what happens smile


George Little
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