Service Neutral Size

[jeff43222]

I recently started using SER for services, only to discover that the supply houses don't carry 3-conductor cable where all of the conductors are the same gauge (it's often 2-2-4 or something like that). Usually for services I would use the same size wire for the neutral that I use for the ungrounded conductors, since the neutral must be sized according to the maximum unbalanced load (220.61). Not knowing how balanced the load might be, I always like to play it safe and have the neutral the same size.

If I want to buy SER with all of the conductors the same size, the only stuff they carry also has a ground wire, which I have no use for. But I'm wondering if service neutrals are allowed to be smaller than the ungrounded conductors. Since 220.61 says that "The maximum unbalanced load shall be the maximum net calculated load between the neutral and any one ungrounded conductor.", that alone seems to indicate that the neutral should probably be as big as the ungrounded conductors.

Or am I missing something?

 

[don_resqcapt19]

Jeff,

Since 220.61 says that "The maximum unbalanced load shall be the maximum net calculated load between the neutral and any one ungrounded conductor.", that alone seems to indicate that the neutral should probably be as big as the ungrounded conductors.

Only if there are no 240 volt loads. If there are 240 volt loads such as heat and AC, they do not contribute to the neutral load.
Don

 

[bphgravity]

In most cases, the load on the service grounded conductor is somewhat less than the load on the ungrounded conductors. This is due to line to line loads that don't utilize a neutral connection (water heater, air handler's, A/C units, ect). Plus other loads such as dryers and ranges have reduced neutral loads as compared to the corresponding ungrounded circuit.

The only concern I have is that the service grounded conductor also serves as the effective fault path during ground-faults. The reduction of the service grounded conductor increases the impedance of this circuit which will slow the clearing time of the fault. Plus the increased impedance on this circuit diverts more current through the grounding electrode system.

I think the best practice is to provide full sized or even oversized neutrals in the case of non-linear loads, but the code is the minimum allowance...

 

[jeff43222]

Jeff,

Only if there are no 240 volt loads. If there are 240 volt loads such as heat and AC, they do not contribute to the neutral load.
Don

I try to design for worst-case scenario. I sometimes do work in houses where there's a full panelboard and not a single 240V circuit. In those cases, the neutral would have to be sized according to the load on each side.

The fault impedance argument hadn't occurred to me, but it sounds like all the more reason to use a full-sized neutral. I realize that under many circumstances the neutral will be carrying much less current than either of the ungrounded conductors since they are phase-shifted.

Does anyone know if SER is available with three conductors that are all the same size? I'd kinda like it if I didn't have to pay for a ground wire I don't need.

 

[infinity]

Does anyone know if SER is available with three conductors that are all the same size? I'd kinda like it if I didn't have to pay for a ground wire I don't need.__________________
Jeff
Electrical Contractor (one-man shop)
Minnesota

SER cable does have three equal sized conductors and one smaller conductor wrapped around the three. SEU has two equal size conductors and one smaller conductor wrapped around them.

 

[LarryFine]

SER cable does have three equal sized conductors and one smaller conductor wrapped around the three.

I've only seen SER with a round stranded bare, not spiraled around the insulated conductors.

 

[jeff43222]

I've only seen SER with a round stranded bare, not spiraled around the insulated conductors.

Sounds like the stuff I installed yesterday: three 4/0 insulated conductors, and one bare one (looked like about 2/0), but not spiraled around the others.

 

[George Stolz]

The SER I install is reduced neutral, usually around 70' long. I don't believe that the reduction would increase the fault-clearing time appreciably for that distance, IMO. I suppose it would depend on each installation.

 

[Pierre C Belarge]

Bryan wrote:
"Plus the increased impedance on this circuit diverts more current through the grounding electrode system."

I think that is a misleading statement.


The GEC will carry a certain amount of current based on Ohm's Law, just as the grounded conductor will. The GEC will obviously carry considerably less than the grounded conductor. With the grounded conductor being smaller in size, I agree that the amount of TOTAL current flow will be less during a ground fault, which could affect the time value of the overcurrent device.



As far as the amount of current over the NEUTRAL conductor for a dwelling service, I do not see an issue with properly downsizing the conductor, as diversity of load and 2-pole circuits in most cases will cause less current to flow on the neutral conductor.

 

[jeff43222]

As far as the amount of current over the NEUTRAL conductor for a dwelling service, I do not see an issue with properly downsizing the conductor, as diversity of load and 2-pole circuits in most cases will cause less current to flow on the neutral conductor.

True, but what about the few cases where there are no 2-pole circuits and perhaps the load isn't so diverse? There could conceivably be close to 200A running through one bus and almost nothing on the other. That reduced neutral would be overloaded and unprotected. Granted, such a scenario would be unlikely to occur, but it could. Making all three conductors the same size would prevent the possibility.

I just find it odd that there apparently is no three-conductor SER (with no ground wire) where all three conductors are the same size. The stuff I used the other day had all three conductors the same size, but with a ground wire I had no use for.

 

[paul32]

Jeff,
What are you using the cable for? You say services and 3 conductor cable, so I assume a service entrance conductor, like from the meter to the service drop? I know they do that in the northeast, but I've never seen that around here. You can get aluminum single conductors and still use conduit.

But you also say SER. That is commonly used from an outside service disconnect to the panel, but the ground is used.

SEU is two insulated conductors with the bare strands individually wraped around the other two, and the cable is flat. SER is 3 insulated conductors with a ground (like 3 cond NM) with all 4 conductors twisted around each other. In some sense, the ground is 'around' the other conductors in some manner. Some of the above discussion is a little confusing. When I look at Werner, they have 2-2-2 SEU so you wouldn't undersize the neutral of a 100A service. The larger 4/0-4/0-2/0 is undersized, but I see all new 200A services around here using that size USE (underground).

 

[haskindm]

It would be very difficult to install all of the 120-volt load on one buss. That would meant that every other breaker in a panel would have 0 load while the other breakers were fully loaded. That would be a very dificult installation to pull off. I can't find it right now but I believe we are required to have our loads evenly distributed.

 

[infinity]

I've only seen SER with a round stranded bare, not spiraled around the insulated conductors.

I think that you're right Larry. Been a long time since I used SER. SEU has the bare spiraled conductor.

 

[George Stolz]

I can't find it right now but I believe we are required to have our loads evenly distributed.

Not unless you're wiring a 120V RV. (551.42(D))

 

[iwire]

210.11(B) sort of suggests it as well.

 

[iwire]

Jeff if the reduced neutral SE is all that is readily available in your area and services are not burring up on a regular basis that suggests to me that you should just forget it and move on.

I have never seen SE with a full sized neutral.

 

[George Stolz]

210.11(B) sort of suggests it as well.

I'd say kinda, sorta, but not really, IMO.

This load shall be evenly proportioned among multioutlet branch circuits within the panelboard(s).

I kinda get the impression that all these circuits could be on the same phase, so long as the circuits bore equal shares of the load, IMO.

Good reference, though, I tend to forget that one hiding out there. I cheated and searched for "balance" throughout the NEC.

 

[haskindm]

210.11(B) is the section that I was thinking of but could not find. Thanks. This is an example of what is meant in 90.1(B) where it states that code compliance does not necesarily mean that an installation is "efficient, convenient, or adequate...." I believe part of ensuring that your installation is installed in a "workmanlike manner" would be ensuring that the load is spread over the phases, not necesarily balanced, but at least an attempt was made to distribute the load among the available phases.