Sizing neutral for subpanel.

[dewhite04]

I'm installing a subpanel directly adjacent to the main, which will have 3 connected loads:

1. Electric Vehicle Service Equipment with a max load of 48A @ 240v. Because this is an EVSE, continuous use makes this a 60A load.

2. HVAC condenser with a nameplate MCA of 21.1A @ 240v.

3. A dedicated 15A/120v receptacle for occasional use charging the batteries in a small RV.

So, my calculated load for the subpanel is 96.1A. I plan to feed the main-lugs in the subpanel with (2) #3 THHN conductors landed to a 100A OCPD in the main panel. I would use (1) #8 THHN for the EGC.

My question is about sizing the neutral in this 240/120 split-phase system. If I understand correctly, the neutral needs only to be sized for the unbalanced load(s). The EVSE should always present a balanced load (a neutral won't even be connected to that piece of equipment). Same thing for the HVAC condenser, right? So, the only unbalanced load would be the 15A dedicated receptacle...

This would make a #14 adequate for the neutral? Is there a part of the code that further advises on or restricts this? I don't think I would put a #14 in there, regardless of what the minimum adequate size is. Probably, I would be inclined to put in at least a #8 to allow for additional unbalanced loads in a future reconfiguration of the panel.

Why not bring in a third #3, you ask? Only because this is a relatively small subpanel (GE TL412R1P) and I know from using the same model elsewhere that things get tight in there quick.

Thoughts?

 

[sberry]

You cant use a 14.

 

[wyliesdiesels]

Theres specific rules for reduced neutrals

I wouldnt bother.

Who’s to say that down the road you may have more loads and the reduced neutral is now too small.

 

[sberry]

You could go a size.

 

[alfredeneuman]

2017 NEC 90.8 Wiring Planning.
(A) Future Expansion and Convenience. Plans and specifications
that provide ample space in raceways, spare raceways, and
additional spaces allow for future increases in electric power
and communications circuits. Distribution centers located in
readily accessible locations provide convenience and safety of
operation

 

[mm08822]

I would put in a bigger sub-panel - such as TLM1212RCUP. If you are already out of space, why load up a new panel right away. For $20-30 more, you can get larger sub-panel and make it worth the effort.

Yes, you can reduce the neutral with calculations, assuming the inspector accepts your numbers. I personally wouldn't go less than 70%. If the loads change in the future - your covered. #14 is crazy!

 

[klassenl]

In my area the general rule is after the main you can't reduce the neutral. It may be different in your area.

 

[strutaeng]

Yeah, why not just use a larger subpanel? I installed this one in my garage, and had plenty of space:

https://www.homedepot.com/p/GE-Powe...g-Circuit-Breaker-Panel-TLM1212CCUP/202046233

I used #2 Al with #6 ground. Plenty of space to work the #2 conductors comfortably.

 

[Jim greengo]

So just size everything for a 100a sub panel.

 

[dewhite04]

I would put in a bigger sub-panel - such as TLM1212RCUP. If you are already out of space, why load up a new panel right away. For $20-30 more, you can get larger sub-panel and make it worth the effort.

Yes, you can reduce the neutral with calculations, assuming the inspector accepts your numbers. I personally wouldn't go less than 70%. If the loads change in the future - your covered. #14 is crazy!

I do appreciate that suggestion. It would have been my preference to use a larger subpanel, but the area available on the wall is limited. Rather than continuing to poke at reducing the size of the neutral, I have decided to keep the 15A RV circuit in the main panel and move an additional smaller HVAC condenser circuit with a nameplate MCA of 10.94A @ 240v to this sub. That will make for a total calculated load of 92.04A with only balanced loads requiring no neutral at all. Just (2) #3s and (1) #8.

I'm not much worried about future expansion space because the next stop on this train is an upgrade to the meter, weather-head, service drop, and main panel (something I would certainly hire-out). Hopefully this structure will reach the end of its useful life before that time comes and the entire house (converted for light office use) will be demo'd to make way for a future commercial building in the next few years.

 

[dewhite04]

You cant use a 14.

I don't disagree that it would be a terrible idea. What I was asking for was a code citation so I could better understand what is allowable.

