#3 feeding 1/0 Voltage drop for entire run? - Electrician Talk - Professional Electrical Contractors Forum
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Old 11-05-2012, 07:17 AM   #1
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Default #3 feeding 1/0 Voltage drop for entire run?

I have a 100A service. In the gutter/splitter I have #3s as the feed. This splitter feeds a 100A panel and a 100A disconnect. In the 100A Disconnect I'm using 1/0 for derating and voltage drop. So basically I have 10ft of #3 feeding 1/0. I've heard some say the entire run should be the same but the way I'm looking at it is I'm keeping what I'm getting into the 100A disconnect. Thoughts?
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Old 11-05-2012, 07:34 AM   #2
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Quote:
Originally Posted by Aegis View Post
I have a 100A service. In the gutter/splitter I have #3s as the feed. This splitter feeds a 100A panel and a 100A disconnect. In the 100A Disconnect I'm using 1/0 for derating and voltage drop. So basically I have 10ft of #3 feeding 1/0. I've heard some say the entire run should be the same but the way I'm looking at it is I'm keeping what I'm getting into the 100A disconnect. Thoughts?
After reading your post 3 times, I think maybe, if I understand what you are saying, I agree. I reserve the right to change my mind as details are explained.
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Old 11-05-2012, 07:38 AM   #3
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whats on the load side of the disconnect and how far away is it? It must be pretty far to go three sizes up. I think your good as far as the NEC is concerned.
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Old 11-05-2012, 07:47 AM   #4
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The 100A disco is feeding a panel that's 200ft away and I'm calculating for 80A. There's also some temp derating going on but to my understanding doesn't stack on top of the voltage drop calculation.
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Old 11-05-2012, 07:55 AM   #5
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Does the CEC limit voltage drop?
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Old 11-05-2012, 07:56 AM   #6
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1/0 seems a little large for the load/distance. However there is nothing wrong with upsizing just the long portion of the run, its done all the time
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Old 11-05-2012, 08:00 AM   #7
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You just add the two voltage drops together to get the total. I get 0.4 volt drop on the 10' of #3 with an 80 amp load and 3.5 volts on the 200' of 1/0.
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Old 11-05-2012, 08:30 AM   #8
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don't forget to upsize your EGC also.
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Old 11-05-2012, 08:47 AM   #9
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I think it would look better if you brought the 1/0 to whatever termination point it needs to go without splicing but I think its legal either way.

I did one for the dept of transportation where they specified #8 copper for a 15amp circuit which seemed big except the homerun was over 1200 feet (120 volts).



if i'm not mistaken, z*i=voltage drop

200ft x 2 = 400ft
z = .13 per 1000 ft (table 9), .00013/ft
400ft * .00013 z/ft = .052 z
.052 z * 80 amps = 4.16 v voltage drop
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Old 11-05-2012, 10:28 AM   #10
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Quote:
Originally Posted by k_buz
Does the CEC limit voltage drop?
Not sure what you mean.
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Old 11-05-2012, 10:34 AM   #11
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Quote:
Originally Posted by uconduit
I think it would look better if you brought the 1/0 to whatever termination point it needs to go without splicing but I think its legal either way.

I did one for the dept of transportation where they specified #8 copper for a 15amp circuit which seemed big except the homerun was over 1200 feet (120 volts).

if i'm not mistaken, z*i=voltage drop

200ft x 2 = 400ft
z = .13 per 1000 ft (table 9), .00013/ft
400ft * .00013 z/ft = .052 z
.052 z * 80 amps = 4.16 v voltage drop
I got something similar to what Don got, close to 3.5v. I've used table D3 in the past and also this website
http://www.csgnetwork.com/voltagedropcalc.html
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