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I just recently installed a 50A two pole GFCI circuit breaker that was part of a 240VAC spa panel. For 50A I was using 6 AWG wires. When connecting up this type breaker you connect the two poles of the breaker to the two hot load connections and then the neutral wire of the load also connects to the "white" marked terminal of the two pole breaker. From the breaker there is a short coiled white wire that gets connected to the neutral bus bar in the electrical box.

I noticed that the coiled wire is decidedly smaller gauge that the #6 AWG wires I was using for the circuit connections. From a guess without measuring the size I would say it was 10 AWG.

So here is my question. This coiled wire will be asked to carry the neutral current from the load back to the neutral bus bar. Why is it not a wire normally rated for 50A?

  • Because ground wires do not need to be sized to match neutral sizes, in general. And particularly with GFCIs there is no reasonably likely scenario where the full load current would be shunted to ground without tripping the breaker. – Hot Licks Nov 29 '15 at 22:31
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    @HotLicks -- the jumper in question's a neutral, and can carry the full load current on a 2pole/240VAC GFCI without tripping it (this would happen on a MWBC if everything was on one leg of the circuit, for instance.) – ThreePhaseEel Nov 30 '15 at 1:25
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It is likely #8 wire. Why this is allowed is not clear. #8cu THHN IS rated for 50A, but #8 NM cable is not. It is most likely that this wire is rated 75 deg C so it is fine for 50A.

Also, the neutral is only carrying the 120V loads, and even then only the imbalance, which on a hot tub are definitely far less than the full load amperage draw since the heavy loads, such as the heaters, are 240v.

  • I believe the code says a manufactured 50 amp UL approved pigtail can be as small as #12. Perhaps the jumper is considered as a pigtail. – Kris Nov 29 '15 at 21:29
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    @Kris, That's one I never heard. Interesting. I wouldn't doubt it. – Speedy Petey Nov 29 '15 at 21:30
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The size of wire required to carry a certain amount of current is primarily affected by two things:

  1. How much power one can afford to waste in the form of voltage drop per unit length of wire. The greater the power one can afford to waste per unit length, the smaller a wire one can use by this criterion.

  2. How much power one can dissipate per unit length without the wire getting unacceptably hot. The more heat can be carried away, or the greater the acceptable temperature rise, the smaller a wire one can use by this criterion.

Normal current-carrying figures assume a conductor wrapped in a protective outer jacket and then buried in material that is thermally non-conductive but flammable. A wire in free air within a breaker box would be able to get rid of heat far more effectively than one in an insulated wall, and less likely to ignite anything nearby. Further, if a wire is short, the allowable voltage drop per unit length will generally be much higher than it would with a longer wire. Thus, the normal factors which limit allowable currents as a function of size, or compel minimum wire sizes as a function of current, are less applicable for short wires in a box than they would be for longer wires in a wall.

  • All good points -- but it's also fair to say that AWM (or THHN) can survive higher temps than say NM. – ThreePhaseEel Nov 30 '15 at 1:26
  • @ThreePhaseEel: Even if the wire could survive higher temperatures, I don't think one would want the wire to reach a temperature which would be even remotely risky for even the wimpiest insulation. – supercat Nov 30 '15 at 2:16
  • that's contradicted by the ampacity tables in the NEC -- they clearly give credit to higher temp ratings when determining the ampacity of individual wires, and I strongly suspect UL does the same thing. – ThreePhaseEel Nov 30 '15 at 2:17
  • @ThreePhaseEel: A variety of factors may affect the maximum acceptable temperature that a wire might reach. The point where the insulation fails will obviously be one limiting factor, but in many cases it will be desirable to keep wires well below that temperature for other reasons. – supercat Nov 30 '15 at 2:25
  • true -- it's really a matter of the acceptable temperature rise – ThreePhaseEel Nov 30 '15 at 3:35
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I believe there are different ratings by AWG for chassis wiring and power distribution, since length is a big part of ampacity. I'd figure the NEC would have to do with power distribution ampacity, the UL listing would deal more with chassis wiring. This jumper would be chassis wiring.

The neutral is only going to carry the imbalance of the 120v loads but worst case if all the loads are on one leg, it could carry the full load.

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The breaker, baring a factory-installed conductor, is UL approved for its intentional usage. The code refers to such as "tapped conductors" and in the field they are known as pig-tails.

While there are a lot of rules about tapped conductors, there is nothing specific about a GFCI double pole breaker in general that I can find

For comparison purposes ONLY here is a code article about tapped conductors.

II. Branch-Circuit Ratings

210.19 Conductors — Minimum Ampacity and Size

A (4) Other Loads. Branch-circuit conductors that supply loads other than those specified in 210.2 and other than cooking appliances as covered in 210.19( A)( 3) shall have an ampacity sufficient for the loads served and shall not be smaller than 14 AWG.

Exception No. 1: Tap conductors shall have an ampacity sufficient for the load served. In addition, they shall have an ampacity of not less than 15 for circuits rated less than 40 amperes and not less than 20 for circuits rated at 40 or 50 amperes and only where these tap conductors supply any of the following loads:

  • (a) Individual lampholders or luminaires with taps extending not longer than 450 mm (18 in.) beyond any portion of the lampholder or luminaire.

  • (b) A luminaire having tap conductors as provided in 410.117.

  • (c) Individual outlets, other than receptacle outlets, with taps not over 450 mm (18 in.) long.

  • (d) Infrared lamp industrial heating appliances.

  • (e) Nonheating leads of deicing and snow-melting cables and mats.

  • sorry, didn't quite catch that you were just using the quote for comparison purposes. – ThreePhaseEel Nov 30 '15 at 3:39
  • Do you think I should remove the comparison? – Kris Nov 30 '15 at 3:47
  • I don't think so, now that I get the post -- I'm just a bit tired :P – ThreePhaseEel Nov 30 '15 at 3:49

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