Sizing the Neutral

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auffant said:
I don't know much about NEC rules, but the reason to have a neutral wire the same size as the lines is primarily for 3 phase systems where because of faults or load distribution there could be a situation where loads are not balance between the phases. In an ideal 3 phase system there is no current flowing thru the neutral, but if that system became very unbalance the current flowing thru the neutral could be greater than the phase current thus requiring a full sized neutral.
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Without the effect of harmonics, the grounded conductor current will not exceed that of the phase conductors.
 
Well here's how I would look at it. The maximum unbalanced load current is carried over the feeder neutral. The fault current is carried over the service neutral. These are two different types of current. There may be a code interpretation problem about which rule to follow. Either 250.66 or 220.61. NEC 250.66 would be suitable to carry the fault current over the service neutral. NEC 220.61 would be suitable for the maximum unbalanced load over the feeder neutral. They would both have to be terminated at the neutral terminal bar inside the service enclosure. It's not like you would have to splice the two conductors together. Just connect them to a bar.
 
Article 250 must be the most misunderstood or misinterpreted part of the code. Some things about it just don't make sense. For instance, requiring that the feeder neutral be sized according to NEC 250.122. The feeder neutral and the EGC are two different things on the load side of the service. The feeder neutral only carries the maximum unbalanced current. I would think that the service neutral would be sized in accordance with NEC 250.122. After all, whenever there's a ground fault the fault current flows through the service neutral. It becomes an Equipment Grounding Conductor. Why the service neutral should have the same sizing as the Grounding Electrode Conductor(NEC 250.66) I do not know.
 
roger said:
No, if they are 3 phase motors the service neutral wouldn't come into play except for clearing a fault which is the reason for 250.24(C)
Roger
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It could be a VFD with a three-phase output and a single-phase line-neutral input in which case there would be both fundamental and harmonic currents in the neutral. A 1-3 UPS would have similar characteristics.
 
erickench said:
For instance, requiring that the feeder neutral be sized according to NEC 250.122. The feeder neutral and the EGC are two different things on the load side of the service.
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The reason that the feeder neutral must be sized no smaller than the required EGC in accordance with 250.122 is that in the event of a hot to neutral fault in the feeder the neutral conductor must be capable of carrying the fault current so that the feeder overcurrent device can trip.

I would think that the service neutral would be sized in accordance with NEC 250.122. After all, whenever there's a ground fault the fault current flows through the service neutral. It becomes an Equipment Grounding Conductor. Why the service neutral should have the same sizing as the Grounding Electrode Conductor(NEC 250.66) I do not know.
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You can't use 250.122 to size the service neutral conductor because there is no overcurrent device protecting the service conductors other than the fuses on the primary side of the utility transformer. This is the reason for sizing the service neutral using 250.66.

Chris
 
raider1 said:
The reason that the feeder neutral must be sized no smaller than the required EGC in accordance with 250.122 is that in the event of a hot to neutral fault in the feeder the neutral conductor must be capable of carrying the fault current so that the feeder overcurrent device can trip.



You can't use 250.122 to size the service neutral conductor because there is no overcurrent device protecting the service conductors other than the fuses on the primary side of the utility transformer. This is the reason for sizing the service neutral using 250.66.

Chris
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IMHO, a most excellent response, Chris
 
erickench said:
Yes but when you try to compare the sizing for 250.66 with 220.61 it seems that 250.66 would give you a smaller conductor.
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You need to use the larger of the two.

250.24(C)(1) requires that the grounded conductor be NOT SMALLER than the required grounding electrode conductor as specified in Table 250.66.

Chris
 
erickench said:
Then why have 250.24 (C) (1)? Why not just leave it as 220.61?
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220.61 does not require you to take a neutral the the service disconnect, it is for sizing purposes of the neutral not for fault clearing as 250.24(C)(1) is.

Roger
 
roger said:
220.61 does not require you to take a neutral the the service disconnect, it is for sizing purposes of the neutral not for fault clearing as 250.24(C)(1) is.

Roger
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Agreed, in my area it is common for irrigation pivots to have a 3 phase 480 volt service that does not need a grounded conductor for the motor but does need it for clearing ground faults.

Chris
 
But would'nt the fault current be higher than the maximum unbalanced load current? The service conductors are not protected as someone said. You would have to use a larger conductor size.
 
erickench said:
But would'nt the fault current be higher than the maximum unbalanced load current? The service conductors are not protected as someone said. You would have to use a larger conductor size.
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There would not be any unbalanced load in a line to line or phase to phase service.

The grounded conductor brought to the service equipment required in 250.24(C) is not necessarily a neutral, it is for fault clearing only and must be sized per T 250.66.

Your concentrating on a neutral load which 250.24(C) is not about.

Roger
 
Whether it's an actual neutral i.e.wye system, or center tap from a utility transformer you're still going to have alot of current. Now I've heard it said that the utilities will allow their cables to run hot. But the service entrance conductors running from the property line box to the Service itself is still the customers property.
 
erickench said:
Whether it's an actual neutral i.e.wye system, or center tap from a utility transformer you're still going to have alot of current.
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If there are no neutral loads there would be no current on a grounded conductor. Imagine a service in the middle of a field feeding a 240 V single phase pump, the only curent flow would be on the two ungrounded conductors, the grounded conductor would be waiting on standby until a fault occured.

erickench said:
Now I've heard it said that the utilities will allow their cables to run hot. But the service entrance conductors running from the property line box to the Service itself is still the customers property.
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I don't understand the point you're trying to make here.

Roger
 
The service entrance conductors are not protected. You would have to use a large size conductor in a grounded system. Then again you're not supposed to have any type of disconnect on a grounded conductor unless it opens simultaneously with the ungrounded conductors.
 
erickench said:
The service entrance conductors are not protected. You would have to use a large size conductor in a grounded system. Then again you're not supposed to have any type of disconnect on a grounded conductor unless it opens simultaneously with the ungrounded conductors.
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I still don't understand what you are trying to say.

I will try one more time to though.

The reason we have to take a grounded conductor from the POCO source to the service equipment per the requirement of 250.24(C) is for fault clearing on the load side of the service equipment. A fault on the line side will have to fend for itself and probably will not fair very well.

Roger
 
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