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

[jaggedben]

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

No, I don't think it does. They all say 'EQ' on the breaker. They are the same type.

 

[jim dungar]

No, I don't think it does. They all say 'EQ' on the breaker. They are the same type.

The first breaker mentioned by the OP, in post #7, were the MEQ and the EQ.
In post #11 the breakers mentioned were the EQ and QP.

 

[jaggedben]

The first breaker mentioned by the OP, in post #7, were the MEQ and the EQ.
In post #11 the breakers mentioned were the EQ and QP.

Oh right, the Murray vs. Siemens model numbers, almost identical. Maybe they have different internals, maybe not.

In any case, that wasn't the substance of Siemens' response. No one (except you?) is contending that the 150A and 175A versions of the same breaker (EQ or MEQ) are a different type than the 200A, or that that is the reason you can't downsize.

(Post #11 just does not apply to the breakers on the OP.)

 

[jim dungar]

Oh right, the Murray vs. Siemens model numbers, almost identical. Maybe they have different internals, maybe not.

In any case, that wasn't the substance of Siemens' response. No one (except you?) is contending that the 150A and 175A versions of the same breaker (EQ or MEQ) are a different type than the 200A, or that that is the reason you can't downsize.

(Post #11 just does not apply to the breakers on the OP.)

I am not contending anything. I do not know Siemens part numbers, although I recognize the EQ line from ITE back in the 80's.
My point was simply general advice. Just because breakers are physically interchangeable does not mean they are interchangeable for series ratings. You need to follow the manufacturer's UL tested combinations.

 

[SparkyBirdman]

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.

I just want to add another level of ludicrousness to your reply: had a solar job similar thing-installing a 60A solar breaker into a new 200A sub panel with 100A feeder breaker from the main. The electrical plan checker said that the solar is still above the rating of the main bussing-assuming that no loads were turned on inside the entire house and all of the solar produced backfed to the grid through the main bussing. He said I would have had to change the sub panel breaker to a 40A breaker to not derate the main.

I understand the Code's intention with the 120% rule, but to even keep this rule in the code as it is, with two decades of real world numbers out there about ACTUAL installed solar output and the very, very remote and extremely unlikely event that these homes will EVER pull 200A of demand from the utility while more than 40A Solar is being produced is asinine to me. I have performed load calculations for hundreds of homes installing generators and solar and NEVER have I had a home pull up to my calculations (and thats using the smaller of the two calc methods)-majority of the time when turning on all items in the house, it is 30% LESS than my calculations. I can't imagine what a homeowner is doing that they are pulling 241+Amps on a 200A service. Even the utility knows a residence on a 200A service isnt going to pull that, otherwise they would required a 400A service.

 

[ggunn]

A couple of things...

The code is what it is, and we are not allowed to speculate on what the loads on a panel or service will actually be, similar to how we are not allowed to lower the OCPD or conductor rating on an inverter output circuit based on how much DC we have connected to it.

That said, in the case you cite a 60A PV breaker would be allowed in the subpanel per 705.12(B)(3)(2) provided it is installed at the opposite end of the bus from the feed from the main because the OCPD protecting the bus is only 100A. (1.2)(200A) - 100A = 140A of headroom on the bus. This is true even if the subpanel has a 200A main breaker; the 100A breaker feeding it is its OCPD.

That said, however, you still need to comply with 705.12(B)(3) in the main panel as well; if it is a 200A panel with a 200A main breaker, 705.12(B)(3)(2) is not satisfied; you would only have (1.2)(200A) - 200A = only 40A headroom in the main panel, although you may be able to qualify the main panel bus under one of the other sections of 705.12(B)(3).

 

[electrofelon]

I just want to add another level of ludicrousness to your reply: had a solar job similar thing-installing a 60A solar breaker into a new 200A sub panel with 100A feeder breaker from the main. The electrical plan checker said that the solar is still above the rating of the main bussing-assuming that no loads were turned on inside the entire house and all of the solar produced backfed to the grid through the main bussing. He said I would have had to change the sub panel breaker to a 40A breaker to not derate the main.

