Sub-fed lugs, backfed power, and neutral sizing

Status
Not open for further replies.

drive1968

Senior Member
I was discussing a project with my inspector and we have a disagreement on the proper way to size a neutral. A home I am working on has a main panel with 150 amp breaker current protection from the poco. The main panel also has 30 amps of 240V being backfed from solar panels. So in theory, the main panel could have a total of 180 amps in combination from the poco and solar panels.

I will be using sub-feed lugs rated at up to 225 amps to feed the loadcenter sub-panel (bus rated up to 200 amps) in the garage. I was planning on using 2/0 copper for the hots and 1/0 copper for the neutral to supply the sub-panel. The inspector said 1/0 wasn't allowed for the neutral because my local area doesn't allow downsizing of the neutral, even when based on calculated load. My thinking is that because the solar is 240V with no neutral current, that 150 amps is the most neutral current that could ever occur on the neutral before the main breaker would trip. He said it is possible that 180 amps could be moving through the neutral and the 150 amp breaker couldn't protect against that.

I have no problem using 2/0 for the neutral, but I am trying to understand the correct theory. Any thoughts?
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
If you have an amendment in the area then you may have to use a full sized neutral, however if the calculated load allows then you may go as small as the GEC for the service and as small as the EGC for the feeder. Remember the calculated load must allow this. Art. 250.24 (C)(1) for the service.
 

mull982

Senior Member
if the calculated load allows then you may go as small as the GEC for the service and as small as the EGC for the feeder. Remember the calculated load must allow this. Art. 250.24 (C)(1) for the service.

Where does the code reference different neutral sizes for the service and feeder as you mentioned?

I'm assuming that the feeder is for the solar panel however would this even require a neutral?
 

drive1968

Senior Member
There is a local amendment that doesn't allow downsizing the neutral, so I can't use the calculated load to justify the smaller neutral. The feeder I am referring to is supplying the sub-panel loadcenter. The back-feed from the solar to the main panel doesn't require a neutral to carry any load, except to carry performance data from the inverter.

My thinking is about the sub-panel potentially receiving 180 amps (150 from the poco and 30 from the solar) from the sub-feed lugs on the main panel. Since 1/0 copper is only good for 175 amps, I chose to use 2/0 for the two hots supplying the sub-panel. I figured 1/0 was okay to use for the neutral because no neutral current is being carried to or from the solar system, so only up to 150 amps would be carried on the neutral because of the poco feed. It seems to me that 1/0 would be sufficient for the neutral, even though I can't use calculated load to justify the smaller wire.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
I agree but it seems moot if there is an amendment. I am not a solar guy but I am trying to imagine why the service conductor would need more than 150 amps. If the breaker panel is 150 amps will 180 amps will be fed thru that service. If it does carry 180 amps then I believe you would need to upsize your service to 200 amps. Teach me something...plz.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
Where does the code reference different neutral sizes for the service and feeder as you mentioned?

Art. 220.61(A) tells us the neutral load for service & feeders can be calculated as such
220.61 Feeder or Service Neutral Load.
(A) Basic Calculation. The feeder or service neutral load shall be the maximum unbalance of the load determined by this article. The maximum unbalanced load shall be the maximum net calculated load between the neutral conductor and any one ungrounded conductor.

art. 250.24(C)(1) states the service neutral info. This basically gives us a minimum for the service.

I have not found the feeder article that states the grounded conductor must not be smaller then the egc but logical tells me if there is a fault to neutral the neutral would have to be the minimum required to carry the fault. T. 250.122 for the egc is that minimum.

Suppose I had a 60 amp sub panel that had a cal. neutral load os 5 amps. If I used a #14 gauge neutral and there was a fault the OCPD may not open. Thus a #10 neutral would still be required for the 60 amp sub panel.
 

iwire

Moderator
Staff member
Location
Massachusetts
I agree but it seems moot if there is an amendment. I am not a solar guy but I am trying to imagine why the service conductor would need more than 150 amps. If the breaker panel is 150 amps will 180 amps will be fed thru that service. If it does carry 180 amps then I believe you would need to upsize your service to 200 amps. Teach me something...plz.

Dennis, not 180 amps through the 'service' but a possible 180 amps on the panel bus.


