3 hots one not

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gmreynol

gmreynol

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I am running 3 #12 THHN ungrounded conductors and 1 #12 neutral or grounded conductor in emt to a J-box. From there I am then running (3)individual 12-2 mc to 3 single phase heaters pulling 700 watts each. Each heater receives a phase(A,B or C) and shares the one neutral or ungrounded conductor. Service is 120/208 Y. My question is the neutral or grounded conductor a current carrying conductor? Also, if the loads were not equal would that change anything?
Thanks
 
Re: 3 hots one not

If you mean counted for derating purposes, no. Unequal loads would make no difference.(310.15(B)(4)(a)
 
Re: 3 hots one not

Thank you for your responses(310.15(B)(4)(a) reads neutral conductor that carries only unbalanced current from other conductors of the "same" circuit shall not be counted. This is where I get lost. It seems to me I have three circuits in this case?
 
Re: 3 hots one not

gmreynol,
(310.15(B)(4)(a) reads neutral conductor that carries only unbalanced current from other conductors of the "same" circuit shall not be counted.
Click to expand...
this is true and what you technically have is one "Multiwire circuit.
(See the definition of Branch Circuit, Multiwire)

Notice the wording in 210.4(A) A multiwire circuit shall be permitted to be considered as multiple circuits

Roger

[ September 20, 2004, 08:45 PM: Message edited by: roger ]
 
Re: 3 hots one not

Great Thanks! Alot of electrician go by a rule of thumb of no more than 9 #12 thhn conductors at 20 amps per pipe run, ungrounded and grounded/neutral conductors. It appears to me that a 9 ungrounded and 4 neutrals could be installed in the same raceway and derate at 70 percent i.e
(a,b,n) (a,c,n) (a,b,c,n) (b,c,n)
 
Re: 3 hots one not

gmreynol,
It appears to me that a 9 ungrounded and 4 neutrals could be installed in the same raceway
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depends on the size of the conduit, see chapter 9 and Annex C :)

and derate at 70 percent i.e
(a,b,n) (a,c,n) (a,b,c,n) (b,c,n)
Click to expand...
Nope, we can't do this, see 310.15(B)(4)(b)

In a wye system when only two phases are present the grounded conductor is not a neutral, so you would have 12 current carrying conductors in your example, (a,b,n) (a,c,n) (a,b,c,) and (b,c,n).

Roger

[ September 20, 2004, 10:08 PM: Message edited by: roger ]
 
Re: 3 hots one not

In 310.15(B)(4)(b)it refers to the common wire as a neutral, is this a worded wrong? By definition it appears to me 2 phase conductors and a common... would be a neutral?
 
Re: 3 hots one not

I?ll jump back on my podium.

We should always bear in mind that we speak two languages. We can use what I like to call ?Conversational English? when speaking with co-workers, customers, and family members. In that language, the same word can often mean many things, and a set of many words can often mean the same thing. In that language, the terms ?neutral,? ?cold leg,? ?common,? ?white wire,? and a host of others are used interchangeably.

But if we want to discuss what is or is not allowed per code, then we need to use what I will call ?Professional Language,? or ?Code Language.? In this language, the thing described above in many different terms should properly be called the ?grounded conductor.? In this language, the word ?neutral? may be used interchangeably with ?grounded conductor,? but no other term should be used.
 
Re: 3 hots one not

gmreynol,
When you are using only two ungrounded phases and a grounded conductor from a 3 phase wye system, the grounded conductor is always required to be counted as a current carrying conductor. This is because under this condition, if both ungrounded conductors have 15 amps on them, the grounded conductor will also have 15 amps. On a three phase system, the current in the grounded conductor does not cancel unless you have current from all three ungrounded conductors.
Don
 
Re: 3 hots one not

Gmreynol,
In 310.15(B)(4)(b)it refers to the common wire as a neutral,
Click to expand...
actually it is refering to the Neutral Conductor of a wye system being a Common Conductor when connected to two phases since it is no longer a Neutral.

This would also be true if connected to one phase.

To expand on Don's post (only I will use 5 amps) it is as follows

To figure the neutral load on a wye system the formula is as follows;

______________________________________
\/ (AI^2+BI^2+CI^2) - (AxB)+(BxC)+(CxA)

In otherwords it is the square root of
A phase squared + B phase squared + c phase squared - A phase x B phase + B phase x C phase + C phase x A phase

Give all three phases of value of 5 amps and figure the Neutral load as shown above.


5x5 + 5x5 + 5x5 = 75 - 5x5 + 5x5 + 5x5 = 75

75 - 75 = 0 the square root of Zero is Zero, so the Grounded Conductor is indeed a Neutral Conductor.

Now using just two phases A & B figure it again.

5x5 + 5x5 = 50 - 5x5 = 25

50 - 25 = 25 the square root of 25 is 5, so in this example you can see the Grounded Conductor carries the same current as the two phase conductors.

Charlie b could probably show it better. :)

Roger
 
Re: 3 hots one not

Originally posted by roger:Charlie b could probably show it better
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Thanks for the vote of confidence, but I think you explained it very well. ;)

I will offer the clarification that the formula you used and the statement Don made (i.e., about counting the grounded conductor) both apply, as Don explicitly stated, for a 3-phase system. If you are dealing with two ?hots? and one ?not-hot? in a single phase 120/240Vsystem, the math works differently.
 
Re: 3 hots one not

:confused: I feel a little confused about what I have read on a few of the other respones concerning this area.

310.15(b)(4)a
I always thought this is refering to a multiwire branch circuit. Whether it be one two or three phases that supply only line to neutral load. Sharing the same neutral.

310.15(b)(4)b
I thought there is a difference between a 3 wire circuit and a multi wire circuit.

310.15(b)(4)c
I also thought there is a difference between a 4 wire three phase circuit and a multi wire circuit.

The reason I bring all of this up is Derating seems to have a lot of unanswered questions.

9-20A circuits 1,3,N,2,4,6,N,8,10,12,N,7,N all of which feed receptacles. What size wire should be used? Also which of the above neutrals carry unbalanced current of the same circuit, since this would be the ones that arent counted?
 
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