Ground and Neutral Wires

[inetdog]

No Single or Poly Phase Power Meter uses Neutral.

+1
By measuring current in all but one of the leads you know all of the currents, since they must sum to zero.
But the "true" measurement requires knowing all the line to line voltages in a system where the neutral is not used or all of the line to neutral voltages in a system where the neutral is used for some loads.
The commercial revenue grade meters instead assume that the neutral is midway between/among the lines and therefore they do not need any connection to the neutral at all.

[Sunking]

I have CT's on both legs of my SolarEdge inverter's output, and whatever current it draws at night is equal on both. As far as I can tell, there is no 120 v load that uses the neutral conductor, so I can't offer any insight into why it is required.

No Single or Poly Phase Power Meter uses Neutral.

[sensij]

I have CT's on both legs of my SolarEdge inverter's output, and whatever current it draws at night is equal on both. As far as I can tell, there is no 120 v load that uses the neutral conductor, so I can't offer any insight into why it is required.

[Sunking]

My guess is the neutral is for CPU monitoring. I once put in a 300KVA UPS system where the building supply was a 480v ungrounded delta. The UPS wanted 480Y with neutral. Upon investigation we found the UPS only used the neutral for metering and monitoring. So we used 250watt ground transformers to synthesize the neutral. Only drawback is the the UPS then had to feed a delta load but that was easy with the load side 480 to 208/120 distribution transformer.

THX. Done a lot of large scale UPS up to 2 Mva, all of them delta fed, cannot recall any needing a Neutral reference, all need Ground reference, but none requiring Neutral. A grounded circuit conductor would imply load current, and with current comes voltage drop and loss of reference.

I can see and understand a Ground or Zero Voltage reference, but a grounded circuit conductor provide no reference as it would vary with current. Only that makes makes any sense is for Control, but that requires the AC to be converted to DC which means either a transformer or SMPS. and it makes no difference if you use 240 or 120. Adding 120 just adds cost and complexity. I am sure there is a reason, but darn if I can put my finger on it. Nothing adds up.

[Andy D]

Not sure how that would be possible as there should be no upstream GFCI breakers. Only thing up stream should the Main Service Breaker. Even if it were a sub panel you would not feed it with a GFCI feeder as it would be operating constantly from false trips. Anyway I was just curious because there are no 120 volt loads I know of.

My guess is the neutral is for CPU monitoring. I once put in a 300KVA UPS system where the building supply was a 480v ungrounded delta. The UPS wanted 480Y with neutral. Upon investigation we found the UPS only used the neutral for metering and monitoring. So we used 250watt ground transformers to synthesize the neutral. Only drawback is the the UPS then had to feed a delta load but that was easy with the load side 480 to 208/120 distribution transformer.

[solar-man]

Thanks for the comments. I have reached out to some local installers for help. Once I get their feedback I will post it here.

[SunEagle]

That would be my guess.

There were some areas in the manual that were written in French and referenced Canada electrical requirements.

That included a CSA Certificate of Compliance.

[Sunking]

Not sure if the PE or "equipment grounding conductor" is a carry over from European equipment or not.

That would be my guess.

[SunEagle]

Huh? PE = Protective Earth which is not a term used in the USA or NEC. Only place you see that term is UK, and they don't use 120 volts so now I am even more confused. Oh well I really do not care and will step out of this.

The manual is specific to US installations and mentions the required NEC section 690 requirements. It also refers to DC AFCI compliance UL 1699B standard. Not sure if the PE or "equipment grounding conductor" is a carry over from European equipment or not.

[Sunking]

Seems strange but the installation manual for that inverter clearly shows both a neutral and ground wire coming from the house panel being terminated on distinct terminal blocks (PE for ground & N for neutral) along with L1 & L2.

Huh? PE = Protective Earth which is not a term used in the USA or NEC. Only place you see that term is UK, and they don't use 120 volts so now I am even more confused. Oh well I really do not care and will step out of this.

[SunEagle]

What neutral? Your inverters are 240 volts right? You have no need to carry a Neutral as you have no 120 volt supply/load connections.

Seems strange but the installation manual for that inverter clearly shows both a neutral and ground wire coming from the house panel being terminated on distinct terminal blocks (PE for ground & N for neutral) along with L1 & L2.

There is also a wiring diagram (Figure 16) on page 26 for a 240v/120v split phase AC grid connection showing the Neutral wire from the inverter connected to the center or ground terminal of the 240/120v transformer.

My guess would be for the OP to just run the neutral wire through the AC disconnect switch to the neutral bus in the main panel.

[inetdog]

Don't ask me why, but the field engineers for the transformerless inverters say that because the DC side is not grounded, the need for a GEC eliminated altogether.

The details of this depend strongly on which version of the NEC is in effect. The statement is pretty close to true for areas subject to the 2014 NEC. Note however that a GEC is still needed somewhere for grounding/bonding the frame and other exposed metal of the panel array.

[solarix]

Not familiar with Solectra inverters per se, but assuming it is a transformer type with grounded DC conductors, the inverter ground should have both an EGC (safety ground) as well as a GEC (lightning protection). The EGC is used to ground all metallic enclosures and conduit etc. to protect against ground faults (shorts to ground) and needs to not carry current (except during a fault). Thus you still need a neutral as the inverter does have some minor circuitry using it (if nothing else than to monitor the phase voltages). The neutral should wire from the inverter through any disconnects and meters (without connecting to them) to the breaker panel. Can be a reduced size from the hot wires though. The GEC is handled 2 ways in the code. Either can be outside the conduit or inside the conduit. Outside is preferable as the potentially high currents of a lightning hit are less induced into the active wiring, but being exposed, needs to be 6ga to meet the durability requirement. Inside is sometimes preferable on long runs and can double as the EGC, but because of the "choking effect" of a conductor in a metal conduit, code requires the conduit to be bonded on both ends. With a disconnect and meter in the AC path, this can be a real hassle. Don't ask me why, but the field engineers for the transformerless inverters say that because the DC side is not grounded, the need for a GEC eliminated altogether.

[Sunking]

Also because even a small, as power goes, current in the EGC could cause an upstream GFCI to trip.

Not sure how that would be possible as there should be no upstream GFCI breakers. Only thing up stream should the Main Service Breaker. Even if it were a sub panel you would not feed it with a GFCI feeder as it would be operating constantly from false trips. Anyway I was just curious because there are no 120 volt loads I know of.

[inetdog]

Interesting I stand corrected. At the moment cannot see any reason for Neutral, as Ground from the N-G bond point in the service disconnect would do the same thing. Sounds like an extra conductor with no purpose to me. I gotta be missing something.

It seems to me that there are at least two possible reasons for requiring the neutral.
1. To maintain the effective separation of function of neutral and EGC by not even using the EGC as a minimal current voltage reference. This is in line with not allowing a three wire dimmer to use EGC instead of neutral for its neutral reference. Also because even a small, as power goes, current in the EGC could cause an upstream GFCI to trip.
2. To make sure that the voltage balance determination is made correctly if there is a high resistance neutral and unbalanced loads upstream of the inverter output. L1-G/L2-G balance does not assure L1-N/L2-N balance.

PS: Some inspectors are currently insisting that the reference-only neutral be counted as a CCC, or even that that neutral cannot be sized smaller than the line conductors which are actually carrying the inverter output current.
I do not think that [2014] has made this particular point any clearer.