Transformer "secondary" overcurrent protection

[SunFish]

I'm working on a solar project with a single 480/277 VAC inverter. The service is 120/208 so we're stepping down the inverter output from 480/277 to 120/208. We're planning to do a supply side connection so I have a fused disconnect as my OCPD on my 120/208 "primary" side. Note I'm still considering the utility side of the transformer as the primary side even though power is flowing from the inverter to the utility instead of utility to load.

The question is, am I required to have overcurrent protection on the "secondary" side, between the inverter and the transformer? All components are installed outdoors.

 

[electrofelon]

Yes, you need an OCPD to protect the transformer secondary conductors (240.21(C)) and sometimes the transformer itself (although in your case you are compliant with 450.3(B) because the primary protection is not over 125%.)

There is that 705 rule requiring you to consider the required 450.3 transformer protection going "both ways" but the exception makes that moot.

 

[sokbok]

I think this needs to be explored more.
I dont think an OCPD is needed on the inverter side of the transformer in this instance.

While 240.21(C) does state that secondary conductors need OCPD, it uses the traditional terminology of "load" to describe the transformers usage.

Both 690.9(D) and 705.30(C) more accurately reflect the situation here then 240.21(C). These aforementioned sections recognize the current limited nature of the inverter and state that OCPD is not necessary and while vague there is a safe assumption that the secondary conductors are included in this.

The 62.5 Core 1 inverter also does have a full load rated AC Disconnect built into it.

The hardware is all being installed outside which seems to also confirm that an OCPD is not needed: 240.21(4) (C)

The real question is if a Neutral conductor is needed from the Core 1, which will depend on the type of transformer being used.

Electrofelon I always appreciate your insights, but I dont think an OCPD is needed on the secondary.

 

[sokbok]

sorry that inverter is a Tripower not Core 1, but same situation

 

[electrofelon]

Doesn't (C)(4) still require an OCPD at the termination of the secondary conductors?

 

[Carultch]

The 62.5 Core 1 inverter also does have a full load rated AC Disconnect built into it.

The hardware is all being installed outside which seems to also confirm that an OCPD is not needed: 240.21(4) (C)

The real question is if a Neutral conductor is needed from the Core 1, which will depend on the type of transformer being used.

What do you mean by "hardware is all being installed outside"? And how would that make a difference for 240.21(C)? The outside-unlimited secondary rule still requires a secondary OCPD. It just doesn't limit the length.

The CORE1 inverters have a disconnect built in on the AC side, but not an integrated AC OCPD.

This particular inverter requires a Y-grid with the neutral grounded where the grid is derived. You would need to have a 277/480V WYE on the transformer's inverter side. The grid side can be Delta or WYE.

This particular inverter has two options for what to do about the neutral. Either you can connect an ordinary neutral to it, or you can use the factory-supplied jumper to use the EGC for the instrumentation purposes instead. I.e. voltage metering to verify that the grid is within spec. Unfortunately, their documentation doesn't help you understand how to know whether you get to take credit for this option.

 

[Carultch]

View attachment 2553721

I notice it uses language that implies a singular inverter, as opposed to a group of several inverters paralleled. I would interpret that omitting an OCPD on the inverter side of the transformer, would only apply when there is one inverter alone. Lacking other information making an exception, a panelboard paralleling several inverters would therefore require a main. Unless it is a topology of transformer that allows indirect protection of the secondary from the primary OCPD.

 

[sokbok]

I notice it uses language that implies a singular inverter, as opposed to a group of several inverters paralleled. I would interpret that omitting an OCPD on the inverter side of the transformer, would only apply when there is one inverter alone. Lacking other information making an exception, a panelboard paralleling several inverters would therefore require a main. Unless it is a topology of transformer that allows indirect protection of the secondary from the primary OCPD.

Yes. Makes sense to me. I think only 1 inverter can have OCPD omitted between transformer even though 240.21 (C) adds some confusion.

I dont understand 240.21(C)(4)(1-4). The way I read it is that if all 4 of the conditions are met for the outdoor and protected transformer secondary conductors, then OCPD is only needed at one location. Thoughts?
Kinda confusing stuff.