The Code reason behind the "return" to full-size Neutral?

Paco Solar

Member
Location
PA
Occupation
Solar Engineer
Hello Folks,
Scenario: Supply-side PV interconnection ("IX") inside a Utility CT cabinet (load-side of CT's....permitted by this Utility).
Focus of question: The conductor segment from the fused PV Disconnect to the above IX point (about 5 LF)

For my typical designs, I will downsize the Neutral in the previous (upstream) wire segments (i.e. from inverters to AC-disco), as permitted by Code and by Inverter Mftr (Enphase).
My practice has been, for this last segment, to terminate the EGC in the AC-Disco (where it is bonded to the N), and only continue the (fully sized) Neutral (and phase conductors) on to the supply side IX point. (often in the Service Gear/MCB...)

My understanding is we make the Neutral in this segment Full-size, since the Code allows it to be reduced, but not less than the size of the EGC, and since there is no EGC in this segment, there is no "minimum" N size, so it can't be reduced....but that seems like an argument from silence.

Can anyone point to Code sections that would more formally require the above return to "full-size" for this portion of the Circuit, or is it just good practice?

thank you
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
While the NEC doesn't require that a PV supply side interconnection be treated just like an additional service, it is moving in that direction (e.g. 2020 NEC 250.25), and nothing requires deviation from that treatment to my knowledge.

Then 230.42(C) would govern the minimum size of the grounded conductor, and it references 250.24(C), which in turn references 250.102(C)(1).

So that table gives you your minimum size.

Cheers, Wayne
 
While the NEC doesn't require that a PV supply side interconnection be treated just like an additional service, it is moving in that direction (e.g. 2020 NEC 250.25), and nothing requires deviation from that treatment to my knowledge.

Then 230.42(C) would govern the minimum size of the grounded conductor, and it references 250.24(C), which in turn references 250.102(C)(1).

So that table gives you your minimum size.

Cheers, Wayne
What is your opinion on the applicability of 705.95(B)? It says "connected to a single phase or three phase interactive inverter". Does the conductor need to be "directly" connected to the inverter for this to apply?
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
What is your opinion on the applicability of 705.95(B)? It says "connected to a single phase or three phase interactive inverter". Does the conductor need to be "directly" connected to the inverter for this to apply?
I don't follow your question, 705.95(B) refers to "shall be sized equal to or larger than the equipment grounding conductor." But the OP's question is about upstream (utility-side) of the service disconnect, where there's no EGC, so that wouldn't apply.

Cheers, Wayne
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
What is your opinion on the applicability of 705.95(B)? It says "connected to a single phase or three phase interactive inverter". Does the conductor need to be "directly" connected to the inverter for this to apply?

250.102(C) is applicable between the supply side 'tap' point and the disconnecting means. It will generally override 705.95(B). Note that this section of conductor isn't 'solely used' for the purposes mentioned in 705.95(B) because it is also used as the ground fault current path. Which is an important practical difference in this case. Also I think whenever you have two applicable code sections in the NEC that each contain a minimum size requirement, the larger required size prevails.
 
I don't follow your question, 705.95(B) refers to "shall be sized equal to or larger than the equipment grounding conductor." But the OP's question is about upstream (utility-side) of the service disconnect, where there's no EGC, so that wouldn't apply.

Cheers, Wayne
Well my question is what conductors does or can 705.95 apply to. It isn't very clear. I guess, like you say, because there is no EGC at this point being discussed, we just assume this cannot apply......? Okay yeah so I certainly agree 250.102c makes sense for the conductors being discussed, but seems like 705.95 should have different wording to send you to 250.102 as applicable.
 

Paco Solar

Member
Location
PA
Occupation
Solar Engineer
thank you Gents. This is quite helpful. My takeaway is that while the Neutral in the wire segment discussed can not be reduced by virtue of 705.95.B, it does not need to be sized to match the ungrounded conductors but can be sized per 250.102.C, which is significantly smaller.

Some inverter companies have told me that they don't allow the N to be reduced, but want it "full size," which I've always taken to mean the same size as the phase conductors. Perhaps those N wires that directly (physically) connect to the inverter are what the Inverter folks want "full-sized" rather than reduced by 705.95. BUT, by the time we get several inverters through an AC combiner, and the PV Service Disco, I'm wondering if that Neutral can be treated differently (i.e. allowed to be reduced per 250.102.C) and not be an issue with the Inverter folks. I guess I should ask them.
 
thank you Gents. This is quite helpful. My takeaway is that while the Neutral in the wire segment discussed can not be reduced by virtue of 705.95.B, it does not need to be sized to match the ungrounded conductors but can be sized per 250.102.C, which is significantly smaller.
IF I may be picky. I would say theoretically the process goes like this:

1) Calculate the size for the neutral per 220.61
2) Find that the value above is less than the minimum conductor size allowed per 250.24(C), so go with the minimum allowed.
Some inverter companies have told me that they don't allow the N to be reduced, but want it "full size," which I've always taken to mean the same size as the phase conductors. Perhaps those N wires that directly (physically) connect to the inverter are what the Inverter folks want "full-sized" rather than reduced by 705.95. BUT, by the time we get several inverters through an AC combiner, and the PV Service Disco, I'm wondering if that Neutral can be treated differently (i.e. allowed to be reduced per 250.102.C) and not be an issue with the Inverter folks. I guess I should ask them.
That drives we crazy, the waste. Ive worked on a lot of multi megawatt projects with full neutrals everywhere :mad: :mad: :mad: :mad:
This is somewhat old but may have some useful information:
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Yeah, it's not like the inverter engineers don't know the max possible current on the neutral and couldn't put it on the data sheet for us.
Inverters are inherently balanced, so there should not be any current on the neutral. Many inverters do not require a neutral at all.
 
