Sizing a neutral (grounded)

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fleetfeet

Member
Location
Broomfield, Colorado
Is it possible to run one neutral with three (3) full boats in EMT raceway? My instructor said he's seen it done and it's wrong. Couldn't you just upsize the wire; enough to allow current to flow through to the power source? I know that it's conventional to run a separate neutral with each full boat, but I was just wanting to know if it's "COMPLETELY" wrong.
 
LarryFine

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Re: Sizing a neutral (grounded)

I believe this is only permissable for feeders, as per 215.4, and for outside lighting as per 225.7(B).

In theory, as long as the neutral is sized for the largest current it could possibly be subject to (meaning the largest total of overcurrent protection on any given phase), it could work, but I wouldn't do it.
 
LarryFine

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Re: Sizing a neutral (grounded)

Originally posted by pierre:
What is a "full boat"?
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I believe that means three hots, one per phase. Either that, or it means three of a kind and two of a kind.
 
C

cselectric

Senior Member
Location
Wisconsin
Re: Sizing a neutral (grounded)

Yep, a full boat is three hots and a neutral. Also refered to as a network, depending on what part of the country you're in.

(how did trade slang get so regionalized? I've worked in three different markets, and every time I've had to relearn what everything is called :roll: )
 
LarryFine

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Re: Sizing a neutral (grounded)

I've always referred to it as three phases and a neutral. Silly me. :p
 
D

dillon3c

Senior Member
Re: Sizing a neutral (grounded)

Originally posted by LarryFine:
referred to as three phases and a neutral. :p
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Darn,I get it now..

You mean three little indians and a papoose..

Edited:--> or was that a canoe..

[ September 03, 2005, 12:34 AM: Message edited by: dillon3c ]
 
don_resqcapt19

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Re: Sizing a neutral (grounded)

Larry,
I believe this is only permissable for feeders, as per 215.4, and for outside lighting as per 225.7(B).
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While you are correct that those are the only two code sections that specifically permit this type of installation, what section(s) prohibit it in other cases?
Don

[ September 03, 2005, 11:04 AM: Message edited by: don_resqcapt19 ]
 
ryan_618

ryan_618

Senior Member
Location
Salt Lake City, Utah
Re: Sizing a neutral (grounded)

In my opinion this would be allowed, assuming the nuetral was propoerly sized.

I think it is a foolish thing to do, but I don't think it is prohibited.
 
R

redfish

Senior Member
Re: Sizing a neutral (grounded)

What about adjustment factors in article 310.15(4)and table 310.15(B)(2)(a)? Would they not apply in this case? :)
 
bob

bob

Senior Member
Location
Alabama
Re: Sizing a neutral (grounded)

Is it possible to run one neutral with three (3) full boats in EMT raceway? My instructor said he's seen it done and it's wrong.
Click to expand...
Is your question about the EMT or running 1 neutral with 3 phases?

Couldn't you just upsize the wire; enough to allow current to flow through to the power source?
Click to expand...
What do you mean?

I know that it's conventional to run a separate neutral with each full boat, but I was just wanting to know if it's "COMPLETELY" wrong.
Click to expand...
I don't think its done "all" the time.
Are your referring to installing a neutral with each phase as is done many times with HID lighting?
Red
If the neutral is a non current carring conductor no need to adjust.

[ September 03, 2005, 08:36 PM: Message edited by: bob ]
 
augie47

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
Re: Sizing a neutral (grounded)

I think it is a foolish thing to do,
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why?
maybe I';m missing somethning, but if I understand correctly its a common wiring practice around here.
 
paul

paul

Senior Member
Location
Snohomish, WA
Re: Sizing a neutral (grounded)

Here, a full boat means Ph A, Ph B, Ph C, & Neutral wire. It sounds like you're asking if you can run one neutral with three Ph A, three Ph B, and three Ph C wires all in the same raceway. I would say no.
 
F

fleetfeet

Member
Location
Broomfield, Colorado
Re: Sizing a neutral (grounded)

For each group of phase A,B,C, it is common practice to run a neutral (same size). However, for the sake of conduit fill (and costs), can you run one neutral with three groups of A,B,C phase wires (11 wires, counting the grounding conductor), and just upsize the wire for the neutral.

