residential sub panel calculation

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rlundsrud

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chicago, il, USA
What is the best method for doing a load calculation for a residential sub panel. The panel supplies parts of the house that main panel also supplies, so I am guessing you couldn't use square footage. Do you simply add up the known loads and make a determination from that? I only ask as I came across a sub panel that had an undersized neutral and when I added the loads it shouldn't have been derated. The house is still under warranty, so before I tell the homeowner to get it rectified, I want to be sure I didn't miss something. I doubt the EC that did this house would know how to derate it as he had bonded the neutral in the sub panel, so he clearly isn't very knowledgeable. I don't mean to overthink this, I just want to be certain before I cause a stir.

Thanks
 
rlundsrud said:
What is the best method for doing a load calculation for a residential sub panel. The panel supplies parts of the house that main panel also supplies, so I am guessing you couldn't use square footage. Do you simply add up the known loads and make a determination from that? I only ask as I came across a sub panel that had an undersized neutral and when I added the loads it shouldn't have been derated. The house is still under warranty, so before I tell the homeowner to get it rectified, I want to be sure I didn't miss something. I doubt the EC that did this house would know how to derate it as he had bonded the neutral in the sub panel, so he clearly isn't very knowledgeable. I don't mean to overthink this, I just want to be certain before I cause a stir.

Thanks
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IMO there is no right or wrong way - well I guess if you have frequent tripping of overcurrent protection you must have been wrong.

I would suggest adding up all known fixed loads like permanently installed lighting, fastened in place appliances, etc. then add any known not fastened in place appliances and this is typically the minimum necessary, But type or use of some items could still have some impact on what you may be able to get away with. One family may not load it much, but the next one to move in may have different ways and it will not be enough.
 
kwired said:
IMO there is no right or wrong way - well I guess if you have frequent tripping of overcurrent protection you must have been wrong.

I would suggest adding up all known fixed loads like permanently installed lighting, fastened in place appliances, etc. then add any known not fastened in place appliances and this is typically the minimum necessary, But type or use of some items could still have some impact on what you may be able to get away with. One family may not load it much, but the next one to move in may have different ways and it will not be enough.
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That's what I thought, I can't imagine why the EC that did the electric on the house decided to use a #8 neutral with #6 conductors (it is protected by a 60 amp OCPD). I just didn't want to call him out on it and then find out I was mistaken because of some obscure method for sizing a sub panel neutral. Thank you for the reply.
 
rlundsrud said:
That's what I thought, I can't imagine why the EC that did the electric on the house decided to use a #8 neutral with #6 conductors (it is protected by a 60 amp OCPD). I just didn't want to call him out on it and then find out I was mistaken because of some obscure method for sizing a sub panel neutral. Thank you for the reply.
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Call him out on what? I would have done the exact same thing. Neutrals that only carry the imbalance of the load don't need to be and are not required to be full size.

There are only a few times that a full size neutral for a feeder is required and only a tiny fraction of those few times when it is actually needed.
 
kwired said:
IMO there is no right or wrong way - well I guess if you have frequent tripping of overcurrent protection you must have been wrong.

I would suggest adding up all known fixed loads like permanently installed lighting, fastened in place appliances, etc. then add any known not fastened in place appliances and this is typically the minimum necessary, But type or use of some items could still have some impact on what you may be able to get away with. One family may not load it much, but the next one to move in may have different ways and it will not be enough.
Click to expand...

rlundsrud said:
That's what I thought, I can't imagine why the EC that did the electric on the house decided to use a #8 neutral with #6 conductors (it is protected by a 60 amp OCPD). I just didn't want to call him out on it and then find out I was mistaken because of some obscure method for sizing a sub panel neutral. Thank you for the reply.
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Given what kwired posted, I still have to question what you thought... and/or continue to think.

First, the only thing affecting the calculated load of the subpanel is loads in the same area served by both subpanel and main panels that are determined by footage. These are typically limited to general lighting and receptacles. Code gives no prescribed way to segregate these, but proportional to connected load (or Code theory such as 180VA per receptacle) would be the most logical.

With that said, there is potentially an easier way to determine whether the neutral is properly sized. The maximum unbalanced neutral current calculated load plus the line-to-line loads cannot exceed the rating of the panel, supply ocpd or ampacity, whichever is least (i.e. weakest link). A quick check can be performed by simply subtracting any line-to-line loads from the weakest link value, and that would be the maximum required neutral ampacity for that subpanel.

Given subpanel is fed with #6 hots and #8 neutral, there's only a 15A difference in ampacities @ 75?C. If there are any line-to-line loads of 15A or greater, the neutral is properly sized as long as the #6 is properly sized....
 
