120/240 1Phase MWBC

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Bill Davis

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McDonough, GA
I want to be sure about something before I step out on a limb here.
310.15(B)(4)(a) In what instance does this apply? I have witnessed the installation of 8 hots 4 nuetrals and a ground all #12 thhn in a 1" conduit
This is a 120/240 volt single phase system. Do these neutral need to be counted in the deration process?
Some of these circuits serve lighting loads and some serving device load. In what instance can we not count the neutrals?
 
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Bill Davis said:
I want to be sure about something before I step out on a limb here.
310.15(B)(4)(a) In what instance does this apply? I have witnessed the installation of 8 hots 4 nuetrals and a ground all #12 thhn in a 1" conduit
This is a 120/240 volt single phase system. Do these neutral need to be counted in the deration process?
Some of these circuits serve lighting loads and some serving device load. In what instance can we not count the neutrals?
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Welcome Bill, and congrats on your new position! I was a contractor and now I?m not and lovin it as well.

This can be confusing at a glance but read it by itself with out distractions. Notice 310.15(B)(4), ?A neutral conductor that carries only the unbalanced current from other conductors of the same circuit shall not be required to be counted when applying the provisions of 310.15(B)(2)(a).?
 
MWBCs

MWBCs

Yes, welcome Bill. As you are looking at these MWBC, be sure to check out 210.4 in whichever Code is applicable for your area. '08 has some significant changes.
 
Bill Davis said:
310.15(B)(4)(a) In what instance does this apply?...This is a 120/240 volt single phase system. Do these neutral need to be counted in the deration process?
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I understand that if you have a 3 wire circuit 120/240 1PH the neutral needs to be counted. If there were 2 neutrals (one for each circuit) these would not have to be counted.
 
tryinghard said:
I understand that if you have a 3 wire circuit 120/240 1PH the neutral needs to be counted. If there were 2 neutrals (one for each circuit) these would not have to be counted.
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Oops! You got that backwards.
 
Think I've got it.

Think I've got it.

This must be the most over thought section I've delt with. It seems some inspectors from Atlanta were teaching the urban legend in their continuing education classes. I will not count the neutrals in deration of the conductors, and understand that a dedicated neutral is not a neutral.
Thanks Fellows, its nice to have your support. I wish I'd signed up years ago.
Contact me if I might be able to assist you.
Bill Davis
 
tryinghard said:
I?m may have a brain freeze goin, :roll:, what am I missing because I still understand it this way. :-?
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A MWBC consisting of 2 hots and a neutral from a single phase 120/240 source counts as 2 current carrying conductors.

Two 2-wire circuits consisting of 2 hots and 2 grounded conductors from a single phase 120/240 source counts as 4 current carrying conductors.
 
tryinghard said:
I?m may have a brain freeze goin, :roll:, what am I missing because I still understand it this way. :-?
Click to expand...
Let's see:

I understand that if you have a 3 wire circuit 120/240 1PH the neutral needs to be counted.
Click to expand...
In this case, any ampere flowing on the neutral is one not flowing on a line conductor, so two conductors' worth of current is the most these three conductors might ever see. The neutral need not be considered a CCC.

If there were 2 neutrals (one for each circuit) these would not have to be counted.
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In this case, every ampere flowing on a line conductor will be matched by the current on its corresponding grounded conductor, so four conductors' worth of current can flow. Obviously, there are four CCC's here.


Is the brain thawing a bit yet? :)
 
tryinghard said:
I?m may have a brain freeze goin, :roll:, what am I missing because I still understand it this way. :-?
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I know these drawings may be getting old but here goes.

The grounded conductor is a Curent Carrying Conductor in two wire circuits.

not_a_neutral.JPG


and is not a Curent Carrying Conductor in the illustrations below.

unbalanced__neutral.JPG


true_neutral.JPG


Roger
 
iwire said:
Well it's not like electricity changed it's behavior recently. :cool:
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Whew, I'm sure glad that it hasn't. :grin:

Roger
 
tryinghard said:
Okay, the ice age is nearly over. :-? So a neutral in a 3 wire circuit that originates from a 3PH wye system counts?
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based on an unbalanced load or balanced for that matter, No they do not count if the rule is consistent.

