Three phase 120/208v feeders in MC

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Pplayz18

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Millville nj
I have a three phase 120/208v 400amp service that I need to supply two 200 amp panels in MC with. I need to know what size MC wire to use and how to come up with that wire so I know for next time. The panels are approx. 45 feet away...

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roger

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Fl
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I have a three phase 120/208v 400amp service that I need to supply two 200 amp panels in MC with. I need to know what size MC wire to use and how to come up with that wire so I know for next time. The panels are approx. 45 feet away...

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Start with article section 310.15 and let us know what you come up with.

Roger
 

Pplayz18

Member
Location
Millville nj
Using that, assuming MC wire is basically the same as thhn and ran in copper I should get away with 3/0? So the voltage has nothing to do with it?

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Pplayz18

Member
Location
Millville nj
I've been reading that in a three phase 3 wire service the neutral wire is actually considered a load carrying conductor. If this is true would I have to derate the wire amperage in 310.15 by 80% and therefore need a larger size wire?

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augie47

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I've been reading that in a three phase 3 wire service the neutral wire is actually considered a load carrying conductor. If this is true would I have to derate the wire amperage in 310.15 by 80% and therefore need a larger size wire?

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In a majority of situations it is not counted as a current carrying conductor.

Here's part of a sheet that rob prepared to discuss that situation:

[FONT=&quot] 3Ø- 208Y/120 or 480Y/277 volt system-different circuit types:[/FONT][FONT=&quot]
A) 2 wire circuit w/ 1 ungrounded, 1 neutral = 2 CCC's
B) 3 wire circuit w/ 2 ungrounded, 1 neutral = 3 CCC's
C) 4 wire circuit w/ 3 ungrounded, 1 neutral = 3 CCC's*

Notes:
A) A normal 2 wire circuit has equal current flowing in each of the circuit conductors so they both count as CCC's.
B) In this circuit the neutral current will be nearly equal to the current in the ungrounded conductors so the neutral counts as a CCC
C) In this circuit the neutral will only carry the imbalance of the current between the three ungrounded conductors so it is not counted as a CCC, with an exception, *if the current is more than 50% nonlinear (see below for NEC article 100 definition) then the neutral would count as a CCC.

[/FONT]
[FONT=&quot] Nonlinear Load. A load where the wave shape of the steady-state current does not follow the wave shape of the applied voltage.
Informational Note: Electronic equipment, electronic/electric-discharge lighting, adjustable-speed drive systems, and similar equipment may be nonlinear loads.

[/FONT]
 

Pplayz18

Member
Location
Millville nj
Ok I know a lot of that makes sense to a lot of different people but I've read it 10 times, along with other forums from other people on the matter of the neutral being counted but I'm just not grasping the full concept.

Your saying that the majority of the times it isn't counted but from what I've been reading it seems it is. In my current situation I'm still unsure weather to or not and why.....

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infinity

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Ok I know a lot of that makes sense to a lot of different people but I've read it 10 times, along with other forums from other people on the matter of the neutral being counted but I'm just not grasping the full concept.

Your saying that the majority of the times it isn't counted but from what I've been reading it seems it is. In my current situation I'm still unsure weather to or not and why.....

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Take closer look at post#7, letter C) and the note that addresses non-linear loads and whether or not the neutral is a CCC.
 

Smart $

Esteemed Member
Location
Ohio
Ok I know a lot of that makes sense to a lot of different people but I've read it 10 times, along with other forums from other people on the matter of the neutral being counted but I'm just not grasping the full concept.

Your saying that the majority of the times it isn't counted but from what I've been reading it seems it is. In my current situation I'm still unsure weather to or not and why.....

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Derating for number of current-carrying conductors (CCC's) is based on conductor self-heating from the current running through it and the ability to dissipate that heat to the surrounding environment. Greater number of CCC's hinder dissipation... when they are all conducting rated current.

A neutral conductor (working in the capacity of a true circuit neutral) only carries the unbalanced current of line conductors. The net heating effect cannot exceed that which the three line conductors would generate conducting rated current... with linear loads. With non-linear loads, it is practiced as if net heating effect can exceed that of the three line conductors (look up "triplen harmonics" for starters). Non-linear loads are those that essentially have electronically generated power supplies (PC's, electronic lamp ballasts, etc.)
 

Pplayz18

Member
Location
Millville nj
Take closer look at post#7, letter C) and the note that addresses non-linear loads and whether or not the neutral is a CCC.
How would I go about calculating that 50% though? Seems most of the time you'd be safe without counting the neutral as current carrying...

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Smart $

Esteemed Member
Location
Ohio
How would I go about calculating that 50% though? Seems most of the time you'd be safe without counting the neutral as current carrying...
If all the information you have is what you've divulged here, you can't.

If that be the case, someone somewhere determined a 400A service is sufficient... and additionally divvied the loads up to the (2) 200A panels. That person should have that info...
 

Pplayz18

Member
Location
Millville nj
If all the information you have is what you've divulged here, you can't.

If that be the case, someone somewhere determined a 400A service is sufficient... and additionally divvied the loads up to the (2) 200A panels. That person should have that info...
Ok. That's what I figured. This is what happens when a boss underpays you and expects you to do his job.

I certainly don't make enough to be doing those calculations to determine the answer to that.

Nice to know for my own future reference though as I've also been in the process of obtaining my license...

Thanks alot

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