310.10(H) - Conductors in Parallel

[Dale001289]

NEC has established 1/0 AWG as the minimum size allowable for conductors in parallel. There are a exceptions: 1- for control power wiring and 2- existing installations 'under engineering supervision', but can someone out there enlighten me as to the technical reasoning behind using 1/0 as a minimum?

Thanks in advance

 

[Carultch]

NEC has established 1/0 AWG as the minimum size allowable for conductors in parallel. There are a exceptions: 1- for control power wiring and 2- existing installations 'under engineering supervision', but can someone out there enlighten me as to the technical reasoning behind using 1/0 as a minimum?

Thanks in advance

One reason I suspect, is that the relative manufacturing tolerance is less in larger sizes, which means due to this factor, there is less differences in path resistance and less non-uniform current division among the paths.

For instance, suppose you have a manufacturing tolerance of +/- 1 kcmil. This isn't necessarily realistic, but it is a number we can use to make my point mathematically. On a #10 wire, this is about +/- 10%. On a 1/0 wire, this is +/- 1%. So the relative errors due to manufacturing variability in proportion to total size are less for larger sizes.

 

[Dale001289]

One reason I suspect, is that the relative manufacturing tolerance is less in larger sizes, which means due to this factor, there is less differences in path resistance and less non-uniform current division among the paths.

For instance, suppose you have a manufacturing tolerance of +/- 1 kcmil. This isn't necessarily realistic, but it is a number we can use to make my point mathematically. On a #10 wire, this is about +/- 10%. On a 1/0 wire, this is +/- 1%. So the relative errors due to manufacturing variability in proportion to total size are less for larger sizes.

Sounds reasonable - I never would have thought of it, thanks

 

[petersonra]

I think it was about practicality. There is typically no good reason to parallel smaller wires.

 

[Dale001289]

I think it was about practicality. There is typically no good reason to parallel smaller wires.

Certainly logical - but also subjective.
Suppose I want to feed a 75kVA Xfmr (3ph, 480-208/120V) and just happen to have 1000' of 'extra' 3/C 6AWG W/Gnd laying around (And - I'm cheap). So rather than purchase 2AWG I decide to run dual 6's instead.
Why is this wrong other than the fact it violates the Code?

 

[petersonra]

Certainly logical - but also subjective.
Suppose I want to feed a 75kVA Xfmr (3ph, 480-208/120V) and just happen to have 1000' of 'extra' 3/C 6AWG W/Gnd laying around (And - I'm cheap). So rather than purchase 2AWG I decide to run dual 6's instead.
Why is this wrong other than the fact it violates the Code?

it is wrong cause it violates the code.

 

[tom baker]

If you chart the size vs ampacity, in sizes greater than 1/0, its no longer proportional. The ampacity of 500 mcm is not double that of 250.
So with larger services you get more ampacity per $$ by running in parallel.

 

[electrofelon]

I think mainly because conductors in parallel introduce the risk of uneven current distribution. Limiting it to larger wire sizes limits use to more specific situations. Possibly its a personnel safety thing too: someone taking something apart and thinking one of them is dead (I am not a fan of trying to make electrical work safe for people who dont know what they are doing, but the code clearly tries to brother in law proof things).

 

[Dale001289]

I think mainly because conductors in parallel introduce the risk of uneven current distribution. Limiting it to larger wire sizes limits use to more specific situations. Possibly its a personnel safety thing too: someone taking something apart and thinking one of them is dead (I am not a fan of trying to make electrical work safe for people who dont know what they are doing, but the code clearly tries to brother in law proof things).

Good answers - all. I now have enough ammo to defend this article even against the most stubborn contractors on site!


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[petersonra]

Good answers - all. I now have enough ammo to defend this article even against the most stubborn contractors on site!


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the only defense it needs is "the code says so".

 

[Dale001289]

the only defense it needs is "the code says so".

I agree 100%. Unfortunately it doesn’t always work that way!


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[ggunn]

I agree 100%. Unfortunately it doesn’t always work that way!

You have contractors intentionally installing noncompliant wiring simply because they don't understand or agree with the reasoning behind the code?

 

[Dale001289]

Nothing installed yet. This topic came up during a meeting with construction.

I am one of those who prefers to provide a technical explanation rather simply saying “No you can’t”.


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[petersonra]

Nothing installed yet. This topic came up during a meeting with construction.

