Existing MLO subpanel as backed-up loads panel

[j-rts]

Hi! First post, so go easy on me.

I've got an Eaton CH404040SH where the secondary main circuit breaker feeds an MLO Eaton BR-type subpanel. There is 21kA of available fault current from the utility. My understanding is that the BR-type branch breakers in the subpanel are series rated to 25kaic with the CSR-type main circuit breaker in the SES, even though that main breaker is in a separate enclosure.

Now the slightly more complicated part - we're asked to install a solar and battery system, using the existing MLO subpanel as the backed up loads panel. So a bunch of equipment gets added between the CSR main breaker in the SES and the BR branch breakers in the MLO subpanel (a fused disconnect, an unfused disconnect, and the Tesla Backup Gateway). Does the series rating still apply? Or do we have to replace the MLO subpanel with a panel that has a main circuit breaker?

 

[Carultch]

Hi! First post, so go easy on me.

I've got an Eaton CH404040SH where the secondary main circuit breaker feeds an MLO Eaton BR-type subpanel. There is 21kA of available fault current from the utility. My understanding is that the BR-type branch breakers in the subpanel are series rated to 25kaic with the CSR-type main circuit breaker in the SES, even though that main breaker is in a separate enclosure.

Now the slightly more complicated part - we're asked to install a solar and battery system, using the existing MLO subpanel as the backed up loads panel. So a bunch of equipment gets added between the CSR main breaker in the SES and the BR branch breakers in the MLO subpanel (a fused disconnect, an unfused disconnect, and the Tesla Backup Gateway). Does the series rating still apply? Or do we have to replace the MLO subpanel with a panel that has a main circuit breaker?

Series ratings apply to any combination of overcurrent devices that are in series, regardless of location. So whether your subpanel has its own main breaker that series-rates with its branch breaker, or whether your subpanel is an MLO panel whose branch breaker in the main panel series-rates with its branch breaker, you can take credit for series ratings.

Series ratings are very product-specific. Usually a breaker product family would be series-rated with a given list of breaker families within the same brand, or with a given class of fuses. I've yet to see series ratings between different brands of breakers.

Make sure you are complying with applicable 705.12 rules for interconnection. Upstream OCPD's that act as a main to the subpanel, will count as a main breaker for an MLO subpanel. Interconnection rules also propagate to all upstream panelboards.

 

[don_resqcapt19]

Series ratings apply to any combination of overcurrent devices that are in series, regardless of location. So whether your subpanel has its own main breaker that series-rates with its branch breaker, or whether your subpanel is an MLO panel whose branch breaker in the main panel series-rates with its branch breaker, you can take credit for series ratings.

Series ratings are very product-specific. Usually a breaker product family would be series-rated with a given list of breaker families within the same brand, or with a given class of fuses. I've yet to see series ratings between different brands of breakers.

Make sure you are complying with applicable 705.12 rules for interconnection. Upstream OCPD's that act as a main to the subpanel, will count as a main breaker for an MLO subpanel. Interconnection rules also propagate to all upstream panelboards.

Probably never will see a series rating across different brands of breakers as the testing is costly and having a series rating with a different brand could act to decrease sales.

 

[jaggedben]

Have you ever seen series ratings enforced on subpanels in resi work? I haven't.

 

[Carultch]

Have you ever seen series ratings enforced on subpanels in resi work? I haven't.

It's uncommon that it even matters anyway, when the available fault current is less than 10 kA, and all breakers are sufficient without depending on series ratings.

 

[jaggedben]

It's uncommon that it even matters anyway, when the available fault current is less than 10 kA, and all breakers are sufficient without depending on series ratings.

True. I honestly don't know what the available fault current is on the resi projects I do.

 

[ptonsparky]

21kA available from POCO. What is it by the time it reaches your panel?

 

[PWDickerson]

Where did you get that 21kA figure from? Was that reported to you by the utility as the current available at the transformer secondary terminals, or was that calculated to include the impedance of the service conductors as well, or...? Every now and then when we have a large (for residential) tranformer and a short service conductor run, I calculate the short circuit amps at the service disconnect, and it is usually way under 21kA. I would be curious to know the transormer z rating, and the service conductor size and circuit length.

 

[ptonsparky]

Welcome to ask a question and we will give answers for something else.

