Max size circuit breaker in main panel

[600SL]

I'm having a issue understanding the maximum size CB I can put in my Siemens main panel to feed a sub panel. The notes are presented below. The right hand side show's the maximum breaker size is 100A. But the note bellow the table states QPH and HQP type breakers are limited to 100 amps if installed on a circuit capable of delivering fault current > 10,000 amps. Not sure what that means but I would like to know if I can use a 125A breaker with this box to power my sub panel. Not the end of the world if I'm limited to 100amps but a heat pump with E head while welding and compressor coming on could push the limits.

 

[micromind]

The 'circuit capable of delivering less than 10,000 amps' is referring to the maximum current the power company (POCO) can send to the panel with a dead short.

This amount of current depends on the POCOs transformer size and wire size and length to your meter.

Generally speaking, if your service is the only one on the POCO transformer, the maximum current will be less than 10,000 amps.....often a lot less.

If there are multiple services on the transformer, quite often it's less than 10,000 amps but not always.

If there's a rating on the transformer, like 15, 25, 37, 50, 75, 100 and the number is 75 or less, it's almost certain that it's less than 10,000 amps. 100 and up might be higher and might not.

In general, not very many residential panels are supplied with more than 10,000 amps of fault current.

 

[wyliesdiesels]

you should be good. doubt your service can deliver more than 10Ka on a dead short

 

[600SL]

The 'circuit capable of delivering less than 10,000 amps' is referring to the maximum current the power company (POCO) can send to the panel with a dead short.

This amount of current depends on the POCOs transformer size and wire size and length to your meter.

Generally speaking, if your service is the only one on the POCO transformer, the maximum current will be less than 10,000 amps.....often a lot less.

If there are multiple services on the transformer, quite often it's less than 10,000 amps but not always.

If there's a rating on the transformer, like 15, 25, 37, 50, 75, 100 and the number is 75 or less, it's almost certain that it's less than 10,000 amps. 100 and up might be higher and might not.

In general, not very many residential panels are supplied with more than 10,000 amps of fault current.

Good info, Thanks

I guess unless I climb the pole and look at the data plate, I will have no way of knowing. But the transformer was just up graded for my 400 amp service. So there is me with 400 amps and my neighbor with I ASSUME 200amps on this transformer. The new transformer they put in look's exactly the same physical size as the one they took out but I didn't measure it.

So one question still remains. Does the right hand column of the notes in my main breaker box, limit me to 100 amps or does that limit only apply for the QPH and HQP type breakers in this situation. For example can I use a 125A QP type breaker.

 

[larry4406]

I’m dumb and not a sparkie.

I recall a stab rating coming into play when I installed a 100A breaker to feed our barn.

 

[600SL]

you should be good. doubt your service can deliver more than 10Ka on a dead short

Thank you. See my last question in post #4.

 

[600SL]

I’m dumb and not a sparkie.

I recall a stab rating coming into play when I installed a 100A breaker to feed our barn.

That's my concern. My GE box in the house is a little more clear. It allows a 125A breaker in the top couple spaces only.

 

[mm08822]

The cb you will be using is the QP series. The cb family maxes out at 125A. Unless the labeling specifically prohibits this size or a total load combination on a buss stub (typical with QT cbs) then 125A is acceptable.

For Siemens panels, the restrictions are typically listed below the panel cat # on the label.
As an example below for buss stab limitations:

 

[600SL]

The cb you will be using is the QP series. The cb family maxes out at 125A. Unless the labeling specifically prohibits this size or a total load combination on a buss stub (typical with QT cbs) then 125A is acceptable.

For Siemens panels, the restrictions are typically listed below the panel cat # on the label.
As an example below for buss stab limitations:

Thank you. I just found the same part of the label on mine. Looks like its the same load center. I don't see anything about 125A posted on either mine or the example you posted. But the note, "Sum of the breaker rating is not to exceed 110 amps per branch circuit bus stab. So is the maximum breaker size 110A. And if I were to put a 110A breaker in, would that mean I cant put any breakers on the opposite side of the panel as the sum would now exceed 110A?

 

[mm08822]

No, a QP1125 is not a QT series cb. QTs are tandem cb.

125amp is your limit which the max for that cb series. QP are full size cb's @ 1" per pole.

You (partially) quote the panel label but leave out a key qualifier ......QT. Ask if you don't understand.

 

[600SL]

No, a QP1125 is not a QT series cb. QTs are tandem cb.

125amp is your limit which the max for that cb series. QP are full size cb's @ 1" per pole.

You (partially) quote the panel label but leave out a key qualifier ......QT. Ask if you don't understand.

OK I think I understand it. The note only applies to QT breakers and has nothing to do with QP breakers, which I will be using. So the limit for QP breakers is essentially the maximum size available which it 125Amp.

Therefore on the right side of the box I can put a 125A QP breaker in position 2 and 4 and on the left side of the box I can put a 60A QP breaker in position 1 and 3, even though positions 1 and 2 share the same stab as well as positions 2 and 4.

I was able to get the whole label in the two pictures below.

Thank you

 

[mm08822]

Correct. But I like to put light loads (small cbs) across from the bigger ones when possible. I don't lose any sleep if I can't.