Sizing the Neutral

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roger said:
Eric, don't take this wrong but, since it is so confussing to you, why don't you send in a proposal and maybe when the CMP explains their action it might be more clear to you than the way we are trying to explain it.
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Well I just might do that in the future. You see taking a professional engineers exam is not like taking a master electricians test. It's as if we were back in a college classroom taking a written test. They don't teach us the NEC in college. We have to learn that on our own. I've purchased some of Mike Holts books and I'm learning. But I have to question everything. The best way for me to learn is to analyze everything. I've made a career out of finding mistakes on drawings and specs.
 
Eric,
The code required size of the grounded conductor is based on the maximum possible grounded conductor load for the installation. That is not going to be the rating of the main OCPD in most cases. Line to line loads do not add neutral current and are not included in calculating the size of the grounded service conductor. In some installations there is almost no calculated grounded conductor load and that is where you apply 250.24 so that you have a service grounded conductor that will carry the fault current. The code assumes that the fault is on the load side of the service OCPD and a grounded conductor sized to the larger of what is required by 220.61 or T250.66 will be able to carry enough current to cause the OCPD to open the circuit without being damaged. However all bets are off for a fault on the line side of the OCPD as there is no protection for these conductors. In most cases the fault has to burn itself clear as the utility primary OCPD is not designed or intended to protect the transformer or its secondary conductors. In this case you could argue that providing a full sized grounded conductor would be safer.
 
Yes it would be good engineering practice to size the grounded neutral conductor the same as ungrounded conductors. We did it in the Transit Authority. I only take this up with you electricians to see if I missed anything. When I was a young engineer I found out that alot could be learned from talking and working with technicians because they dealt with the real world. I noticed Scott Harding and John Travers profiles in Mike Holts book "Grounding versus Bonding". Are they on this Forum? I thought I saw them in the videos on Mike Holts website.
 
You already have some answers so I'm just going to comment on specific statements...
erickench said:
Yes I was a bit confused at first because the earth has such a high resistance and that very little current would flow through it. But if you would examine NEC 250.66 you would note that the area(cmils) of the conductor is 1/4 of that of the ungrounded service conductor. Therefore the resistance is increased by a factor of four. This creates heat.
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You have to have current flow to generate the heat. Refer to your following example and my comment...

erickench said:
Now let as suppose you had a 100A 3-wire single phase system. The circuit breaker is going to allow a maximum of 100A to go through. Assume that one of the ungrounded conductors is open(no load). The maximum current flowing through the neutral would be 100A. You can't just assume a partial load as being the maximum. It would be good engineering practice to assume the maximum permissible amps flowing through the circuit breaker and through the neutral as well.
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You are 100% correct... but you should also note that in your example, the grounded neutral conductor would be sized to carry the 100A, and keep in mind that it only sees the full 100A in the maximum imbalanced state of operation.

erickench said:
So what is the purpose of 250.24(c)(1) if you know that you are going to have so many amps and that you would have to size the grounded(or neutral) conductor the same as the ungrounded conductors?
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Because scenarios exist where the grounded neutral does not need to carry a current equal to any one ungrounded conductor. Many loads (3? and 1? Line-to-Line) do not contribute to grounded neutral current. Therefore the maximum current the grounded neutral conductor can be expected to carry is less than that of any one ungrounded conductor. Bcause this is a variable which cannot be defined numerically for all instances, code has be written to allow the adjusted smaller size, but yet maintains that it shall never be too small. This has been true for many code cycles, and I'm sure the CMP's would have heard by now if the method implemented did in fact permit too small a grounded neutral conductor!!!

erickench said:
Why was this written into the code? Service conductors do have overload protection but how is the protection going to work if the code says that you are permitted to have a smaller grounded(or neutral) conductor? That cable is going to fry.
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Reason and real world experience has proven otherwise.

erickench said:
Of course in the real world we could deal with this problem very easily just by sizing the grounded conductor(or neutral) to be as large as the ungrounded conductor.
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I've seen many an installation spec'd exactly this way. Perhaps there are other engineers as skeptical as you :rolleyes:. On the other hand, sometimes it's simply a matter of already having that size wire on hand ;)
 
Well right now sizing the neutral is more like a guessing game. Things change. So who can be sure if they sized it properly to suit the changing load in the future? The best bet is to assume the worst case scenario. It's better than ripping out the service conductors than reinstalling them again.
 
erickench said:
Well right now sizing the neutral is more like a guessing game. Things change. So who can be sure if they sized it properly to suit the changing load in the future? The best bet is to assume the worst case scenario. It's better than ripping out the service conductors than reinstalling them again.
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Not a guessing game, it is calculated.