If the only unbalanced load is a dedicated 15A/120v receptacle, why couldn't the neutral feeder be a #14?

 

[dewhite04]

2017 NEC 90.8 Wiring Planning.
(A) Future Expansion and Convenience. Plans and specifications
that provide ample space in raceways, spare raceways, and
additional spaces allow for future increases in electric power
and communications circuits. Distribution centers located in
readily accessible locations provide convenience and safety of
operation

2017 NEC 90.5 Mandatory Rules, Permissive Rules, and Explanatory Material.
(A) Mandatory Rules. Mandatory rules of this Code are those that identify actions that are specifically required or prohibited and are characterized by the use of the terms shall or shall not.

 

[Bert_]

NEC requires it to be sized for unbalanced load, which in your case is 15A. But the neutral cannot be smaller than the equipment ground. So for your 100A feeder #8 copper is as small as you could go.

Given that your panel is pretty well loaded future circuits will probably be minimal. #8 thhn is good for 50A anyway, that will handle quite a several 120V circuits.

I reduce neutrals a size or two all the time for feeders over 60A. No reason to be afraid of it.

 

[mm08822]

NEC requires it to be sized for unbalanced load, which in your case is 15A. But the neutral cannot be smaller than the equipment ground. So for your 100A feeder #8 copper is as small as you could go.

Given that your panel is pretty well loaded future circuits will probably be minimal. #8 thhn is good for 50A anyway, that will handle quite a several 120V circuits.

I reduce neutrals a size or two all the time for feeders over 60A. No reason to be afraid of it.

You’re right, I found it in 215.2(A)(2) earlier today and asked myself what the hell did I read yesterday about calculating neutral loads for sizing. Finally found it in 220.61(A). Worst part is 220.61(A) never mentions the limitations of 215.2(A)(2).

Just to make it better, 215.2(A)(2) refers you to 250.122 which leads you to the table for equipment grounding conductor sizing. Somewhere there needs to be a sentence stating the dual purpose of that table.

 

[sberry]

It seems I remember number 8 in there some where for some reason. It shouldn't be smaller than short circuit for 100A.

 

[dewhite04]

NEC requires it to be sized for unbalanced load, which in your case is 15A. But the neutral cannot be smaller than the equipment ground. So for your 100A feeder #8 copper is as small as you could go.

Given that your panel is pretty well loaded future circuits will probably be minimal. #8 thhn is good for 50A anyway, that will handle quite a several 120V circuits.

I reduce neutrals a size or two all the time for feeders over 60A. No reason to be afraid of it.

Thanks for your reply! That's the basic information I was seeking, although I've now simplified the plan by putting only balanced loads in this subpanel, with no neutral being required.

It seems I remember number 8 in there some where for some reason. It shouldn't be smaller than short circuit for 100A.

I guess just a lucky guess on my part that #8 would have been a good size to go with.

You’re right, I found it in 215.2(A)(2) earlier today and asked myself what the hell did I read yesterday about calculating neutral loads for sizing. Finally found it in 220.61(A). Worst part is 220.61(A) never mentions the limitations of 215.2(A)(2).

Just to make it better, 215.2(A)(2) refers you to 250.122 which leads you to the table for equipment grounding conductor sizing. Somewhere there needs to be a sentence stating the dual purpose of that table.

This is the exact information I was looking for. I'll know for the future to size neutral feeders to the unbalanced loads, but in no case smaller than the EGC required in table 250.122. After looking at the code sections you cited, I share your concern that 220.61(A) should at least contain a note pointing to 215.2(A)(2). It also seems like table 250.122 should have its name changed or a note added to reflect its alternative use for determining the minimum size for grounded conductors.

For any future passersby, the relevant sections read:

215.2(A)(2) Grounded Conductor. The size of the feeder circuit grounded conductor shall not be smaller than that required by 250.122 except that 250.122(F) shall not apply where grounded conductors are run in parallel.

220.61(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.

It seems that reading 220.61(A) without knowing about 215.2(A)(2) could lead someone to think that #14 would be OK for the neutral feeder in my original example...