Secondary OCPD With String Inverters

[wwhitney]

There is no OCPD required between the panels and the transformer as long as the total maximum output current of the inverters does not exceed the current rating of the transformer per 705.30(C).

The section on Power Transformers in 705.30 has been changing every year, both in content and section letter. 705.30(C) indicates you mean the 2020 NEC, and it says in part "Secondary protection shall not be required for a transformer secondary that has a current rating not less than the sum of the rated continuous output currents of the power sources connected to that secondary."

But does that the secondary conductors or the secondary coil of the transformer itself? The 2023 NEC version makes it clear it meant the secondary coil itself. 705.30(F)(2) says "Transformer secondary conductors shall be protected in accordance with 240.21(C)."

So that's what the thread is about, how to comply with 240.21(C) when you have multiple non-parallel sets of secondary conductors, each sized to one or more of the inverters.

Cheers, Wayne

 

[wwhitney]

A disco on the primary side of the transformer will satisfy the six handle rule.

I'm thinking that the controlling issue is 225.30, and that there aren't any transformer specific requirements about number of disconnects.

So in the picture in the OP, the question is whether that transformer on its pad is a separate structure from the rack holding all the panelboards and inverters. If it is, then 225.30 limits the number of feeders from transformer to the rack. For the 2020 NEC, 225.30(B) allows up to six.

So my 16 separate disconnects idea doesn't work unless the transformer and the inverter rack can be considered one structure. I assume there's a reason that the transformer can't be right next to the inverter rack?

Cheers, Wayne

P.S. Maybe I should submit a PI to 225.30 to allow more than six feeders as long as only "six operations of the hand" are required to disconnect them all? Is an enclosure with multiple OCPDs, each with separate line and load terminals, all of which can be shut off with one operation of the hand, commercially available?

 

[pv_n00b]

You may need to size the conductors so that they would be protected by the OCPD on the primary side of the transformer, though.

Primary protection of secondary conductors is only allowed in very limited applications, see 240.21(C)(1)

 

[pv_n00b]

For utility-tied systems, the above is all true whether there's a load at the end, or an interactive source at the end. So I see no reason the tap rules should be any different for the two cases. The short circuit and ground fault protection requirements are the same either way.

The two cases are different because inverters are not loads. Or am I missing some inside knowledge here?

 

[pv_n00b]

So it seems like there are only these three options if somebody really wants to do MLO.
1. Fusing at the secondary terminals of the transformer
2. Keep conductors less than 10ft and make it MLO switchboard
3. Call it industrial installation, keep conductors less than 25ft, make it MLO switchboard, and run full transformer rated conductors

One more option is to put in a separate OCPD, a CB or fused disconnect, in the feeder between the transformer and the MLO AC combiner. It does not match your picture but it is an option. Other than that I think you have them all. I'll say that qualifying for the "industrial installation" category is going to be up to the AHJ since the term is not well defined.

 

[wwhitney]

The two cases are different because inverters are not loads. Or am I missing some inside knowledge here?

As I understand it, the safety issue that comes from tap conductors not having OCPD at their point of supply is simply short circuit and ground fault (SC/GF) protection. The limits on how long taps can be and how they need to be protected have to do with the acceptable compromises on that SC/GF protection. And what is at the far end of the conductors, loads or PV inverters, has no bearing on SC/GF.

So the fact that you have PV inverters, rather than loads, at the end of your tap conductors is irrelevant. What matters is that you have conductors connected to the grid that do not have the correct SC/GF protection between the conductor and the grid.

Cheers, Wayne

 

[jaggedben]

As I understand it, the safety issue that comes from tap conductors not having OCPD at their point of supply is simply short circuit and ground fault (SC/GF) protection. The limits on how long taps can be and how they need to be protected have to do with the acceptable compromises on that SC/GF protection. And what is at the far end of the conductors, loads or PV inverters, has no bearing on SC/GF.

So the fact that you have PV inverters, rather than loads, at the end of your tap conductors is irrelevant. What matters is that you have conductors connected to the grid that do not have the correct SC/GF protection between the conductor and the grid.

Cheers, Wayne

For a residential site the ratio of PV inverter fault current to utility fault current is so low that the denominator might as well be zero, like you say. For a multi-MW site whose interconnection was engineered for the purpose...Maybe not so much?

Regardless, 705.12(2) requires both to be considered when sizing taps. Although it's a little unclear how to apply this to transformer secondaries that don't have overcurrent protection.

That said, I agree that the issues are short circuit and ground fault protection, not overload.

 

[wwhitney]

For a residential site the ratio of PV inverter fault current to utility fault current is so low that the denominator might as well be zero, like you say. For a multi-MW site whose interconnection was engineered for the purpose...Maybe not so much?

Yes, rather than saying that what's on the far (away from grid) end of the tap doesn't matter, I should have said that the case the far end is a PV inverter is at least as problematic as the case that it is a load, so there's no reason the tap rules should be more lenient for a PV inverter at the end.

Regardless, 705.12(2) requires both to be considered when sizing taps. Although it's a little unclear how to apply this to transformer secondaries that don't have overcurrent protection.

Codewise, 705.12(B)(2) directly references 240.21(B), which is only for taps, while 240.21(C) is for transformer secondaries. So we get to ignore the PV inverters for this discussion.

And for 240.21(C)(4) Outside Secondary Conductors, there is no minimum ampacity for the secondary conductor. Which means that considering the PV inverters wouldn't change anything.

Arguably 705.12(B)(2) should also reference 240.21(C), since several of the other rules in 240.21(C) do reference the OCPD protecting the primary conductors multiplied by voltage ratio of the transformer.

Cheers, Wayne

 

[jsashley2]

I've seen this image (or one very much like it) multiple times before. I'm trying to understand the code compliance, implications, and options with it. Specifically with the transformer secondary conductors and their protection.

Assume they're all 125kW string inverters. So the inverters (16x125kw = 2MW) aren't greater than the transformer (2MVA) rating. Because of 690 and 705, I don't need secondary transformer protection. But there's still the secondary condcutor protection requirements in 240.21(C), and then 408.36 if it's a panelboard and not a switchboard.

The only option that seems to make sense to allow multiple circuits off the secondary like this is the 240.21(C)(2), but that's limited to 10ft conductors, and there's no way these conductors in the picture would be less than 10ft when you go through the conduit path.

Or perhaps I'm reading that section entirely wrong, and when 240.21(C)(4), outside conductors, says that it needs to land on a single OCPD, it really only means that this set of conductors need to land on a single OCPD. The conductors to the other 3 panels are irrelevant and treated separately. As long as each set lands on a single OCPD that aligns with 240.21(C)(4), then you're good. Is it as simple as that?


I've even seen some places show these panels being MLO, and I'm not sure how that is able to fly. Maybe the "industrial installation" clause in 240.21(C)(3), but it would require full size secondary conductors to each panel, which could be problematic with lug connections there. 2500A+ conductors landing in 800A panel... and then there's also 408.36 panelboard protection issue. Right?


View attachment 2565082

i don't know where a MLO panel would be allowed. all panels for a string inverter cluster like this have always had main breakers. what state did you see that was allowing MLO to be built?