My point is that when and if we see this practice, is it entirely wrong (ie. hazardous, code violation, inefficient, etc.). Or, for the sake of theory, is is "workable".
 
R

redfish

Senior Member
Re: Sizing a neutral (grounded)

225.7 (B) Common Neutral The ampacity of the neutral conductor shall not be less than the maximum net computed load current between the neutral and all ungrounded conductors connected to any one phase of the circuit.
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That's going be a pretty big grounded conductor, and after you are through derating to 50% I don't think that it is going be any cheaper to run all of these current carrying conductors in the same raceway. What size branch circuits are you considering? Post the size so we can figure it out . :)
 
F

fleetfeet

Member
Location
Broomfield, Colorado
Re: Sizing a neutral (grounded)

20 amp branch circuits...thanks. I've had journeyman tell me that it would short out, and/or that it will not work because A/C needs it's own ungrounded and grounded path (back and forth) to work.

I don't want to make this complicated.
 
paul

paul

Senior Member
Location
Snohomish, WA
Re: Sizing a neutral (grounded)

Here's a problem for you. Figure that your 9 circuits are 20 amps a piece and you're running XHHW. You've also got to run a ground wire per owners specs. What size of grounded, grounding, and ungrounded conductors do you pull and what size of GRC are you going to run? I won't get into box sizing, kernies for your splices or labor at this point
 
iwire

iwire

Moderator
Staff member
Location
Massachusetts
Re: Sizing a neutral (grounded)

Originally posted by paul:
Here's a problem for you. Figure that your 9 circuits are 20 amps a piece and you're running XHHW. You've also got to run a ground wire per owners specs. What size of grounded, grounding, and ungrounded conductors do you pull and what size of GRC are you going to run?
Click to expand...
OK I'll bite, my answers are code minimums and based on three phase being available.

Here is what I come up with using one neutral for all 9 circuits.

9 - Ungrounded conductors - 10 AWG CU XHHW

1 - Grounded conductor - 2 AWG CU XHHW

1 - Grounding conductor - 10 AWG CU XHHW

RMC - 1.25"

We end up with 10 current carrying conductors resulting in a 50% reduction in ampacity.

Also the one neutral could have a maximum current of 60 amps (3 - 20 amp circuits of the same phase) so we end up needing a 2 AWG CU neutral.

(By the way if three phase was not available the single neutral would end up being a 3/0!)

_________________________________________________

Now lets think more conventionally.

Using two-wire circuits here is what I come up with.

9 - Ungrounded conductors - 10 AWG CU XHHW

9 - Grounded conductors - 10 AWG CU XHHW

1 - Grounding conductor - 10 AWG CU XHHW

RMC - 1.25"

Using two wire circuits we have 18 current carrying conductors which per Table 310.15(B)(2)(a) get a 50% downward adjustment from Table 310.16s rating.

Because of that 50% reduction we had to increase the size of the circuit conductors so now 250.122(B) kicks in and we must increase the size of the grounding conductor as well.

So in this instance we have a total conductor count of 19 - 10 AWG XHHW, requiring a 1.25" RMC.

_________________________________________________

Now lets see what happens if I use 3 phase multiwire branch circuits.

9 - Ungrounded conductors - 12 AWG CU XHHW

3 - Grounded conductors - 12 AWG CU XHHW

1 - Grounding conductor - 12 AWG CU XHHW

RMC - 1"

By using three phase multiwire branch circuits I only had 9 current carrying conductors to deal with and a total of 13 - 12 AWG XHHW conductors to fit in the raceway.

IMO, this is a great example of how much labor and material is saved by using multiwire branch circuits. :)

A reduction in conductor size from 10 AWG to 12 AWG. :)

A reduction in raceway size from 1.25" RMC to 1" RMC. :)

The reduction in raceway size from 1.25" RMC to 1" is significant as you can hand bend 1" RMC but will need a power bender to bend 1.25" RMC.

If it is my choice I will stick with conventional multiwire branch circuits.

[ September 04, 2005, 07:47 AM: Message edited by: iwire ]
 
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