Extent of new sub-panel load calculation

[Calrissian45]

For large office buildings, is it standard practice to calculate the load for the entire building to determine if a new load will require an upgrade of any of the upstream panelboards, feeders, and breakers? If not, then how far back should the load be calculated for a new sub-panel and what would trigger the need to calculate the load all the way back to the service entrance?

 

[mgookin]

For all projects, if you add load, you need to demonstrate that you're not overloading the existing system. That's pretty much for all engineering - elec, structural, plbg, hvac, etc.

 

[Calrissian45]

I've never seen other electrical engineers calculate the entire building load when they've added loads. So are they neglecting their responsibility or is it appropriate to make a professional judgment that the system can handle the new load without calculating the entire system load?

 

[Calrissian45]

Is the only way to demonstrate that by calculating the electrical load for the entire building?

I've never seen other electrical engineers do that so I'm wondering if professional opinion is sufficient to determine if the system has sufficient capacity for the new load.

 

[Fitzdrew516]

For large office buildings, is it standard practice to calculate the load for the entire building to determine if a new load will require an upgrade of any of the upstream panelboards, feeders, and breakers? If not, then how far back should the load be calculated for a new sub-panel and what would trigger the need to calculate the load all the way back to the service entrance?

Depends really... I used to work in retail and for example all we would ensure was that our 200A feed which was given to us by the landlord was not overloaded. We did have an inspector ask for a load summary for the whole landlord switchgear one time though. If you have the utility bills for the past 12 months you can use 125% of that plus your new calculated load to show you're not overloading the service. NEC 220.87.

So if the highest peak demand over the past year was 100A. You would be at 125A plus your new loads. As long as that number is below the existing you're good to go.

 

[Calrissian45]

Depends really... I used to work in retail and for example all we would ensure was that our 200A feed which was given to us by the landlord was not overloaded. We did have an inspector ask for a load summary for the whole landlord switchgear one time though. If you have the utility bills for the past 12 months you can use 125% of that plus your new calculated load to show you're not overloading the service. NEC 220.87.

So if the highest peak demand over the past year was 100A. You would be at 125A plus your new loads. As long as that number is below the existing you're good to go.

I don't have meter data or utility bills so my lead electrical engineer suggested that I connect a current meter for a few hours during the middle of the day to determine peak usage. Am i correct that that would not be sufficient to determine peak usage and in this case I am forced to calculate the load of the whole building?

 

[ron]

I don't have meter data or utility bills so my lead electrical engineer suggested that I connect a current meter for a few hours during the middle of the day to determine peak usage. Am i correct that that would not be sufficient to determine peak usage and in this case I am forced to calculate the load of the whole building?

Not correct

220.87
Exception: If the maximum demand data for a 1-year period is not available, the calculated
load shall be permitted to be based on the maximum demand (measure of average power
demand over a 15-minute period) continuously recorded over a minimum 30-day period using
a recording ammeter or power meter connected to the highest loaded phase of the feeder or
service, based on the initial loading at the start of the recording. The recording shall reflect the
maximum demand of the feeder or service by being taken when the building or space is
occupied and shall include by measurement or calculation the larger of the heating or cooling
equipment load, and other loads that may be periodic in nature due to seasonal or similar
conditions.

 

[Calrissian45]

Not correct

220.87
Exception: If the maximum demand data for a 1-year period is not available, the calculated
load shall be permitted to be based on the maximum demand (measure of average power
demand over a 15-minute period) continuously recorded over a minimum 30-day period using
a recording ammeter or power meter connected to the highest loaded phase of the feeder or
service, based on the initial loading at the start of the recording. The recording shall reflect the
maximum demand of the feeder or service by being taken when the building or space is
occupied and shall include by measurement or calculation the larger of the heating or cooling
equipment load, and other loads that may be periodic in nature due to seasonal or similar
conditions.

Perfect! I missed that statement. Thanks for the help.

 

[petersonra]

For large office buildings, is it standard practice to calculate the load for the entire building to determine if a new load will require an upgrade of any of the upstream panelboards, feeders, and breakers? If not, then how far back should the load be calculated for a new sub-panel and what would trigger the need to calculate the load all the way back to the service entrance?

I don't know much about office buildings, but I have not seen a whole lot of load recalculating going on anywhere else.

It probably should be done but load calculations are so conservative that it would generally have to be a really big change before it would matter.