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#21911 - 02/12/03 03:19 PM cabling effect
ikauffma Offline
Member

Registered: 02/12/03
Posts: 11
Loc: Waynesboro, PA USA
I have a house that is a pre-fab, the main feeds were ran after the house was put in place and go into j-boxes going to the different circuits. The feeds ( 10-15 of them ) are bundled together using zip-ties and are layed on top of the foundation wall and run the length and width of the house. I have read about the "cabling effect" where NM 14,12 or 10 awg zip-tied together over long distances can sustain a dramatic loss in the amp rating for the circuits involved. Has anybody else heard of this and if so what can you do about it?

Thanks for any posts!

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#21912 - 02/12/03 04:00 PM Re: cabling effect
iwire Offline
Moderator

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
You can bundle cables togther, but you will have to derate as shown by 310.15(B)(2)(a)

You said 10-15 cables, I figure 12 2 wire circuits makes 24 current carrying conductors, derate to 45% so

240.4(B) allows next standard OCPD

#14 = 11.25 amps Still on 15 amp breaker
#12 = 13.5 amps now on 15 amp breaker
#10 = 18 amps now on 20 amp breaker

I have added the following:

Curt pointed out (see below)I was wrong with the application of 240.B, going up to the next standard breaker is prohibited for outlet circuits which some of these are likely to be.

In this case the #14 and #12 would be on 10 amp OCPDs and the # 10s on 15 amp OCPD


[This message has been edited by iwire (edited 02-15-2003).]
_________________________
Bob Badger
Construction & Maintenance Electrician
Massachusetts

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#21913 - 02/13/03 09:40 AM Re: cabling effect
ikauffma Offline
Member

Registered: 02/12/03
Posts: 11
Loc: Waynesboro, PA USA
iwire;

Thanks much for the reply and calculations. I'll check out the allowed limits and see if there is a way to fan these runs out either way. I'd like my system to be as efficient and safe as possible, but I'll take what I can get!

Thanks Again!

Ian

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#21914 - 02/13/03 01:16 PM Re: cabling effect
George Offline
Member

Registered: 02/23/02
Posts: 380
ikauffma ----

Before you go and do a lot of work...

The concern is heat damage to the wires not some interaction that will cause a drop an ampicity.

If the wires have been there for any length of time and there are no obvious heat related problems, don't go looking for one.

I suspect that the wires will not get any hotter the way they are bundled than a wire in a foam insulated wall.

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#21915 - 02/13/03 02:05 PM Re: cabling effect
iwire Offline
Moderator

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
George,
Most inspectors in my area would pick up on 10 to 15 romex bundled and ask for it to be seperated or show them the derating.

Whether or not you beleive it to be a problem, it is the code.
_________________________
Bob Badger
Construction & Maintenance Electrician
Massachusetts

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#21916 - 02/13/03 10:45 PM Re: cabling effect
caselec Offline
Member

Registered: 04/14/02
Posts: 557
Loc: San Jose, CA
Iwire,
Don't forget that 240.4(B) does not permit using the next higher standard overcurrent device if the conductors are part of a multioutlet branch circuit supplying receptacles for cord and plug connected loads.

Curt

[This message has been edited by caselec (edited 02-14-2003).]
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Curt Swartz

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#21917 - 02/14/03 10:24 AM Re: cabling effect
George Offline
Member

Registered: 02/23/02
Posts: 380
iwire ---

This appears to be an existing situation that does not require inspection.

I believe the question was concerned with waste of electricity no code compliance. That is why my answer was different than yours.

Consider the following:

We have a 120v subpanel with a 20amp main breaker. The panel has 10 20amp circuits. All of the circuits are run through 1 6" conduit.

The perscriptive code requires the conductors be derated. I agree that the code and the inspector would require derating. But derating is foolish in this case.

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#21918 - 02/15/03 08:21 AM Re: cabling effect
iwire Offline
Moderator

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
Curt,
The circuits were not described in detail so to do any calculations I had to make some assumptions in my post I figured 12, 2 wire circuits, if he has some multiwire circuits it might help him, as for each 2 circuits on multiwire you lose 2 current carrying conductors (the neutrals). I did not bother to figure all the options.

George,
When some one asks a question here I assume they want an answer as far as code, in this particular case there should not be any heating unless most or all the cables are fully loaded which is probably not going to happen, as in your example it could not happen, but whether or not you or I believe it to be a problem it's still a violation that could be failed by an inspector.

That said, I know there is a real world out there, and this kind of installation is not that uncommon

Thanks Bob
_________________________
Bob Badger
Construction & Maintenance Electrician
Massachusetts

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#21919 - 02/15/03 09:40 AM Re: cabling effect
caselec Offline
Member

Registered: 04/14/02
Posts: 557
Loc: San Jose, CA
Bob

I was referring to multioutlet circuits not multiwire circuits. Since most circuits in a dwelling are multioutlet circuits you can not use the next higher standard size overcurrent protections device.

Curt
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Curt Swartz

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#21920 - 02/15/03 09:57 AM Re: cabling effect
iwire Offline
Moderator

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
Curt, thanks now I got it, so it would be

#14 = 11.25 amps now on 10 amp OCPD
#12 = 13.5 amps now on 10 amp OCPD
#10 = 18 amps now on 15 amp OCPD

I will make note of this on my first post.

Bob



[This message has been edited by iwire (edited 02-15-2003).]
_________________________
Bob Badger
Construction & Maintenance Electrician
Massachusetts

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