Appling 705.95(B) to a Parallel Circuit

moonlite

Member
I would like to get your help with the NEC section 705.95(B) Neutral Conductor for Instrumentation, Voltage Detection or Phase Detection. Since most 3-phase inverters do not run current through the neutral in the output circuit, this code makes good sense. However, in section 705.95(B) there is not the same exception as in section 215.2(A)(1), which states, “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.”

With that said, when sizing the neutral where 705.95(B) applies and the inverter 3-phase output circuit is made of parallel runs, can we apply sections 215.2(A)(1) and 310.10(H) to this circuit?

For example, let’s use the circuit between a 1200A inverter breaker and the utility-interactive inverter. Using Table 250.122, a 1200A overcurrent protection device (OCPD) requires an EGC size of a #3/0 AWG copper conductor. Since four parallel raceways are used, we need to divided the cross sectional area of the #3/0 AWG by 4 to determine the minimum cross sectional area for the four parallel grounded conductors, where #3/0 AWG = 167.8 kcmil.

167.8 kcmil /4 = 41.95 kcmil.

Since #1/0 AWG (105.6 kcmil) is the minimum size for a grounded conductor to be run in parallel per 310.10(H) and #1/0 AWG is greater than the required size for each of the four runs, is a #1/0 AWG grounded conductor code compliant in this example?

jaggedben

Senior Member
Well, if it's just a one inverter circuit then it's not a feeder, so it'd be up to your AHJ if they want to be liberal.

If you stick a second OCPD at the inverter, then you can call it a feeder. That might or might not be worth the money.

moonlite

Member
Lets modify the example to be 30 SMA Tripower 24kW inverters, where the outputs of all the inverters are collected in a 1200A panelboard. The 1200A panelboard has no loads, only the inverter branch connections and the aggregate output to the 1200A inverter breaker at the interconnection point at the main service.

jaggedben

Senior Member
Then in my opinion you are completely allowed to treat it like a feeder and not apply 250.122(F) to the neutral.

You would probably be well advised, before pulling wire, to double check with SMA that the neutral for the Tripowers should qualify for 705.95(B).

moonlite

Member
Out of the inverters available today, I believe the only 3-phase output type (208V or 480V) that will operate with current on the neutral is the Fronius. At least they did a couple years ago. I have not design with Fronius for a while. Inside this inverter, it is configured as three small L-N unit inverters. When the solar irradiance is low and increases or high and decreases, the master unit will turn on or shut down the other two units based on the available kWDC from the array and their operating hours. Therefore, when only one or two units are generating current, the neutral will have a significant amount of current running through it. Fronius = Full size neutral. All of the other 3-phase inverters only use the neutral, if required, for voltage sensing.

Carultch

Senior Member
Out of the inverters available today, I believe the only 3-phase output type (208V or 480V) that will operate with current on the neutral is the Fronius. At least they did a couple years ago. I have not design with Fronius for a while. Inside this inverter, it is configured as three small L-N unit inverters. When the solar irradiance is low and increases or high and decreases, the master unit will turn on or shut down the other two units based on the available kWDC from the array and their operating hours. Therefore, when only one or two units are generating current, the neutral will have a significant amount of current running through it. Fronius = Full size neutral. All of the other 3-phase inverters only use the neutral, if required, for voltage sensing.

I think that is specifically the Fronius 12 kW inverters that operate at 277/480V, which will function in the manner you described.

Their 3-phase counterparts within the Fronius IG-Plus product line that operate at 120/208V, have the three internal inverters connected to one another in a delta topology, where it will supply current phase-to-phase, instead of phase-to-neutral. They still work with their master unit shutting off inverting units at low amounts of power, but this doesn't cause neutral current due to the way the inverting units are not connected to put current onto the neutral.

ggunn

PE (Electrical), NABCEP certified
Then in my opinion you are completely allowed to treat it like a feeder and not apply 250.122(F) to the neutral.

You would probably be well advised, before pulling wire, to double check with SMA that the neutral for the Tripowers should qualify for 705.95(B).
I did that and SMA tech support confirmed that the neutral on a Tripower is for voltage reference only. But verify that, of course.