stab amp maximum rating

[hardworker]

If a panel has a main breaker on the same stab as a breaker that is feeding a load, do you consider the main breaker ampage as part of the equation.

Slot #1 and #3 have a breaker for 70 amps and across from it on #2 and #4 the main breaker is a 100amp. If the stab maximum rating is 150 amps is there a problem? One is a load and the other is a line. Does this make a difference?

 

[jumper]

Here is a thread about a similar application. Note what post #9 says.

http://forums.mikeholt.com/showthread.php?t=143058

 

[Smart $]

If a panel has a main breaker on the same stab as a breaker that is feeding a load, do you consider the main breaker ampage as part of the equation.

Slot #1 and #3 have a breaker for 70 amps and across from it on #2 and #4 the main breaker is a 100amp. If the stab maximum rating is 150 amps is there a problem? One is a load and the other is a line. Does this make a difference?

Yes, it makes a difference. In your example, if a 70A load is connected to breaker #1-3, that 70A would be the same 70A coming in on breaker #2-4. The max current through those stabs is what passes through the 100A breaker. It is not the same as two loads at 100A and 70A.

 

[jumper]

Yes, it makes a difference. In your example, if a 70A load is connected to breaker #1-3, that 70A would be the same 70A coming in on breaker #2-4. The max current through those stabs is what passes through the 100A breaker. It is not the same as two loads at 100A and 70A.

That is what I thought, but does not Don seem to say otherwise in post 9 in that link above. It was late and I could not wrap my head on why he said that or did I misread what he said?

 

[jim dungar]

First, there is the matter of the current flowing 'into the buss' through the stab, in your case the 100A main breaker supplies current to all of the other loads, it therefore limits the current on its stab to 100A, regardless what else may be connected to the stab.

Second, there is the issue of the current flowing 'out of the buss' through the stab. The current through the stab will be the sum of the loads on the breakers connected to it.

 

[don_resqcapt19]

That is what I thought, but does not Don seem to say otherwise in post 9 in that link above. It was late and I could not wrap my head on why he said that or did I misread what he said?

My statement was incorrect.

 

[suemarkp]

Need to watch the older panels too. I have a CH 200A main breaker panel that limits the branch breaker stabs to 140A. I few years later I bought a newer version of the same panel, and it had a beefier bus and allowed 200A per stab.

So the older 200A main breaker panel can have problems if you have a 100A and 70A branch breakers installed in the same row. Their 125A MLO panels still have the same limit -- 140A per stab. With a main of 100A, you could only have a branch breaker across from that main of 40A or less.

 

[hardworker]

Am I correct in saying no matter what the sum total on a given stab, if the main is 100amp, that maximum any stab could have is 100amp or the main would trip and prevent any ampage above the 100.

 

[Smart $]

Need to watch the older panels too. I have a CH 200A main breaker panel that limits the branch breaker stabs to 140A. I few years later I bought a newer version of the same panel, and it had a beefier bus and allowed 200A per stab.

So the older 200A main breaker panel can have problems if you have a 100A and 70A branch breakers installed in the same row. Their 125A MLO panels still have the same limit -- 140A per stab. With a main of 100A, you could only have a branch breaker across from that main of 40A or less.

That last part is not correct. If the stabs are rated 140A total, you could put anything up to 140A across from the main (which over 100A has "don't do it" written all over it... but not for the reason being discussed). Note when the MAIN is "backfeeding", it is not using the stabs as a branch circuit breaker. The branch breaker rating sum limitation applies only to breakers on the other half of the effected stabs.

 

[Smart $]

Am I correct in saying no matter what the sum total on a given stab, if the main is 100amp, that maximum any stab could have is 100amp or the main would trip and prevent any ampage above the 100.

Sort of... that works only if the breaker sum per stab rating is greater than the main rating. Attempting to reword could get you in trouble if the breaker sum rating per stab is less than the main rating.

 

[hardworker]

Sort of... that works only if the breaker sum per stab rating is greater than the main rating. Attempting to reword could get you in trouble if the breaker sum rating per stab is less than the main rating.

I don't think the stab rating would be less if the main is backfed thru a stab. Am i correct?

