Existing MLO subpanel as backed-up loads panel

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wwhitney said:
My understanding is that if OCPD A and B are series rated, and you add OCPD C in between A and B, that affects the series rating. During a fault event, the series rating of A and B is based on a fixed impedance between them. The OCPD C would be a dynamic impedance during the fault as it starts to open, and that will affect the opening behavior of A and B.

But maybe I've lost track of the configuration in this thread here, and when you said "between the existing equipment" you didn't mean "between the series rated OCPDs".

Cheers, Wayne
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Yes, OCPDs in between. OP said a fused disconnect.
 
wwhitney said:
Good to know, is that a general principle, so true for other manufacturers?

Cheers, Wayne
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Actually I was slightly wrong. IT says "Breaker" not OCPD. I wonder if it is ok to do it with a fuse?

Any FULLY RATED breaker can be applied upstream, downstream, or in the middle of any of the series ratings stated in the tables
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from this:


Frustratingly, I havent been able to find a series rating application guide or instructions from any other manufacturer.
 
Restating the question and the points that have been made to help myself think through the scenario - we have three OCPDs in series - a CSR main breaker, a fuse, and a BR branch breaker. If the fuse weren't there, the series rating says that the CSR main breaker would trip (clearing the fault) and that the BR branch breaker would be able to withstand the let-through current (without being catastrophically destroyed). So the question at hand is - Could adding a fuse between two series rated breakers slow down the trip behavior of the main breaker, thereby increasing the amount of let-through current beyond what the downstream breaker can safely withstand?

I think that the (perhaps unsatisfying) conclusion we're coming to is: Maybe - predicting fault behavior is really hard, which is why series ratings rely on testing and listing between very specific combinations of equipment. The safe answer is that the fuse will affect the fault behavior and series rating between the two breakers can no longer be applied.

Thanks everyone for the discussion! Feel free to continue it, but this will probably be my last response.
(Am I supposed to mark the discussion as "closed" or "answered" in some way?)
 
jaggedben said:
nobody cares about this in resi.
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Inspectors here will definitely ask about it if they notice a 167kVA on the pole. I had it come up on a service change out at a triplex years ago, it was in a heavy commercial downtown area. I had to get 42k rated mains for the meter pack, I ended up getting approval to use the 10k ones to get the power back on and had to go back 18 months later when the 42k mains arrived LOL.
 
electrofelon said:
One thing to remember, and that most people either don't know or ignore, is that single phase center tapped transformers typically have a higher L-N than L-L AFC. You should be multiplying your L-L by 1.5.
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At/near the transformer. As you get further away conductor impedance ends up making L-N AFC lower than L-L AFC.
 
wwhitney said:
OK, so adding a fused disconnect between two series rated breakers may affect the fault behavior and I think renders the two breakers no longer series rated.

Cheers, Wayne
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But is possible (or even likely) the end breaker may be series rated with certain fuse types and if those type are in the disconnect might be fine anyway.
 
tortuga said:
Inspectors here will definitely ask about it if they notice a 167kVA on the pole. I had it come up on a service change out at a triplex years ago, it was in a heavy commercial downtown area. I had to get 42k rated mains for the meter pack, I ended up getting approval to use the 10k ones to get the power back on and had to go back 18 months later when the 42k mains arrived LOL.
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For service equipment that makes some sense to me. Nitpicking subpanels, not so much.
 
kwired said:
But is possible (or even likely) the end breaker may be series rated with certain fuse types and if those type are in the disconnect might be fine anyway.
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This is a good point. Also why not replace the fused disconnect with something that takes an Eaton breaker.
 
kwired said:
At/near the transformer. As you get further away conductor impedance ends up making L-N AFC lower than L-L AFC.
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That's right! Especially since the N conductor is generally smaller. But at the xformer, the shorter half winding has less impedance and will pass more fault current.
 
What really bugs me is that the engineer refuses to take into account the 100' distance between the xformer and the service disconnect. What is the rational for that? I went to the utility link the OP posted, and it looks like he has a 100 kVA pole-mounted transformer. L-L Isc with 200' of 4/0 AL conductor (100' each way) in the circuit is about 8800A. I am confident the engineer knows a lot more than I do about how to calculate this correctly, but on the surface, it seems like he is being overly conservative.
 
PWDickerson said:
What really bugs me is that the engineer refuses to take into account the 100' distance between the xformer and the service disconnect. What is the rational for that? I went to the utility link the OP posted, and it looks like he has a 100 kVA pole-mounted transformer. L-L Isc with 200' of 4/0 AL conductor (100' each way) in the circuit is about 8800A. I am confident the engineer knows a lot more than I do about how to calculate this correctly, but on the surface, it seems like he is being overly conservative.
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One possible explanation is that the engineer may not know for sure that the 100' distance really will be 100'. The equipment location could change, or the 100' distance estimate is conservative. Consider the accuracy of the information the engineer has about this length, and whether equipment locations are confirmed.
 
electrofelon said:
Maybe I am misunderstanding the situation, but if not then your EE has some serious misunderstandings. For one, two devices do not have to be in the same enclosure for a series rating to apply. And two, he is refusing to calculate the reduction in available fault current from conductor impedance? Hopefully as a professional, he is open to being corrected. Suggest he come on the forum if he has any questions or doubts.
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my personal experience with AZ projects is that most jurisdictions in AZ do not allow designer to consider fault current loss in the service entrance conductors. i have no understanding why this is, but have seen this enforced in most projects i have completed anywhere in Arizona.
 
rlqdot said:
my personal experience with AZ projects is that most jurisdictions in AZ do not allow designer to consider fault current loss in the service entrance conductors. i have no understanding why this is, but have seen this enforced in most projects i have completed anywhere in Arizona.
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That is ridiculous. Do they ignore transformer impedance as well?
 
electrofelon said:
That is ridiculous. Do they ignore transformer impedance as well?
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i didn't say it made any sense, i am just reporting the design requirements that most AZ jurisdictions i have encountered enforce. they point the engineer to the fault current tables published by the utility (Salt River Project, Arizona Power & Light, etc.) and say whatever the published fault is at the secondary terminals of the transformer the utility selects for your service is the fault current you start with at the line terminals of your service entrance equipment. then, like always, the designer calculates available fault current for downstream equipment from that point on.
 
rlqdot said:
i didn't say it made any sense, i am just reporting the design requirements that most AZ jurisdictions i have encountered enforce. they point the engineer to the fault current tables published by the utility (Salt River Project, Arizona Power & Light, etc.) and say whatever the published fault is at the secondary terminals of the transformer the utility selects for your service is the fault current you start with at the line terminals of your service entrance equipment. then, like always, the designer calculates available fault current for downstream equipment from that point on.
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I have sometimes used this method as a worst case scenario when I don't know what all is underground between my point of interconnection and the utility transformer.
 
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