Inverters are inherently balanced, so there should not be any current on the neutral. Many inverters do not require a neutral at all.
I have been out of the loop and not really looked into this since the thread from years ago I linked to..... But I am guessing there are still some manufacturers who either require neutral and or are vague about how to size it. Why? Why require a neutral at all if it doesn't seem to be needed?
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
I have been out of the loop and not really looked into this since the thread from years ago I linked to..... But I am guessing there are still some manufacturers who either require neutral and or are vague about how to size it. Why? Why require a neutral at all if it doesn't seem to be needed?
Most of the three phase commercial inverters I have used in the past few years do not require a neutral; many of them ship with the neutral connector strapped to ground If you choose to run a neutral you have to remove the strap. Our resi guys are mostly using 240V micros that do not have a neutral terminal. We run a minimum sized neutral from the service to the AC combiner for monitoring equipment, but it's overkill.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
Inverters are inherently balanced, so there should not be any current on the neutral.

I'm not sure that's entirely true in all cases. It's odd that the manufacturers won't state it clearly. My understanding is that some 480/277 inverters invert between L and N in three stages (one per phase), in which case the balancing is a matter of engineering against possible failure modes with imbalance, rather than 'inherent'.
 

Paco Solar

Member
Location
PA
Occupation
Solar Engineer
Thank you everyone! So, is it correct to say that when we talk about a "full-size" Neutral, as defined by the NEC, we mean a Neutral that is sized per 250.24 (C), not a Neutral that is the same size as the ungrounded conductors. (Maybe that term "full-size" is just too vague, and not a Code-defined word?).
 

Carultch

Senior Member
Location
Massachusetts
Thank you everyone! So, is it correct to say that when we talk about a "full-size" Neutral, as defined by the NEC, we mean a Neutral that is sized per 250.24 (C), not a Neutral that is the same size as the ungrounded conductors. (Maybe that term "full-size" is just too vague, and not a Code-defined word?).
To me, "full size neutral" means a neutral at least the same size as the phase conductors. It isn't an NEC-defined term. The reason you'd make it full size, is that there is a realistic chance that the neutral carries the full current of one of the phases under a normal circumstance. Either ordinary 60Hz current, triplen harmonic current, or both.

Some inverters actively use the neutral to carry the full current, and would therefore require a full size neutral. The go-to example I can give, is the Fronius IG Plus 12 kW inverters, when running at partial load conditions, in a setting called mix-mode. You would see full current on either 1 or 2 phases at a time, the same full current on the neutral, and no current on the remaining phase(s). I'm not aware of any inverters that are made in the current year that do this, but in theory, they could exist.

It is most common today, that the 3-phase inverters only use the neutral as a voltage reference, and therefore it carries insignificant current. This is when you get to reduce it to the size of the EGC (9 times out of 10), or whichever other name for the green wire applies (e.g. SSBJ). That is the reason for the update in 2020: to cover the cases when it isn't an EGC by strict definition, but is a conductor governed by 250.102C instead of 250.122.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
It is most common today, that the 3-phase inverters only use the neutral as a voltage reference, and therefore it carries insignificant current. This is when you get to reduce it to the size of the EGC (9 times out of 10), or whichever other name for the green wire applies (e.g. SSBJ). That is the reason for the update in 2020: to cover the cases when it isn't an EGC by strict definition, but is a conductor governed by 250.102C instead of 250.122.
In the cases of SMA and SolarEdge 3 phase inverters, which are the ones I use pretty much all the time, the neutral conductor is optional. If there is no neutral conductor to the inverter, the neutral lug is strapped to the ground lug in the inverter which is bonded to the EGC.
 

pv_n00b

Senior Member
Location
CA, USA
There are 3-phase inverter topologies that put 3 inverters together in a WYE and some of them will not have even loading. Some may start only one inverter phase when there is low PV input to maximize efficiency. This used to be used in some Fronius 3-phase inverters. This is why it is assumed that the inverter requires a full-sized neutral unless the manufacturer specifically states that it does not. Many people seem to be assuming that no full-sized neutral is required if the manufacturer does not say it is needed, and that's backward.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
There are 3-phase inverter topologies that put 3 inverters together in a WYE and some of them will not have even loading. Some may start only one inverter phase when there is low PV input to maximize efficiency. This used to be used in some Fronius 3-phase inverters. This is why it is assumed that the inverter requires a full-sized neutral unless the manufacturer specifically states that it does not. Many people seem to be assuming that no full-sized neutral is required if the manufacturer does not say it is needed, and that's backward.
Yeah, I remember seeing some Fronius inverters that were built that way. I don't remember any manufacturers explicitly saying that a minimum sized neutral can be used, though; as I remember they just either say they need a neutral or not.
 
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