reason for one neutral per bus bar hole?

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  #1  
Old 08-22-07, 11:14 AM
ddr
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reason for one neutral per bus bar hole?

I know that the rule is one neutral wire per bus bar hole, even if that panel allows more than one grounding wire per hole on the same bar. This is obviously a safety issue, but was wondering what the reason for this rule is.

I did a search here and found one reply that said it had to do with heating/cooling cycles (which I assume equals expansion/contraction?) and that arcing could occur if the wires loosen as a result. I can understand that reasoning, but wouldn’t the same thing apply to neutrals that are spliced downstream of the panel?

For example, if you have a neutral for a light spliced to a neutral for a receptacle that powers a computer area, entertainment center, or even a hallway receptacle that gets used for a vacuum cleaner, wouldn’t the difference in load cause the same heating/cooling cycle and the splice be subject to the same problem?

Are there other reasons that only one neutral per hole is allowed?

Thanks in advance.
 
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Old 08-22-07, 12:01 PM
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neutral splices

The neutrals spliced via wire nuts (and sometimes I've seen them soldered) are not likely to separate since they are twisted and they should be readily accessible if they would. In the bus bar, the screw (though rarely) could loosen. That'd be a problem. I've seen multiple ground wires twisted together (and sometimes soldered) before entering the bar. Again, check your local code.
 
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Old 08-22-07, 12:16 PM
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Probably, I personnally feel to some extent it is to prevent the possibility of two neutrals of different sizes to be inadvertantly placed under the lug. Grounds are not current carriers in normal operation and therefore don't heat up but they must be the same size to have multiple grounds under the lug. I have seen where all the grounds regardless of size for the branch circuits have been twisted together and then all placed in a large lug added on to the neutral bar.

I have also seen two neutrals of different sizes and same sizes placed in a wire nut with a larger size wire pigtail and the pigtail connected to the neutral bar. Just attempts to add circuits or to get by the one neutral per lug rule.

You also have the concern for multiwire shared neutral circuits where losing the neutral due to an open at the panel will place high voltage on the circuit it serves. So you really wouldn't want to share a hole with a multiwire circuit neutral and risk a loose or open connection for what ever reason.

In your screnario wire nuts are listed connectors for multiple ungrounded and grounded conductors. A lug like the one on a neutral bar is listed for one conductor just like the ones for the line hots in the panel. The exception being for grounds which are conductors but only during a fault and there is no heat risk.

Just a few thoughts

Roger
 
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Old 08-22-07, 12:33 PM
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It may be helpful to distinguish between a "splice" and a "termination" . A "termination" in one wire at a connection point. A "splice" is a connection between 2 or more wires.

A "splice", if properly done, is not affected by "divisions" of current at the splice-point. A correct splice essentialy forms individual conductors into a monolith, or single conductor----a splice of three #14 conductors forms a conductor 3 X the area of the "Feed-In" 14 conductor , so the "in" and "out" currents are thru a conducting area larger than any individual conductor.

The most effective type of screw-type termination is a single conductor directly under the screw. It's difficult to have 2 conductors aligned directly under screw, which results in a less effective termination, and the termination is more difficult with conductors of different guages.
 
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Old 08-22-07, 12:35 PM
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A wire nut has a spring in it, which to some extent can absorb the expansion and contraction of heating/cooling cycles. The bus bar has no such spring.
 
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Old 08-22-07, 12:51 PM
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> heating/cooling cycles...wouldn’t the same thing apply to neutrals that are
> spliced downstream of the panel?

This does happen to branch circuit neutrals all the time. Open neutral is probably the most common problem addressed on this forum. Either caused by bad termination, at a backstab receptacle for example; or at a loose splice, such as a wirenut. Wirenuts are very good at dealing with expansion and contraction, however.

> Are there other reasons that only one neutral per hole is allowed?

I would agree with Roger; it's probably to prevent neutral conductors of different sizes from being put together under the same screw. Also, the bus bars are rated for both aluminum and copper terminations so enforcing one-per-screw eliminates all of the problems introduced by the variable expansion rates of dissimilar metals.
 
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Old 08-22-07, 12:52 PM
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here is my guess

a bus bar connection is made with a screw providing the force to hold a conductor against the bus bar, which is another good conductor. The screw, typically made of steel for strength, is NOT a good conductor. Steel is many times higher resistance compared to Cu or Al. If you allow a 'stacked' wire arrangement, now you have the "top" conductor (against the screw) delivering a high percentage of its current THRU the "bottom conductor". If both neutral conductors are supplying rated circuit, then the bottom conductor may be overloaded or at least be exposed to more thermal working. This would not be an issue with grounding wires, since they typically don't carry current. That's guess #1.

Guess #2, is that the "bottom" conductor, not being locked in by the screw point, is more likely to pull out.
 
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Old 08-22-07, 02:30 PM
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I'll throw one more idea into the frey.

Since neutrals are current carrying conductors, it is alway wise to turn off the breaker that neutral serves to avoid getting shocked. If you have 2 neutrals under one termination screw, the chances that somebody turns off both circuits is that much slimmer. Now if you have 2 MWBC neuts (that would be 4 total circuits for you resi guys but it could be up to 6 for me and the 3 phase stuff) under the screw, you can screw up twice as much (or 3 times as much) stuff at once by pulling a neut with the circuits hot.
 
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Old 08-22-07, 10:06 PM
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Thanks for all the input folks, clears things up a lot. (Except for why they don't put some extra neutral holes in there in the first place!)
 
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