150 possible from the utility.

30 amps possible from the solar.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
Dennis, not 180 amps through the 'service' but a possible 180 amps on the panel bus.


150 possible from the utility.

30 amps possible from the solar.
Well wouldn't you need a 200 amp rated panel? Is the rating of the buss the same for a 200 amp as it is for a 150 amp panel?
 

drive1968

Senior Member
Yes, both the main panel and the sub-panel have buses rated at 200 amps. The main panel is protected by the 150 amp main breaker and is fed from the poco drop with #1 copper. The sub-panel wiring will be 2/0 copper hots with either a 1/0 or 2/0 neutral. The sub-panel is fed by the sub-fed lugs from the main, so it is not protected other than by the 150 amp main breaker and the 30 amp backfed-solar breaker.
 

iwire

Moderator
Staff member
Location
Massachusetts
Well wouldn't you need a 200 amp rated panel?

Well that makes senses and as the OP said he has 200 amp bus.

However the NEC does allow a bit of wiggle room if the PV source is not to big.

695.64(B)(2) Bus or Conductor Rating. The sum of the ampere ratings of overcurrent devices in circuits supplying power to a busbar or conductor shall not exceed 120 percent of the rating of the busbar or conductor. In systems with panelboards connected in series, the rating of the first overcurrent device directly connected to the output of a utility interactive inverter(s) shall be used in the calculations for all busbars and conductors.

At one job I had to replace the 100 amp panelboard with a 200 amp panelboard but left the 100 amp OCPD in place so I could tie in one 60 amp PV supply.
 

drive1968

Senior Member
I'm glad you posted the wording of 695.64(B)(2). I remembered that the bus could go over by 20%, but I forgot that conductors were included also. It seems 1/0 should be fine. I'll talk to the inspector about it. Thanks.
 

dbuckley

Senior Member
A home I am working on has a main panel with 150 amp breaker current protection from the poco. The main panel also has 30 amps of 240V being backfed from solar panels. So in theory, the main panel could have a total of 180 amps in combination from the poco and solar panels.

I don't agree that math; this calculation would only hold true if the source impedance (and other stuff) of the two supplies were precisely matched to share the current from the sources in the proportion of of 5:1 utility to solar.
 

mull982

Senior Member
Art. 220.61(A) tells us the neutral load for service & feeders can be calculated as such


art. 250.24(C)(1) states the service neutral info. This basically gives us a minimum for the service.

I have not found the feeder article that states the grounded conductor must not be smaller then the egc but logical tells me if there is a fault to neutral the neutral would have to be the minimum required to carry the fault. T. 250.122 for the egc is that minimum.

Suppose I had a 60 amp sub panel that had a cal. neutral load os 5 amps. If I used a #14 gauge neutral and there was a fault the OCPD may not open. Thus a #10 neutral would still be required for the 60 amp sub panel.

So basically the guidelines or steps when sizing a neutral is to first follow 220.61 to determine maximum unbalance and then compare this to 250.122 to make sure it is greater then minimums established there.

When determining maximum unbalance do you just use one of the maximum L-N currents for one phase or can you actually balance the loads in the panel come up with a calculated unbalance number and use this number to size the neutral? I would think that you can calculate max unbalance between phases and use that number otherwise using the max L-N current for a phase would result in a neutal the size of the panel feeders everytime?

Is my though process correct?
 

iwire

Moderator
Staff member
Location
Massachusetts
I don't agree that math; this calculation would only hold true if the source impedance (and other stuff) of the two supplies were precisely matched to share the current from the sources in the proportion of of 5:1 utility to solar.

Can you say it again in simpleton because I am not getting it.:-?
 

mivey

Senior Member
Can you say it again in simpleton because I am not getting it.:-?
Maybe he is saying that just because the solar can pick up 30 amps of load does not mean it will, and if it does it will "rob" the poco of some of the load it was carrying.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
My thoughts: If the inverter is only supplying 240v, the utility transformer is being used to derive the neutral for 120v loads. (Technically, the inverter is being used to 'slow down' the meter.)

It seems that the utility neutral could have all neutral currents imposed on it, but as we know, line-to-line loads do not add to that. I don't see the neutral ever having to see over 150a.
 
Status
Not open for further replies.
Top