De-rate Main Service Breaker for a higher amperage backfed breaker

[SKSolar]

Does anyone ever de-rated a Murray MSP 200A breaker to make room for a higher than 40A backfed solar breaker?

By NEC 120% rule, a 200A MSP with a 200A main breaker limits the backfed breaker to 40A. The general rule is to de-rate the main breaker to increase the amperage of the backfed breaker.

I have a Murray JA244B1200SEC MSP with a 200A main breaker. I asked Siemens and this is what they said:


I don't really agree with this argument/comment. Any thoughts?

 

[Carultch]

It does feel like they missed the entire point of your question. But just because a 175A breaker could exist in theory, doesn't mean anyone makes one that fits the equipment. There are breakers with adjustable trips, but this is uncommon for anything less than a 400A frame size.

If they don't have a reduced rating breaker to fit the panel, they don't have it. Customer demand might simply not justify the expense of making 175A main breakers for 200A panels. Most people will just round up to a 200A main breaker and call it good enough. When backfeeding and the 120% rule isn't an issue, which is most of the time, there's little (if any) reason not to default to rounding up to 200A.

One solution you can do, is to relocate all the loads to a new subpanel that does have the main breaker you need for meeting the 120% rule, and use the rule for "sum of breakers excluding the main supply" to justify the existing main panel. You can also use power control systems from 705.13 as a solution to the problems of 705.12.

 

[SKSolar]

Thanks Carultch, What do you mean relocate "all" the load to a new sub panel? Look like replacing the MSP ?

I intend to go with the Enhpase Solar plus Battery without backup and all my backfeed is 50A where as I can only install a 40A breaker by the 120% rule. Maybe you are right that Siemens missed my question and I hope someone did already done with de-rating the main breaker. And he can advise me the make and model of the breaker.
Yes, Power Control System is the other option and according to Enphase, that system software is only available next month.

 

[electrofelon]

It is not clear to me whether they are saying the required breaker does not exist or just that it is factory installed so it's "not changeable". Have you tried searching the breaker part number Specifically?

 

[Carultch]

What do you mean relocate "all" the load to a new sub panel? Look like replacing the MSP ?

I don't like the idea of needing to do this, but this is one possible work-around to a bad situation where you don't have enough headroom on the 120% rule to interconnect in an existing panel. While it's a legal solution, a big problem is that the main panel will become a panel with lots of vacant spaces, and future users will just ignore the "do not add loads" label anyway.

The idea is that you make the main panel in its new configuration, a panelboard that can comply under the sum of breakers rule. Could be just 1 qty 200A branch breaker, could be subfeed lugs feeding a panel with a main breaker. Then, the ONLY, or one of the very few branch breakers within it, supplies a feeder to the new distribution panel. At that new distribution panel, you configure it to qualify under the 120% rule and opposite end supply.

It isn't necessarily all the loads, but more than likely will be close to all the loads. It could for instance, be 2 qty 100A breakers, leading to A) a 200A subpanel with the interconnection, and B) a 100A subpanel for everything else. It could be a 150A branch breaker leading to the 200 busbar subpanel with the interconnection, and up to 50A worth of branch breakers remaining in place.

BTW, it's the busbar-by-busbar sum, that matters, so you would add up the L1 breakers in one column, and add up the L2 breakers in another column, and treat these sums independently of each other. 2-pole breakers get added to both columns.

 

[electrofelon]

Why not just change the thing out? That's an easy job for an experienced electrician, no sweat. Seems like often people who specialize in solar are terrified of changing a meter or meter/main. Perhaps send us a pic of the thing...

 

[SKSolar]

Why not just change the thing out? That's an easy job for an experienced electrician, no sweat. Seems like often people who specialize in solar are terrified of changing a meter or meter/main. Perhaps send us a pic of the thing...

Thank you Sir. My Murray MSP used the MEQ9985 200A breaker, I searched Siemens for a compaitible and got nothing. A friend of mine who has the same MSP installed by the same electrician. He now has a Solar System for 8 years by SunRun and is using the 175A EQ9864 . Therefore to me, what Siemens said just doesn't make sense. Here are the pictures of my breaker and my friend's.

 

[electrofelon]

Thank you Sir. My Murray MSP used the MEQ9985 200A breaker, I searched Siemens for a compaitible and got nothing. A friend of mine who has the same MSP installed by the same electrician. He now has a Solar System for 8 years by SunRun and is using the 175A EQ9864 . Therefore to me, what Siemens said just doesn't make sense. Here are the pictures of my breaker and my friend's.

If it fits, I would have no trouble using it.

 

[jaggedben]

I have done this numerous times. Never thought I had to ask Siemens and glad now I didn't.

