Disconnect on Inverter side of Transformer

[kingpb]

1200A 100% rated breaker, can be set to 125% of transformer rating
Transformer 1000KVA, 480-480Y/277V Dyn11
MLO switchboard with 4 feeder breakers on 480Y/277V

I contend the MLO switchboard does not require a main, it can be protected by the upstream 1200A breaker
Looking for confirmation

 

[wwhitney]

As a comment on the physics only, not the NEC requirements, if you have a L-N load (or fault) on your secondary, it would take 1200*sqrt(3) = 2078A to cause 1200A through the primary side breaker. So for the secondary MLO busbars to be protected at their ampacity by the 1200A primary breaker would require that they be rated at least 2078A.

NEC-wise, 240.21(C)(1) does not consider your secondary conductors to be protected by the primary side OCPD, so you will need secondary side OCPD to comply with one of the other sections of 240.21(C).

Cheers, Wayne

 

[david luchini]

1200A 100% rated breaker, can be set to 125% of transformer rating
Transformer 1000KVA, 480-480Y/277V Dyn11
MLO switchboard with 4 feeder breakers on 480Y/277V

I contend the MLO switchboard does not require a main, it can be protected by the upstream 1200A breaker
Looking for confirmation

If the transformer secondary conductor length is not more than 10', then the switchboard would not require a main ocpd.

 

[Carultch]

If the transformer secondary conductor length is not more than 10', then the switchboard would not require a main ocpd.

Can you provide a code reference for why?

 

[jim dungar]

If the transformer secondary conductor length is not more than 10', then the switchboard would not require a main ocpd.

This is only true in some supervised industrial installations. 240.21(C) pretty much requires single OCPD at the end of all conductors.

 

[wwhitney]

Can you provide a code reference for why?

240.21(C)(2)(1)(b), which I overlooked in my answer.

Which requires (2017 text) that the secondary conductors have an ampacity "Not less than the rating of the equipment containing an overcurrent device(s) supplied by the secondary conductors or not less than the rating of the overcurrent protective device at the termination of the secondary conductors."

Unspecified is how your determine "the rating of the equipment containing overcurrent devices." Would it mean that in the OP's MLO switchboard, the sum of the ratings of the overcurrent devices must not exceed the secondary conductor ampacity?

Cheers, Wayne

 

[Carultch]

240.21(C)(2)(1)(b), which I overlooked in my answer.

Which requires (2017 text) that the secondary conductors have an ampacity "Not less than the rating of the equipment containing an overcurrent device(s) supplied by the secondary conductors or not less than the rating of the overcurrent protective device at the termination of the secondary conductors."

Unspecified is how your determine "the rating of the equipment containing overcurrent devices." Would it mean that in the OP's MLO switchboard, the sum of the ratings of the overcurrent devices must not exceed the secondary conductor ampacity?

Cheers, Wayne

I'd go by the sum of the secondary overcurrent devices, if it were the case that you could terminate 240.21(C) conductors in an MLO panelboard or equivalent equipment (of which I'm skeptical, outside of the special case of a listed combination of devices for that purpose). The underlying idea is that the transformer primary OCPD can't directly protect the secondary conductors, except in cases of qualifying topologies of the transformer. The OCPD, or sum of OCPDs plural, is meant to be the failsafe against overload due to a fault.

 

[wwhitney]

If it were the case that you could terminate 240.21(C) conductors in an MLO panelboard or equivalent equipment (of which I'm skeptical, outside of the special case of a listed combination of devices for that purpose).

Well, if 240.21(C)(2)(1)(b) intended that you always had to land 10' transformer secondary conductors on a single OCPD, it wouldn't include the language "the rating of the equipment containing an overcurrent device(s) supplied by the secondary conductor" in addition to the language "the rating of the overcurrent protective device at the termination of the secondary conductors."

But I agree the additional language is a bit spartan as far as guidance on how to apply it.

Cheers, Wayne

 

[jaggedben]

Neither the 10ft tap rule nor the 10ft transformer secondary conductor rule require the conductors to land on a single overcurrent device. If the device they land on is a panelboard, then 408.36 requires that. But if the conductors land on a switchboard then I see no such requirement. Both rules mention calculated load, I presume as the method of protecting the conductors and switchboard (similar to 230.90 Exception 3).

 

[electrofelon]

Neither the 10ft tap rule nor the 10ft transformer secondary conductor rule require the conductors to land on a single overcurrent device. If the device they land on is a panelboard, then 408.36 requires that. But if the conductors land on a switchboard then I see no such requirement. Both rules mention calculated load, I presume as the method of protecting the conductors and switchboard (similar to 230.90 Exception 3).

If we look at the actual question asked by the OP:

"I contend the MLO switchboard does not require a main, it can be protected by the upstream 1200A breaker"

The answer is a switchboard does not require OCPD protection. The secondary conductors are a separate issue.