 

[Smart $]

I don't think the stab rating would be less if the main is backfed thru a stab. Am i correct?

Shouldn't be if it was designed for backfed main, but you should always verify rather than assume. What if panel was designed as MLO, opted in field to backfeed through a breaker, and the stab rating is less than panel rating...???

 

[suemarkp]

OK, so if you have a backfed main (100A) with a stab limit of 140A, would having a 100A branch breaker next to that main be a problem? There is 100A in (main) and 100A out (branch), but the bus bars don't care which direction the current is going. Does this count as 100A or 200A on that set of stabs (would be double the heat dissipation of a single 100A load or source)?

I guess the only other situation I can think of where the bus stab limit could be exceeded would be a MLO panel with a backfed main plus an additional power source such as solar on the same stabs.

Or maybe this isn't an issue when the stab rating is higher than the bus rating. The 200A panel with a 140A stab limit could certainly be a problem, but maybe there is no case where the 125/140A panel would ever have an issue if kept within its 125A rating?

 

[Smart $]

OK, so if you have a backfed main (100A) with a stab limit of 140A, would having a 100A branch breaker next to that main be a problem? There is 100A in (main) and 100A out (branch), but the bus bars don't care which direction the current is going. Does this count as 100A or 200A on that set of stabs (would be double the heat dissipation of a single 100A load or source)?

100A branch breaker(s) across from 100A main breaker would be the same 100A on the shared stabs... not 200A. Apply Kirchoff's Current Law using the stab as the node.

I guess the only other situation I can think of where the bus stab limit could be exceeded would be a MLO panel with a backfed main plus an additional power source such as solar on the same stabs.

But that would be a violation. Can't recall the section number right now, but the additional power source is required to feed the bus at the opposite end from the utility main.

 

[Cow]

What kind of panel is it? Are you using a retainer kit? All the Siemens panels I've used retainer kits in don't allow a breaker across from the main.

 

[suemarkp]

Yes, forgot about that rule and you're right the two breaker hold down positions are at opposite corners. So I wonder then why does CH put that stab limit of 140A on a panel rated at 125A? Seems like there is no way to violate the current rating (putting two 80A breaker next to each other would violate the labeling, but they should never be able to draw more current in total than the main allows). Are they just cutting and pasting their panel labels across all the panels?

 

[hurk27]

I only know from experience of main breaker failures to the stabs when a branch or feeder breaker is located across from the back fed main, it most likely is a restriction based upon the heat build up from the stab connection point when done this way.

From the above I have always just assumed that it was the above as to why many manufactures blocked the space across from the back fed main when the panel was design as a back fed main, this was done for years as many older Murry, GE, and ITE panels all did this as well as a host of others.

Square D avoided this with some older QO's as the main went all the way across both sets of stabs and had 4 fingers making connection to the buss, but some later versions did block the space across from the main when only one side type main was used.

So my though is it was a heat issue.

 

[Smart $]

... So I wonder then why does CH put that stab limit of 140A on a panel rated at 125A? ... Are they just cutting and pasting their panel labels across all the panels?

Questions I cannot answer with certainty, but I surmise the answers are based on manufacturing costs vs. simplicity.

 

[hardworker]

I only know from experience of main breaker failures to the stabs when a branch or feeder breaker is located across from the back fed main, it most likely is a restriction based upon the heat build up from the stab connection point when done this way.

From the above I have always just assumed that it was the above as to why many manufactures blocked the space across from the back fed main when the panel was design as a back fed main, this was done for years as many older Murry, GE, and ITE panels all did this as well as a host of others.

Square D avoided this with some older QO's as the main went all the way across both sets of stabs and had 4 fingers making connection to the buss, but some later versions did block the space across from the main when only one side type main was used.

So my though is it was a heat issue.

I am no expert at this, but I would think if it is rated for a certain amount of amps, a stab should be able to take the heat if you do not exceed the amp rating??? What does code say about this?

 

[Smart $]

I am no expert at this, but I would think if it is rated for a certain amount of amps, a stab should be able to take the heat if you do not exceed the amp rating??? What does code say about this?

Code says nothing about stab ratings (other than compliance with manufacturer instructions).