While I agree that the #8 will be adequate, IMO, for the install to meet Code, you would need to show some calculations to confirm that.
If, as Smart$ stated, you have some Line-Line loads that would well be all that was needed to assure the #8 was large enough.
Albeit highly unlikely, the possibility of an equal line neutral load exists unless shown otherwise.
 
augie47 said:
While I agree that the #8 will be adequate, IMO, for the install to meet Code, you would need to show some calculations to confirm that.....
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You're old, but not a big enough stick in the mud to require that though.:D

The chances of that neutral getting overloaded are less than winning the lotto or discovering alien life.
 
ActionDave said:
You're old, but not a big enough stick in the mud to require that though.:D

The chances of that neutral getting overloaded are less than winning the lotto or discovering alien life.
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You are correct on all points.. old, would not require and alien life...

but, if one went 'by the book' ......................................
 
When I added up the actual loads, the total was close to 90 amps, so it really should have been a 100 amp panel. And even if it was close to 60 amps, the homeowner should expect his home, (which is only 4 months old) to be done correctly and demand the neutral was replaced.
 
rlundsrud said:
When I added up the actual loads, the total was close to 90 amps, so it really should have been a 100 amp panel. And even if it was close to 60 amps, the homeowner should expect his home, (which is only 4 months old) to be done correctly and demand the neutral was replaced.
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Does that include the application of demand factors?

Any line-to-line loads?

What are the loads?


I for one will not support your conclusion until all the pertinent facts are presented.
 
rlundsrud said:
When I added up the actual loads, the total was close to 90 amps, so it really should have been a 100 amp panel. And even if it was close to 60 amps, the homeowner should expect his home, (which is only 4 months old) to be done correctly and demand the neutral was replaced.
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What loads? Also, unless you can prove otherwise, there is nothing wrong with the #8 neutral.
 
ActionDave said:
What loads? Also, unless you can prove otherwise, there is nothing wrong with the #8 neutral.
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In my world it does not work that way. The neutral is reduced and someone would need to show me why that was allowed.

As far as this specific situation, I agree a lot depends on the actual loads.
 
Disclaimer first:

I'm only a broadcast technician who reads this site to help me understand stuff 'on the supply side of the breaker panel' so to speak..generators, transfer switches, things like that.

But I have to ask WHY anyone would even bother with a sub-panel fed by a 60-amp breaker in a new home ? 100, I could see, but 60 ?

Thank you for your patience.

Gary
 
rlundsrud said:
That's what I thought, I can't imagine why the EC that did the electric on the house decided to use a #8 neutral with #6 conductors (it is protected by a 60 amp OCPD). I just didn't want to call him out on it and then find out I was mistaken because of some obscure method for sizing a sub panel neutral. Thank you for the reply.
Click to expand...

rlundsrud said:
When I added up the actual loads, the total was close to 90 amps, so it really should have been a 100 amp panel. And even if it was close to 60 amps, the homeowner should expect his home, (which is only 4 months old) to be done correctly and demand the neutral was replaced.
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I also have to ask "what loads"? If this is feeding a kitchen, the laundry, or some electric heat or other bigger load, I can maybe see that high of a load calculation. If it is just feeding some general lighting and general receptacles for most anything 60 amps @ 120/240 can feed a lot of lighting and receptacles and never be loaded all that heavy very often in a dwelling. Exception are rooms with electric space heaters or window air conditioners, those begin to add up fast if there is one in every room
 
Gary Glaenzer said:
Disclaimer first:

I'm only a broadcast technician who reads this site to help me understand stuff 'on the supply side of the breaker panel' so to speak..generators, transfer switches, things like that.

But I have to ask WHY anyone would even bother with a sub-panel fed by a 60-amp breaker in a new home ? 100, I could see, but 60 ?

Thank you for your patience.

Gary
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Ease of running the wire and the fact that most circuits for convenience recpts and general lights won't use 50A for an entire house. You don't need to trailer out a Howitzer to go duck hunting.
 
rlundsrud said:
When I added up the actual loads, the total was close to 90 amps, so it really should have been a 100 amp panel. And even if it was close to 60 amps, the homeowner should expect his home, (which is only 4 months old) to be done correctly and demand the neutral was replaced.
Click to expand...

What re "actual loads"? The maximum rating of every device/receptacle supplied from the subpanel? As if it were all continuously loaded?
 
Actual loads are just that, actual. If the space is unoccupied, you cannot usually know the actual loads.
Now calculated loads will involve assumptions, and will still not involve maximizing the loads on each circuit that is not dedicated to a particular known load.
 
I started this thread as I merely wanted to be sure that there wasn't some obscure code for calculating the load for a sub panel. I assure you I know how to add up existing loads, both L-L and L-N. I can even count the fingers on one hand twice and usually give the same answer. The EC that did this house should honestly have his license pulled and his workmanship was terrible and doing things like bonding the sub panel neutral and under sizing the feeder for it was inexcusable. Thank you all for your responses.
 
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