Unless someone comes along and adds loads with no regard for whats existing the neutral can not be overloaded.

It is up to the person adding loads to make sure those loads can be supplied by the existing equipment. You may not agree with that but it what the NEC allows and has a long track record.

I would never say the NEC is perfect but this

I've made a career out of finding mistakes on drawings and specs
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is nowhere near the same as finding huge holes in the NEC. :smile:

The plans and specs you work with are relatively new and have been seen by relatively few. On the other hand the NEC is more then 100 years old, is reviewed daily by many. I bet most glaring mistakes surface quickly. :smile:

For what it's worth in my area virtually every residential service has a neutral 1 or 2 sizes smaller then the ungrounded conductors. It is the way SE cable is sold in this area.
 
Yes it is calculated but the calculation is based on assumptions of how many watts are connected. This can vary. How do you know that you've made the right assumptions? The new owner may come in and just decide to add on more watts than was previously calculated. It's best to assume the maximum. This way you can never go wrong.
 
erickench said:
Yes it is calculated but the calculation is based on assumptions of how many watts are connected. This can vary. How do you know that you've made the right assumptions?
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By what is told to you up front.

erickench said:
The new owner may come in and just decide to add on more watts than was previously calculated. It's best to assume the maximum. This way you can never go wrong.
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Do your best to prevent an idiot from hurting himself and he'll show you that he is a better idiot than anyone could of imagined.


For example, you design a 125 amp feeder for a piece of equipment that specifies using a fused disconnect, you will have to use a 200 amp safety switch, you call for XXX xxx 125 amp fuses, how are you going to stop someone in the future from oversizing the fuses?

You can only worry about the present and design accordingly, then go home and sleep well knowing you did just that

Roger
 
erickench said:
Yes it is calculated but the calculation is based on assumptions of how many watts are connected. This can vary. How do you know that you've made the right assumptions?
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I know that my calculation will be right based on what I was told. If what I was told was wrong we have more problems then just the size of the neutral, the entire service or feeder may be undersized.


The new owner may come in and just decide to add on more watts than was previously calculated.
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No doubt, I would not even use the word 'may' I would say they will likely add load. But it is the responsibility of the person adding that load at that time to ensure the existing electrical installation can in fact have load added.

It's best to assume the maximum. This way you can never go wrong.
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That is not true in the NEC, that is why I mentioned 90.1(B) is worth a look. The NFPA tells us straight out that compliance with the NEC may lead to an electrical system thats not adequate for good service or future expansion.

Forgetting the NEC and moving on to designing it is my opinion to say that we should always assume the maximum is not designing, it is just covering our rear.

To always assume the maximum may price the work out of the customers budget.
 
iwire said:
Forgetting the NEC and moving on to designing it is my opinion to say that we should always assume the maximum is not designing, it is just covering our rear.

To always assume the maximum may price the work out of the customers budget.
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And what would it cost to come in later on and replace the service neutral? Would'nt you have to take apart everything else as well? What does that cost?
You electricians have a tendency to oversize the feeders by applying a %125 continuous load factor even though in a residence you cannot separate the continuous load from the noncontinuous load in the VA number that results from the calculation using NEC Table 220.12. What does that cost? I mean there's nothing wrong with it. It's good engineering practice to assume future expansion. Maybe you should stop oversizing the feeder and increase the size of the neutral to accomodate the maximum load. This would actually save money. You would not be increasing the size of 2-3 ungrounded conductors but just one neutral instead.
 
Conductors for services and feeders are already larger than what the real load will be because of the very conservative nature of the Article 220 load calculations. The neutral is already sized to support the maximum load even where it is smaller than the ungrounded conductors because of the line to line loads. Those loads provide no neutral current and there is no reason to size the neutral to carry an non-existent load.
 
120/240 volt, 3wire, Single Phase Service Neutral Conductor Size

120/240 volt, 3wire, Single Phase Service Neutral Conductor Size

Guys,

I have heard that the neutral can be reduced 12.5% or 70% of the size of the ungrounded conductors. I'm not sure which is right or if either one is. My question is, on a 200 amp service using 4/0 aluminum ungrounded condutors would it be acceptable to use a 2/0 aluminum neutral conductor on a 120/240 volt single phase service supplying a moblile home.
 
rroark0003 said:
Guys,

I have heard that the neutral can be reduced 12.5% or 70% of the size of the ungrounded conductors. I'm not sure which is right or if either one is. My question is, on a 200 amp service using 4/0 aluminum ungrounded condutors would it be acceptable to use a 2/0 aluminum neutral conductor on a 120/240 volt single phase service supplying a moblile home.
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Most likely yes, but to be sure you have to do an Article 220 calculation. There is no rule that automatically permits a reduced neutral.
 
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