Neutral Size

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Tom.Margillo

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If a neutral is required for equipment that is rated 480v 100 amp does the nuetral conductor have to be the same size as the feeder conductors or can it be 1 size less?
 
I started to ask if this was a single phase, 480 volt load, or a three phase, 480/277 volt load. But it would not matter. The answer is the same: you need a neutral rated for 100 amps.
 
Pardon my ignorance. My experience with neutral sizing has always been done to industry standard and not on calculated load (mostly residential), so I never learned what I was not forced to, but why 100 amp on the neutral? What determines it based on the OP description?
 
The original question did not say whether the 480 volt, 100 amp ?equipment? was a single item, or a panel that serves many smaller loads. Lacking more detail, I had to go with the design concept that the neutral might carry the same load as one or more phases. That is, the ?unbalanced load? might be as much as 100 amps. Since the phase conductors (and the overcurrent device) will be sized on the basis of a 100 amp load, then the neutral conductor must be sized on the basis of a possible 100 amp unbalanced load.

If on the other hand this is a single component that draws 100 amps all by itself, and if its use of the neutral is merely to allow for unbalanced load, then perhaps the neutral could be smaller. You would need to know what the manufacturer has to say about the neutral load.
 
On this one I am asking rather than telling, meaning I have a guess but I don't _know_.

The neutral conductor must be properly protected by OCPD, and the only OCPD will be those for the ungrounded conductors.

If my guess is true, then if you had suitable OCPD on the neutral, then you could make it smaller (for example, a 4 pole breaker with common trip, with a lower trip rating for the neutral). I bet that such an oddball OCPD device would be much more expensive than the full sized neutral.

-Jon
 
1793 said:
So, Charlie b, what is your best time to solve the "Cube"?
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I used to be able to do it in under a minute, back in the days that the cube first became popular. Of course, first I had to “send in for” a copy of the solution. Probably cost me a whole quarter, not counting the 10 cent stamp. :grin: :grin:

Actually, I was wondering when anyone would take notice of my changed avatar. I finally figured out how to alter a photograph such that it meets the size limits imposed by this Forum. I am planning on cycling through my “best of” set of photographs over the next several months. ;)
 
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winnie said:
The neutral conductor must be properly protected by OCPD, and the only OCPD will be those for the ungrounded conductors.
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If the neutral is being used in a situation in which it can carry as much current as the phase conductors, then in order to protect it from overcurrent it must be sized using the same process as was used to size the phase conductors. If the neutral is being used in a situation in which it can only carry the unbalanced current from the phase conductors, then it is not in danger of being damaged from an overcurrent condition.
 
charlie b said:
If the neutral is being used in a situation in which it can only carry the unbalanced current from the phase conductors, then it is not in danger of being damaged from an overcurrent condition.
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If you have a piece of equipment with a large phase load and a very small neutral load (say line-line heating elements with a fan motor and controls connected line-neutral), then the neutral current may be far smaller than the phase current during normal operation. But what provides overcurrent protection in the event of a failure or abnormal condition?

-Jon
 
Whatever internal wiring is associated with the motor and the controls, using your example, would have been protected in whatever manner the manufacturer determined was necessary. The equipment would have been UL listed with that wiring and its protective features already built in.

But in this thread we are, of course, talking about the neutral wire that is part of the branch circuit that powers this equipment. Sometimes, that neutral wire is allowed to be smaller than the phase conductors. With regard to such cases, can you conceive of a failure scenario in which enough fault current flows to potentially cause damage to the (reduced size) neutral wires, without also having enough fault current to trip the branch circuit OCPD?

I can?t. Current doesn?t get to flow in the neutral, without first having gone through the load. The resistance of that load will reduce the current flow to the normal value expected by the manufacturer. Even if you short circuit directly from the neutral to the EGC (let?s say it happens at the mid-point of the neutral conductor), it won?t be able to damage the neutral wire, since the fault current will be too low.
 
If the neutral were only there for some pilot lights or controls could you not size the neutral to the size of the EGC or could it be smaller, say #12.
 
I would rather be safe than sorry. If you install the same size grounded conductor as your ungrounded conductors you will be able to sleep better at night.
 
chicar said:
If you install the same size grounded conductor as your ungrounded conductors you will be able to sleep better at night.
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Do you use SE cable for services?

If so your likely reducing the neutral without even knowing it.

Many SE cables have smaller bare conductors which for a service will be the neutral.
 
220.61 feeder or service neutral load. there are few instances where you'd be able to reduce the neutral. in fact if there are some instances where a service will contain an oversized neutral.
 
BossMan20 said:
there are few instances where you'd be able to reduce the neutral.
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I would bet that every, or almost every service for a single family dwelling could have a reduced neutral.
 
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