"neutral conductor" sizing of the inverter

[INGMRS]

Hi,


I am connecting a photovoltaic system to a 3 phase load, and i am not clear with the criteria of sizing the neutral conductor of the inverter, the manufacturer manual does not say anything.


As I undestood, if the voltages diferences between phases is less than 5%, there is not a problem to connect the neutral to the system, but i am not clear the size of the conuctor, i guess it should not be less than the grounding equipment conductor, but not how bigger it should be.


I will appreciate your help with this doubt.


Thanks,

 

[jaggedben]

The inverter manual will tell you if a neutral is required for your application. I believe that it's section 705.95 in the code that allows the neutral to be sized the same as the EGC if it's only used for measuring voltage and such. You may need a letter from the manufacturer to certify if it's that kind of neutral.

 

[INGMRS]

The inverter manual will tell you if a neutral is required for your application. I believe that it's section 705.95 in the code that allows the neutral to be sized the same as the EGC if it's only used for measuring voltage and such. You may need a letter from the manufacturer to certify if it's that kind of neutral.

Hi jaggedben,


Thanks I am going to check, but the manual does not say nothing about the size of the conductor, for the grounding conductor yes, but not for the neutral.


Regards,

 

[electrofelon]

This post may be of interest. I think it boils down to what the manufacturer states about how the neutral is used. Perhaps you can contact them if the manual is silent?

http://forums.mikeholt.com/showthread.php?t=188721

 

[pv_n00b]

You can't go wrong with a full-sized neutral.

If you can tell from the manufacturer's documentation that the neutral is just used for measurement then you can downsize it to the same size as the EGC. Not all manufacturers will put this in their manual and you can't go by what some tech support person says on the phone, it has to be documented so you can show the AHJ if asked. If the AHJ is not convinced it can be expensive to have to rerun a full-sized neutral.

 

[INGMRS]

You can't go wrong with a full-sized neutral.

If you can tell from the manufacturer's documentation that the neutral is just used for measurement then you can downsize it to the same size as the EGC. Not all manufacturers will put this in their manual and you can't go by what some tech support person says on the phone, it has to be documented so you can show the AHJ if asked. If the AHJ is not convinced it can be expensive to have to rerun a full-sized neutral.

Hi,

The manual says about the neutral:

"The inverter must be connected to a three-phase system with the center of the star connected to ground. To connect the inverter to the grid is possible to choose between the four-wire connection (3 phases + neutral) and the three-wire connection (3 phases)."

But, I have a clue, they have indicate the same size of the glands of entrace of cables for the phases, maybe they are assuming that you are going to size the neutral with the same size of the phases.
- Single-core configuration have 4xM40 cable glands for the “R”, “S”, “T” phases and for the “N” neutral cable and a M25 cable gland for the grounding cable.
- Multi-core configuration (optional) have a M63 cable gland for the “R”, “S”, “T” phases and for the “N” neutral cable and a M25 cable gland for the grounding cable.



i know, sounds stupid, it would be better if I await for the response from the manufacturer.





Thanks to all, and regards.

 

[ggunn]

Hi,

The manual says about the neutral:

"The inverter must be connected to a three-phase system with the center of the star connected to ground. To connect the inverter to the grid is possible to choose between the four-wire connection (3 phases + neutral) and the three-wire connection (3 phases)."

But, I have a clue, they have indicate the same size of the glands of entrace of cables for the phases, maybe they are assuming that you are going to size the neutral with the same size of the phases.
- Single-core configuration have 4xM40 cable glands for the “R”, “S”, “T” phases and for the “N” neutral cable and a M25 cable gland for the grounding cable.
- Multi-core configuration (optional) have a M63 cable gland for the “R”, “S”, “T” phases and for the “N” neutral cable and a M25 cable gland for the grounding cable.



i know, sounds stupid, it would be better if I await for the response from the manufacturer.





Thanks to all, and regards.

You have your answer. You can choose not to use a neutral. The three phases must be referenced to neutral/ground but you don't have to connect it.

 

[electrofelon]

You have your answer. You can choose not to use a neutral. The three phases must be referenced to neutral/ground but you don't have to connect it.

I agree, but so bizarre....why do they even give an option? Why would someone spend time and money to run a neuter of its not required :?

 

[ggunn]

I agree, but so bizarre....why do they even give an option? Why would someone spend time and money to run a neuter of its not required :?

Nevertheless, it's a pretty common option in larger three phase inverters.

 

[electrofelon]

Nevertheless, it's a pretty common option in larger three phase inverters.

by larger do you mean central inverters?

A few years ago, my friend told me about a situation with a solectria central inverter. He had already run wire I guess, and didnt run a neutral. Solectria told him how to set it up to run without a neutral, but implied it was "better" to use a neutral and to pull one next time. No one really seems to know definitively what the deal is with this neutral thing.

 

[jaggedben]

Isn't this that UL 1741 thing, where the inverter is supposed to monitor the neutral if there is one? IOW, you can hook it up to a delta system with no neutral, but if you hook it up to a wye you're supposed to connect a neutral.

 

[ggunn]

Isn't this that UL 1741 thing, where the inverter is supposed to monitor the neutral if there is one? IOW, you can hook it up to a delta system with no neutral, but if you hook it up to a wye you're supposed to connect a neutral.

I've seen nothing about that in the installation manuals for the SMA Tripower inverters that can go either way. The manual just says the inverter can be configured to run with or without a neutral.

 

[pv_n00b]

Isn't this that UL 1741 thing, where the inverter is supposed to monitor the neutral if there is one? IOW, you can hook it up to a delta system with no neutral, but if you hook it up to a wye you're supposed to connect a neutral.

You are correct. From what I have seen over the years inverter manufacturers sometimes get around the neutral by requiring the inverter to be interconnected only to a grounded 4 wire service. Then they use the EGC as the reference for the line to neutral voltage measurements. This only works if the neutral is not needed as a current carrying conductor. It makes some sense since in the PV industry it is standard to use an EGC conductor and not the conduit as the EGC. So the reduced size neutral and the EGC are the same size going from the inverter to the POCC and basically just creating a redundant parallel conductor.