I am one of those who prefers to provide a technical explanation rather simply saying “No you can’t”.


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there are a lot of things in the code that are pretty arbitrary like this.

 

[Dale001289]

Yes I see that. What adds to the confusion are the two exceptions.


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[ramsy]

Certainly logical - but also subjective.
Suppose I want to feed a 75kVA Xfmr (3ph, 480-208/120V) and just happen to have 1000' of 'extra' 3/C 6AWG W/Gnd laying around (And - I'm cheap). So rather than purchase 2AWG I decide to run dual 6's instead.
Why is this wrong other than the fact it violates the Code?

If access is restricted to authorized personnel only, under engineering supervision:

There are some interesting advantages to doing so:
1) 3-conductor MC cable is rated for 75°C, has insulated grounding, and idiot proofs wire length & spacing consistency for paralleling.
2) When loads approach 80% of transformer capacity paralleling operates cooler @ 44°C vs 48°C for larger conductor.
3) Smaller grounding bus. Max parallel grounding = un-grounded conductors size, which is much smaller than single conductor.

There are some disadvantages:
1) While smaller termination lugs are more efficient, are they listed beyond max single-wire ampacity designed for lug?
2) A 300ft parallel shows a voltage drop of ~1.8 line-side 480v transformer @ 80% capacity vs ~1.5 volts for larger single conductors.
3) Per Carultch, if manufacturing tolerance of small wire can cause unacceptable parallel variability, must clamp meter under load to check.

 

[Dale001289]

If access is restricted to authorized personnel only, under engineering supervision:

There are some interesting advantages to doing so:
1) 3-conductor MC cable is rated for 75°C, has insulated grounding, and idiot proofs wire length & spacing consistency for paralleling.
2) When loads approach 80% of transformer capacity paralleling operates cooler @ 44°C vs 48°C for larger conductor.
3) Smaller grounding bus. Max parallel grounding = un-grounded conductors size, which is much smaller than single conductor.

There are some disadvantages:
1) While smaller termination lugs are more efficient, are they listed beyond max single-wire ampacity designed for lug?
2) A 300ft parallel shows a voltage drop of ~1.8 line-side 480v transformer @ 80% capacity vs ~1.5 volts for larger single conductors.
3) Per Carultch, if manufacturing tolerance of small wire can cause unacceptable parallel variability, must clamp meter under load to check.

Good stuff.
“Engineering supervision” is allowed in Exception 2....so why can’t it be the same for new installations?
Exception 1 allows control cables to be in parallel for voltage drop to instruments. Go figure....

This one area of the Handbook that could use some enhanced explanation.




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[LarryFine]

Years ago, it was common to see paralleled 10-3 NM supplying residential range circuits.

 

[Carultch]

Years ago, it was common to see paralleled 10-3 NM supplying residential range circuits.

Do you have any idea when that was? I'd like to see the substantiation to when the rule of paralleling conductors limited to 1/0 minimum was first introduced.

There are some disadvantages:
1) While smaller termination lugs are more efficient, are they listed beyond max single-wire ampacity designed for lug?
2) A 300ft parallel shows a voltage drop of ~1.8 line-side 480v transformer @ 80% capacity vs ~1.5 volts for larger single conductors.

Lugs are only rated for one wire, unless listed and marked otherwise. Even if multiple wires is the same total kcmil or same total strands of the same size, 110.14 states that it is still is only rated for one wire by default. It would have to be rated for the full ampacity of the largest wire at its terminal temperature that can fit. Whether the lug body has greater capacity is a moot point, as there is no application when you can fit more amps of wire on it than that.

You bring up a good point with voltage drop. Paralleling is more efficient use of copper when only local factors apply, but ability to curtail voltage drop is generally proportional to total KCMIL, regardless of how you parallel.

 

[JFletcher]

Parallel small conductors exist All Over America as result of older wiring practices... Ever put an afci breaker in an older residence and it trips immediately?
Chances are neutrals of different circuits are tied together ie paralleled. If they are on different legs, then the installer has unintentionally created a multiwire branch circuit with two neutrals versus one.

As for why you cannot run two sets of 14 / 3 to a dryer receptacle, many breakers do not accept/aren't listed for 2 wires, and I seriously doubt that a 14 30 receptacle's terminals are rated for two wires.

I do not know why the cut off limit is 1 / 0, though it had to be somewhere.