 

[ggunn]

Welcome to ask a question and we will give answers for something else.

Of course; it's what we do, innit? By the time we are done with a question the OP has long since run off with his fingers in his ears, hollering "LALALALALALA..."

 

[electrofelon]

Where did you get that 21kA figure from? Was that reported to you by the utility as the current available at the transformer secondary terminals, or was that calculated to include the impedance of the service conductors as well, or...? Every now and then when we have a large (for residential) tranformer and a short service conductor run, I calculate the short circuit amps at the service disconnect, and it is usually way under 21kA. I would be curious to know the transormer z rating, and the service conductor size and circuit length.

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.

 

[j-rts]

Thanks for the comments! The utility publishes short circuit current tables (link), which is where the 21k (technically 20,955) figure came from. Starting with that value at the SES and then considering 11' of 4/0 feeder length, the licensed EE that we work with is showing 18,587A at the panel.

I've asked him to consider the case where we can see the transformer is 100' away and therefore we know the service conductors are much longer than 25', but he refused

Regardless - it sounds like the series rating should apply, even though the main breaker and branch breakers in question are in different panels. That's what I thought, but the licensed EE disagrees, and so here I am looking for a third opinion

 

[jaggedben]

If I had a licensed EE stamping my plans for a max 200A service and saying it doesn't matter I'd go with that. Like I said, nobody cares about this in resi. I try to install the same stuff as existing when I do projects, but that's just so I can swap breakers between panels and stuff like that. Everyone else mixes and matches in resi all the time. Does anyone really believe that having a 125A Siemens subpanel downstream of a 200A Eaton meter/main makes any important difference than if they were the same type?

Now if it was a 1000A three-phase commercial service I might be a bit more diligent, but I don't do that stuff.

 

[j-rts]

Well the problem is the EE is saying it does matter - that since there is 18,587A available at the panel, we either have to:

• A) replace all of the existing BR-type breakers with BRH-type breakers (22kaic rating) OR
• B) replace the existing MLO subpanel with a main breaker subpanel, so that the CSR-type main breaker and the BR-type branch breakers are in the same panel, and then the series rating can be applied

Obviously, both of those options are significantly more cost and labor intensive than simply leaving the panel as-is. And to my mind - they're completely unnecessary, since the upstream protection exists (CSR-type main breaker in the SES, and we'll be adding a utility-required fused disconnect). Both of those OCPDs should prevent a fault from delivering anywhere near 10,000A to the subpanel in question.

In all likelihood, we'll be doing what the guy with the stamp says and replacing the existing MLO subpanel.
But before the next one comes up, I'm going to continue to push on him and try to convince him that it's not necessary to do so.

 

[electrofelon]

Well the problem is the EE is saying it does matter - that since there is 18,587A available at the panel, we either have to:

• A) replace all of the existing BR-type breakers with BRH-type breakers (22kaic rating) OR
• B) replace the existing MLO subpanel with a main breaker subpanel, so that the CSR-type main breaker and the BR-type branch breakers are in the same panel, and then the series rating can be applied

Obviously, both of those options are significantly more cost and labor intensive than simply leaving the panel as-is. And to my mind - they're completely unnecessary, since the upstream protection exists (CSR-type main breaker in the SES, and we'll be adding a utility-required fused disconnect). Both of those OCPDs should prevent a fault from delivering anywhere near 10,000A to the subpanel in question.

In all likelihood, we'll be doing what the guy with the stamp says and replacing the existing MLO subpanel.
But before the next one comes up, I'm going to continue to push on him and try to convince him that it's not necessary to do so.

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.

 

[LarryFine]

Can an EE calculate and approve series ratings of different manufacturers' equipment?

 

[electrofelon]

Can an EE calculate and approve series ratings of different manufacturers' equipment?

Yes they can. Will they? Doubtful

 

[ptonsparky]

Can an EE calculate and approve series ratings of different manufacturers' equipment?

Not an EE, but as an EC I certainly would not accept responsibility for what another has done without close inspection. I would not expect it from one mfg to another.

 

[jaggedben]

Going back to the OP...

How could adding additional overcurrent devices in between the existing equipment possibly compromise the series rating? I would say the original series rating still applies. What is his reasoning for saying otherwise?

 

[wwhitney]

How could adding additional overcurrent devices in between the existing equipment possibly compromise the series rating?

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