I only recently discovered this after talking with one of my installers about counting the n leg or not. The 2002 ugly's says..........
"Maximum Number of Conductors IN <insert raceway type here> for how many wires you can stuff in conduits of various types.

It does not say to not count your N leg.,, but teh opposite.

The section that preceeds the max num of conductors actually has a note that says the following:

NOTE 1:All conductors must be counted including grouding conductors for fill percentage.

What a mess:confused:

Certainly we could argue the NEC superseeds the uglys,,,,,,,,, i hope thats not the case but try explaining that to a room full of Vets.
We all must appreciate the disclaimer on the opposite side of the Ugly's cover though.

Just so you guys know,, I have also ALWAYS counted the N and ground as a conductor because it takes up space , and absorbes the heat and or contributes to congestion in a raceway. ITs the only way i can feel good about the job in the end.
 
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tryinghard said:
Okay, the ice age is nearly over. :-? So a neutral in a 3 wire circuit that originates from a PH wye system counts?
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Yes, for a WYE system, a circuit consisting of two conductors on opposite phases and a shared neutral the neutral is considered a current carrying conductor. 310.15(B)(4)(b).
 
05 nech

05 nech

lets say i have 10 #12 AWG THHN wires. more than 9, so i must use 3/4" EMT which is good for 16 wires. now that i have the correct tube, i have to decide on the circuits.

if i have 5 pairs of ungrounded and grounded, then i have 10 CCCs and must derate ampacity by 50%.

but, if i have 6 ungrounded, 3 grounded and 1 grounding conductors, then i have 3 single ph 120/240 MWBCs and i derate ampacity for 6 CCCs. I now have 3, 20A MWBCs. (neutrals only carry imbalance (4a))

now you may continue on to 110.14 or 240.4(D).........:cool:
 
jcassity said:
based on an unbalanced load or balanced for that matter, No they do not count if the rule is consistent.

I only recently discovered this after talking with one of my installers about counting the n leg or not. The 2002 ugly's says..........
"Maximum Number of Conductors IN <insert raceway type here> for how many wires you can stuff in conduits of various types.

It does not say to not count your N leg.,, but teh opposite.

The section that preceeds the max num of conductors actually has a note that says the following:

NOTE 1:All conductors must be counted including grouding conductors for fill percentage.

What a mess:confused:

Certainly we could argue the NEC superseeds the uglys,,,,,,,,, i hope thats not the case but try explaining that to a room full of Vets.
We all must appreciate the disclaimer on the opposite side of the Ugly's cover though.

Just so you guys know,, I have also ALWAYS counted the N and ground as a conductor because it takes up space , and absorbes the heat and or contributes to congestion in a raceway. ITs the only way i can feel good about the job in the end.
Click to expand...

Neutrals and EGCs count for conduit fill.
You don't count EGCs or neutrals of a MWBC as CCC for derating.
 
Thank you for the correction, although this is not my thread, there is no crime in still being all the more confussed:grin:

Ill come back to see if something is said that finally makes this click.
Now you can see why I use the KISS system and count all conductors no matter where in the circuit we are.
 
I?ve been "around this tree" a few times in my career but obviously have not fully understood how to count the CCC. So I?m back, hope I don?t ring the tree! :)

Thanks Rodger for the visual I?m beginning to see it now. The neutral is a CCC when only carrying current that is unbalanced load from 1PH (310.15(B)(4)(a)). In other words it has current just unbalanced current and is a circuit conductor shared by 2 different phases. Is this correct thinking?

Infinity, how does the neutral carry approximately the same current as the lines of a 3PH wye system?

How about a 3PH delta with 3W circuit from phase A & C sharing a common neutral, is this neutral a CCC?
 
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