I understand the Code's intention with the 120% rule, but to even keep this rule in the code as it is, with two decades of real world numbers out there about ACTUAL installed solar output and the very, very remote and extremely unlikely event that these homes will EVER pull 200A of demand from the utility while more than 40A Solar is being produced is asinine to me. I have performed load calculations for hundreds of homes installing generators and solar and NEVER have I had a home pull up to my calculations (and thats using the smaller of the two calc methods)-majority of the time when turning on all items in the house, it is 30% LESS than my calculations. I can't imagine what a homeowner is doing that they are pulling 241+Amps on a 200A service. Even the utility knows a residence on a 200A service isnt going to pull that, otherwise they would required a 400A service.

I agree the 120% is ridiculously conservative, and just another example of how so many people who have no idea what they are talking about get to make and approve code rules.

 

[wwhitney]

I agree the 120% is ridiculously conservative

Ridiculously conservative?

As I understand it, a panel is tested for temperature rise due to the I²*R heating from busbar resistances, connection resistances, and conductor resistances, at the worst case single source loading. The 120% rule limits those 3 heat sources to 100%, 104%, and 104% of the test conditions, respectively.

What factor would you rather see than 120%? A 130% rule would change those factors to 100%, 109%, and 109% respectively. Are you confident that every panelboard ever made has that extra 9% margin on the connection and conductor resistive heating?

Cheers, Wayne

 

[electrofelon]

Ridiculously conservative?

As I understand it, a panel is tested for temperature rise due to the I²*R heating from busbar resistances, connection resistances, and conductor resistances, at the worst case single source loading. The 120% rule limits those 3 heat sources to 100%, 104%, and 104% of the test conditions, respectively.

What factor would you rather see than 120%? A 130% rule would change those factors to 100%, 109%, and 109% respectively. Are you confident that every panelboard ever made has that extra 9% margin on the connection and conductor resistive heating?

Cheers, Wayne

For residential 200A services , absolutely. Could go way higher.

 

[wwhitney]

For residential 200A services

Now you're considering the nature of the loading by referring to residential. In which case if you are counting on the 200A residential service to never see close to 200A, there's no problem tripping-wise in reducing the main breaker to 175A. And if the residential load calc comes out so much higher than reality, making 175A non-compliant even though it would work fine in practice, that load calculation procedure should be readjusted.

I.e. the issue is not in the 120% rule itself, which needs to be more broadly applicable.

Cheers, Wayne

 

[SKSolar]

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.

Siemens product expert are firm that my MEQ9985 cannot be de-rated. I posted a question to their 175A breaker in Home Depot and this is what they said:

 

[ggunn]

Siemens product expert are firm that my MEQ9985 cannot be de-rated. I posted a question to their 175A breaker in Home Depot and this is what they said:
View attachment 2566979

Sounds to me like they just want to sell load centers. The company I just retired from has replaced main breakers with lower rated ones in many load centers in order to comply with 705.12(B)(3). Replacing a 200A load center with a 175A one would only make things worse unless you have or can get documentation that they both use the same (at least 200A rated) busbars like Eaton CH does.

 

[electrofelon]

Now you're considering the nature of the loading by referring to residential. In which case if you are counting on the 200A residential service to never see close to 200A, there's no problem tripping-wise in reducing the main breaker to 175A. And if the residential load calc comes out so much higher than reality, making 175A non-compliant even though it would work fine in practice, that load calculation procedure should be readjusted.

I.e. the issue is not in the 120% rule itself, which needs to be more broadly applicable.

Cheers, Wayne

Basically, I don't believe that designing something or creating a standard should necessarily be based on things that are theoretically possible. I don't believe bridges are designed to hold bumper to bumper 11 yard concrete trucks in every lane.

 

[jaggedben]

Basically, I don't believe that designing something or creating a standard should necessarily be based on things that are theoretically possible. I don't believe bridges are designed to hold bumper to bumper 11 yard concrete trucks in every lane.

You don't know about the 50th anniversary Golden Gate Bridge walk.

In all seriousness, I agree with you. The code is inconsistent in its approach. For example compare this rule to 230.90 exception 3, where a load calc, instead of actual OCPD, is allowed to protect conductors from overload. Arguably by this sort of logic an intermittent source should be allowed to be disregarded where the load calc - or actual load, as judged by the main breaker never having tripped - should be justification for a much larger intermittent source.

