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#95941 - 10/23/05 09:34 AM What do you think of this practice?
e57 Offline
Member

Registered: 05/27/03
Posts: 2837
Loc: S.F.,CA USA
I have seen this done several times before by other companies, and not sure what to really think about it.....

Say you have a 3 phase 42 circuit panel, with all 20a circuits...
And you run all 42, and thier neutrals out of it in only (4) 1" conduits using this method 50 or so feet to J-boxes and gutters and dropping to #12 for the rest of the circuit.

Table 310.16 for #10 THHN is 40a. (Without load diversity)

Table 310.15(B)(2)(a) for 10-20 conductors is 50% derating. Down to 20a on #10.

Table C1 for #10 is 16 in 1"EMT.

42 circuits, and 14 neutrals is 56 conductors, divided by 16 is 3.5 conduits.

And if in 1 1/4" you could do it in (2) conduits.

What do you think of this practice?

(Cost of Steel and Labor?)
(Cost of Copper, and 56 splices in a bunch od cans?)
(Circuit Quality?)
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Mark Heller
"Well - I oughta....." -Jackie Gleason

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#95942 - 10/23/05 09:45 AM Re: What do you think of this practice?
iwire Offline
Moderator

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
Under the NEC IMO you need three conduits and 10 AWG for 20 amp circuits.

Under the MA rules where I am I can use two conduits and still use 10 AWG.

This is assuming all 42 circuits are 20 amp, run in multiwire fashion and are feeding linear loads.

Bob
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Bob Badger
Construction & Maintenance Electrician
Massachusetts

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#95943 - 10/23/05 09:51 AM Re: What do you think of this practice?
e57 Offline
Member

Registered: 05/27/03
Posts: 2837
Loc: S.F.,CA USA
Bob, how could you do it in three?

Ahh, assuming only linear loads?

This would be in Commercial TI's with loads of computers and lighting, and a high harmonic factor, outside of being non-linear.

[This message has been edited by e57 (edited 10-23-2005).]
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#95944 - 10/23/05 10:14 AM Re: What do you think of this practice?
iwire Offline
Moderator

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
This is what we have to follow

 Quote:
310.15(B)(4)(c) On a 4-wire, 3-phase wye circuit where the major portion of the load consists of nonlinear loads, harmonic currents are present in the neutral conductor; the neutral shall therefore be considered a current-carrying conductor.


How much exactly is the major portion of the load?

50.1%?

75%?

Yes many times some of these circuits are feeding non-linear loads but they also feed linear loads.

Each person will have to decide what is the major portion of load.
_________________________
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Construction & Maintenance Electrician
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#95945 - 10/24/05 01:00 PM Re: What do you think of this practice?
Tesla Offline
Member

Registered: 06/16/04
Posts: 1280
Loc: Sacramento, CA
This is a great issue. The answer greatly establishes a contractor's 'style.'

I never attempt to use minimal raceways coming out of a panel. Self-courtesy and economics drive me to bump 1.25" up to 1.5" if not 2".

Good practice is to provide for expansion. I want to be able to fish in additional conductors to additional breakers at the last moment, without additional pipe.

Maxxing out pipe fill is, in my opinion, poor economics. I don’t design for it, the exception being feeders.

I tend to bump up the size of my gutters as well. Labor expense and safety are my job cost killers; not materials. The difference in cost between 8x8x24 gutters and 6x6x24 gutters is not enough to compensate for the extra labor required in tight space.

In the typical design/ build contract MC will radiate away from these gutters. Naturally, I position them with consideration for ease of both installation and eventual re-entry.

At the panel, #10 THHN stranded makes for a dressy look in reasonable time.

In most of my jobs, #10 is contract specified for homeruns, anyway.

If I am going to homerun out to a gaggle of cans then 1” EMT is my maximum. If I have to run parallel pipes, so be it. I plan on 2 full boats per pipe. The extra pipe capacity makes for sweet extras at the end of the job when time is short and the space is built out.

The wide spread electronic ballasts and switching power supplies makes the neutral a derated conductor. With time, I expect dang near everything to go electronic.


[This message has been edited by Tesla (edited 10-25-2005).]
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#95946 - 10/24/05 04:45 PM Re: What do you think of this practice?
boggerbutt2454 Offline
Member

Registered: 02/27/05
Posts: 44
Loc: Concord NC USA
Down here in the great state of Mecklenburg
County they require you to use the 75 degree 35 amps but 240.4(D)allows only for 30 amps and at 50% is good only for 15 amps. The only way they would let me do this in a 1" conduit or nipple would be if it was 24 inches or shorter. If you are going to use #10 then you could only have 9 current carrying conductors which brings you to 70% derate or 21 amps for the #10. You would need 7-1" conduits, with 8 conductors each.I have seen panels and circuits ran the way you described and have found that these conduits tend to be very hot as well as the panels and cause premature replacemnet of breakers and in some cases the panel itself. personally I feel is a bad install.
The saying around Charlotte NC sums this one up as nothing beats quality like a low price. While saving you and the customer money is important, I feel doing a quality job should always come first.
As a note I just left a service call where someone squeezed 7-#10 strand wires in a 1/2 conduit. I'm not sure but they must have used a tugger.


[This message has been edited by boggerbutt2454 (edited 10-24-2005).]

[This message has been edited by boggerbutt2454 (edited 10-24-2005).]

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#95947 - 10/27/05 06:35 PM Re: What do you think of this practice?
cgw Offline
Member

Registered: 07/29/04
Posts: 133
Loc: Rochester NY
To all:
How are you grounding with this method?

Boggerbutt: Is the use of 75C rating a local requirement or do they think that is the intent of the NEC?

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#95948 - 10/28/05 01:48 PM Re: What do you think of this practice?
Tesla Offline
Member

Registered: 06/16/04
Posts: 1280
Loc: Sacramento, CA
cgw

In my experience the 90 degree table is misunderstood. Most electricians, even EE, don't know how to derate. I've seen it time and again.

There is no 90 degree equipment made. There are only 90 degree conductors, lugs, etc. Various references to requiring 90 degree conductors will be found on all kinds of stuff, but they are still not themselves rated 90 degrees.

The 75 degree rule may be a legacy from older insulators, or it may have crossed over from underground/ exterior ratings. Remember that THHN is typically double rated as THWN and that means wet at 75 degrees.

Having seen enough I just don't use EMT as a grounding path. In the real world: just too many untightened set screws/ KO reducers.

A couple of runs of THHN green are not going to kill me. Lacking same might.
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#95949 - 10/29/05 10:28 AM Re: What do you think of this practice?
boggerbutt2454 Offline
Member

Registered: 02/27/05
Posts: 44
Loc: Concord NC USA
Tesla states pretty much to the letter the reasons our inspectors give us as to why they use the 75 degree column.

I have been pulling equipment grounds in conduit for the last 20 years even in GRC and can't think of a reason why you wouldn't. Grounding and bonding is the one of the most important things you can do for safety.

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