Surely if the main breaker went bad you'd be allowed to replace it with a good one. That's often listed on the label. So I don't see how they can tell you that you can't swap it out for a lower amperage of the same type. Sometimes those are even listed on the label, too. The breaker has the torque specs on it that you need.

And yes, they do make the breaker. So, also, what does that tell you.

 

[Carultch]

And yes, they do make the breaker. So, also, what does that tell you.

Likely, that the person responding to your question, doesn't care to do the research, and is just providing an easy answer instead of a complete answer.

Another thing is that they might have a corporate policy to tell you no, when you plan on modifying your equipment, to avoid the liability.

 

[SKSolar]

Likely, that the person responding to your question, doesn't care to do the research, and is just providing an easy answer instead of a complete answer.

Another thing is that they might have a corporate policy to tell you no, when you plan on modifying your equipment, to avoid the liability.

Thanks. I think what matter is NEC codes say and commonsense - we simply cannot put a breaker greater than the busbar rating.
They have sent me this but I could not find a breaker with QPXXX. Even EQ9684 is not on sale anymore except used one on ebay. There are others QAFXXX, QNXXXX etc

 

[jaggedben]

I can see how they wouldn't want warranty or liability because they can't control torque. But who cares? Who ever claimed a warranty replacement on a 200A meter main? And there's lots of other things everyone has to trust the installer on that aren't factory, so I think it's a realy small piece of liability.

 

[jaggedben]

Don't ask me for your money back if I'm wrong, but I'm about 95% sure that MBK175 is the same thing. You can remove the parallel bus prongs.

 

[jim dungar]

Siemens likely has UL series-ratings using specific models of "main" breakers. They likely will not give you permission to install a different model breaker than what they have tested and show on their official paperwork, as shown in post #11.

Just because it fits, is probably not a legally defensible practice.

 

[jaggedben]

Perhaps, but then what do they make the 175A and 150A breakers for? They are sold as main breakers not branch breakers.
I'm also a bit doubtful that a breaker with the same form factor and (presumably) the same magnetic trip unit would not pass the series test.

I think that either...
a) The Siemens rep didn't know what they were talking about. Happens a lot.
b) Siemens wants to have their cake and eat it, too.

Also gotta add that for every residence with a downsized main breaker for solar that may or may not be approved by the manufacturer, there are thousands of residences with subpanels of different brands that are not series tested.

 

[jim dungar]

Perhaps, but then what do they make the 175A and 150A breakers for? They are sold as main breakers not branch breakers.

What you do to a panel after it has left the factory is between you and the AHJ.

I'm also a bit doubtful that a breaker with the same form factor and (presumably) the same magnetic trip unit would not pass the series test.

Series ratings, of miniature molded case breakers rarely have anything to do with the trip element of the breakers. They are based on bolted fault currents so all of the devices are opening in their instantaneous/magnetic region. Series ratings are about clearing an arc once it has started.

 

[jaggedben]

...

Series ratings, of miniature molded case breakers rarely have anything to do with the trip element of the breakers. They are based on bolted fault currents so all of the devices are opening in their instantaneous/magnetic region. Series ratings are about clearing an arc once it has started.

Sorry if I used an inappropriate term but I think you're repeating exactly what I intended to say. And I would wager a sandwich that the instantaneous/magnetic characteristics of the Siemens MBK200, MBK175, and MBK150 are the same.

 

[electrofelon]

Series ratings, of miniature molded case breakers rarely have anything to do with the trip element of the breakers. They are based on bolted fault currents so all of the devices are opening in their instantaneous/magnetic region. Series ratings are about clearing an arc once it has started.

Basically echoing JB's post, but Do you know if when they series test a breaker line, do they actually test every breaker in it or just the one with the highest setting? For example, the siemens QR is a 250 amp frame, available 100-250A. If the 250 passes do they skip testing the lower ones? (I would think so).

 

[jim dungar]

Basically echoing JB's post, but Do you know if when they series test a breaker line, do they actually test every breaker in it or just the one with the highest setting? For example, the siemens QR is a 250 amp frame, available 100-250A. If the 250 passes do they skip testing the lower ones? (I would think so).

I believe they test each type of internal construction, not necessarily each trip setting. It is possible that a 250A breaker has slightly different internal parts than a 100A device. But the trip ranges would be shown in their selection tables.

But the OP issue deals with different breaker types that have similar external construction.

 

[jim dungar]

Sorry if I used an inappropriate term but I think you're repeating exactly what I intended to say. And I would wager a sandwich that the instantaneous/magnetic characteristics of the Siemens MBK200, MBK175, and MBK150 are the same.

It is not the instantaneous/magnetic characteristics that impact series ratings.
Series ratings are dependent on the internal physical mechanism, arcing path and speed at which the arc is quenched.

Yes, it is likely the internal parts of all MBK breakers are the same, except for the thermal element.