At the very least, why are we stuck at 20% above rating when there are so many 25% factors in the rest of the code. Give us 25% just to be consistent. ;-)

 

[Carultch]

I understand the Code's intention with the 120% rule, but to even keep this rule in the code as it is, with two decades of real world numbers out there about ACTUAL installed solar output and the very, very remote and extremely unlikely event that these homes will EVER pull 200A of demand from the utility while more than 40A Solar is being produced is asinine to me. I have performed load calculations for hundreds of homes installing generators and solar and NEVER have I had a home pull up to my calculations (and thats using the smaller of the two calc methods)-majority of the time when turning on all items in the house, it is 30% LESS than my calculations. I can't imagine what a homeowner is doing that they are pulling 241+Amps on a 200A service. Even the utility knows a residence on a 200A service isnt going to pull that, otherwise they would required a 400A service.

Even if statistical models of user behavior say that it unlikely to consume anywhere near 200A, an adversarial user could deliberately turn on every possible load, and create a condition that is unsafe, and is in the blindspot of the main breaker. We want the breakers to trip before busbar overloading becomes a possibility. I feel that we should be able to take more than 20% partial credit for KCL, when strategically arranging a busbar for subtractive currents, since it will be distributed to different parts of the panel anyhow. But I understand that 120% is an industry compromise we all must comply with, in the current state of the code.

There is a 705.13 solution to this 705.12 problem, to use a power control system to allow you to be more ambitious with the interconnecting capacity. This recognizes that it is an unrealistic situation for more than 240A to ever be present on the panelboard, and allows you to curtail your power as a solution to this, if it ever does become a realistic situation.

 

[wwhitney]

I feel that we should be able to take more than 20% partial credit for KCL, when strategically arranging a busbar for subtractive currents, since it will be distributed to different parts of the panel anyhow.

KCL only affects the busbar currents, which is only one component of the heating in the enclosure. The other components being the busbar connection resistances and the currents on the conductors attached to those connections. So for the above statement to be defensible, you'd need to have a handle on the magnitude of the busbar current heating vs the other 2 sources of heating. Do you have any data on that?

Cheers, Wayne

P.S. Worst case for a single source connected to a 100A panelboard:

100A on two sets of conductors (in/out)
100A on 2 busbar connections (including breaker/lugs)
100A on the full length of busbar

Worst case with two sources, a (100+X)% rule, and a busbar of length N spaces:

100A on two sets of conductors
X A on two sets of conductors
100A on 2 busbar connections
X A on 2 busbar connections
100A on length N-2 of busbar
X A on length 1 of busbar

 

[tortuga]

Does anyone ever de-rated a Murray MSP 200A breaker ../snip/....

I have a Murray JA244B1200SEC MSP with a 200A main breaker.

I have converted many of these Siemens / ITE panels to everything from 100A - 150A, some over 20 years ago.
To do the 100 or 125A conversion I removed one set of wires from the meter.
The reason I had to is one utility here restricts a temp saw pole to 125A for construction sites.
And occasionally i'll be doing a service change and the utility will either limit the main breaker size on their lateral or have us upgrade the lateral, so we'll just match the existing lateral wire size and have a corresponding main.
AHJ has never had an issue.
Be careful to not change the length of the factory conductors as they are parallel.

 

[SKSolar]

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.

Jim,

I have trouble searching for a 175A replacement breaker to de-rate my 200A Murray JA2442B1200SEC/FEC MSP.
As I am going to get an electrician to do the work on the MSP, I need to get the right part for him to do. I believe EQ 9684 would work but not too sure and Siemens website didn't have any info on that. People sells them on ebay for $150 to $250.

You are absolutely right that once a product leaves the manufacturer, it is between me and AHJ.

With your credential, the breaker I have now and the MSP data sheet, would you be able to tell a breaker (with its data sheet) will fit into a MSP ? As you are an expert on power system, what you said carries weight. Can you help to approach Siemens and help me find one that is available in the market so that I can purchase them please. Thanks

 

[jaggedben]

It will fit.