Sizing Feeders for Voltage Drop

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robert howard's picture
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Joined: 2016-12-30

I've been asked to wire a 200A, single-phase 40-circuit panel (i.e., sub-panel) fed from a 200A disconnect. The distance from the disconnect to the panel is 170 ft. I would normally use 4/0, 2/0 direct bury underground conductors. I need to size the wire and allow for a voltage drop. What size do my feeders, neutral, and ground need to be in this situation?

Paul Abernathy's picture
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Joined: 2012-12-21

Greetings Robert Howard,

Firstly, you need to understand that VD is elective and not a mandatory requirement in the National Electrical Code (except for Fire Pumps and other non-general applications). However, the goal would be to make sure the overall VD does not exceed 5% combined or 3% and 2% respectively on either the feeder or the resulting branch circuits.

Based on the info provided it is important to know the following:

1) What is your actual load? Otherwise, the VD calculation would take into account 200A as the rating of your equipment in your question. So that makes a difference.

2) We can assume you are installing AL conductors based on your statement, but since you stated 2/0 also I see you are asking about both CU and AL per Sec. 310.15(B)(7), where applicable.

So let's take some assumptions here that may get you started. First, let's use 4/0 AL and a 200A load.

As 75degC for 170 ft with 200A load your VD would be 2.83% (6.8V drop) and would be acceptable.

With the 2/0 CU equivalent (for 200A) per Sec. 310.15(B)(7), which I am sure you are applying, the VD would be approximately the exact same.

Here is the kicker about VD as well. Sometimes you will find calculators that do the VD and forget that the NEC has minimum size requirements so make sure you always ensure you have a compliant conductor first is selected based on the calculated load before you work a VD calculation and let the size increase (where applicable) dictate from that point.

robert howard's picture
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Joined: 2016-12-30

Thank you for your response. The building is a garage - recently built with a 2nd floor that is unfinished. My customer may turn it into a rental apartment. He is talking about a 2 or 3 ton heat pump or central a/c, an air compressor for the garage, and electric range - possibly a heat pump split system for the garage. At this point that is my best estimate. My inspector came up with 250 MCM ungrounded conductors with a 1/0 neutral with a 1/0 equipment ground.

Paul Abernathy's picture
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Joined: 2012-12-21

Greetings Robert,

I guess my response would be your "inspector" is not the one I would recommend do the calculation. In many cases (not all cases), they are not actually qualified to do such calculations. They can observe installation violations, but typically (again in some cases) lack the actual training to do the calculation.

You really need to perform a load calculation under the guidance of Art. 220 of the National Electrical Code -- remembering that the optional method (where applicable) will yield smaller conductor sizes. There are many load calculators for dwelling units on the internet....or you can find articles on how to do it online and possibly even on this website. Mike Holt contributes a lot of articles and his explanations are usually spot on.

I would be more than happy to assist you in the calculation and show you how to do it but I do not think (even free) soliciting is permitted here and I would hate for Mr. Eby to YANK my posting credentials...lol

robert howard's picture
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Joined: 2016-12-30

Hello sir,

Thank you for your response. I have never had to do this before and admit it is a challenge. It's more in the way of engineering. I can understand my inspector adjusting the wire to 250 MCM for the distance but I'm puzzled as to why and where he came up with a 1/0 neutral? The load is going to be about the same as a small dwelling unit. It's not much at all. It's the distance that is the issue. I'm wary of going with a neutral smaller that 2/0. My inspector got his numbers out of tables in the Code, but I've been unable to find any that come out with a neutral that size.

Again, thanks for your help. These days I need all I can get.

Paul Abernathy's picture
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Joined: 2012-12-21

Greetings Again Robert Howard,

While it would be a guess, I would venture to say that the electrical inspector obtained the 1/0 from table 250.102(C)(2) for AL conductors. If we are talking 250kcmil AL as the ungrounded conductors then based on Table 250.102(C)(2) the grounded (neutral) would be 2 AWG or again based on the actual neutral load to be served.

If the load itself warrants a 2/0 then by all means use it as you can always go larger than 1/0 if you choose, he should not have an issue with that but he (electrical inspector) should also not be designing your system either.

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