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Joined: Dec 2008
Posts: 3
W
New Member
I'm a high school student in my second year of Electrical Construction at VoTech.

I've got a project to do this weekend and I need help. Don't worry, I'm not looking for the answer, just some help figuring it out myself.

I have to create a project on paper to install 4 new subpanels located at different locations in our high school and figure out the wire size needed.

Here's what I'm doing:

I counted steps to my classes so I know (close enough for this project anyway) the wire lengths. I can calculate voltage drop and I know how to figure out what size wire I need based on voltage drop.

We will have 16 wires (feeders) in one conduit and I know how to figure out the wire size based on "more than 3 conductors" in a conduit.

The problem is that I don't know if I need to do both. I mean, do I need to increase the wire size for voltage drop and then increase it again for the number of wires in the conduit or do I figure out the wire size for voltage drop alone and then the wire size for number of wires alone and then pick the one that gives me the largest wire?

Thanks for any help.

-Wants2Learn

2017 / 2014 NEC & Related Books and Study Guides
Joined: Apr 2002
Posts: 7,283
Likes: 3
Member
Hello & welcome to ECN!

First question (although not an NEC item) I have is why would 16 conductors be installed in one conduit, going to four (4) different places?? Did your Instructor require you to do it this way??

Second is what is the amperage of the sub panels/feeders?

Based on the info you posted...I'm going to guess the panels are 3 phase, 4 wire? Or...are you doing single phase, 3 wire, with a full size ground?

If, 3 phase, 4 wire, does the neutral have to be considered a current carrying conductor based on the type of loads in the sub panels?

You can't solve anything without all the required info!

I suggest you gather the missing info and.....
Determine a starting AWG based on the amperage.
Derate from 310.16 amps, based on the number of CCC's per 310.15 (b)(2)(a) table.
Then do Vd calcs after completing the above.

If you need additional help/info/suggestions let me know.

Additional opinions from other members should arrive soon



John
Joined: Dec 2008
Posts: 3
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New Member
Yes, I have all the info you say I need. I didn't post all the details because I don't want somebody else to figure this out for me. I just need to know how to figure it out for myself. But here are your answers:

Yes, the teacher designed the thing and put 16 conductors in the first section of conduit to the first panel. From there 12 go to the second panel, and then 8 to the next, and then 4 to the last.

It's 4-wire, 3 phase and the loads are all single phase so it is a current carrying neutral. 480V. I'm not giving away the distance to the panels or the current because I don't want somebody to post "The answer is X guage wire".

I understand that this wasn't designed to be built, it's just so we can figure out a wire size problem using voltage drop and conduit fill together. I just don't know how to use them together.

What I need to know is if I need to figure out the wire size using voltage drop and THEN make it bigger again according to the rules for conduit fill. Or if the bigger size wire because of voltage drop counts toward making it bigger for conduit fill.

If voltage drop tells me I need to use wire that would normally carry twice the current and conduit fill tells me I need wire that would normally carry twice the current, do I need wire that would normally carry twice the current? or do I combine them and use wire that would normally carry 4x the current?

Joined: Apr 2002
Posts: 7,283
Likes: 3
Member
OK, I got it.

1. Determine AWG based on load ampacity.
2. Derate based on number of ccc in conduit
3. Adjust AWG for correction as req.
4. Do Vd calc on corrected AWG
5. Adjust AWG IF Vd is an issue,
6. Determine conduit size(s).

OK, I was not going to give you the answer(s) unless you asked for them. Just seeing if you were given enough info to do it.





John
Joined: Dec 2008
Posts: 3
W
New Member
Thank you John, that's exactly the information I needed.

I'm sure I'll be back with more questions.

-Wants2Learn

Joined: Apr 2002
Posts: 7,283
Likes: 3
Member
OK, glad to assist.
You'll find a nice (for the most part) bunch of guys here at ECN.

Good luck with the project!



John
Joined: Mar 2005
Posts: 1,213
S
Member
Wants2Learn- To clarify: You don't need to apply derating factors to voltage drop. The #AWG size must ultimately satisfy both the derated ampacity and the voltage drop. (And the latter is not required, merely recommended. Still best to treat it as if it were a requirement, though.)

Good luck!


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