Adding new commercial subpanel

[nguyenvd1]

Hello:

I am looking for advice on determining adequate capacity to add a new subpanel in a commercial building. I have a monthly KWh usage for an entire year from the client. The service is a 400A, 208/120, 3PH system. Lets say the peak month during the year was June, at 14,680 KWh. The panel schedules and as-built drawings do not have the KVA load information. I want to add a 100A, 208/120, 3PH subpanel for the client. Is there a way to determine if I have sufficient capacity to add the 100A subpanel? Thanks.

 

[DanDel]

You have to look at peak demand, not kWh. And then you have to add your calculated load for the new panel. If it's all under 80% of the service (I'd give myself some more for a saftey margin), you should be OK.

 

[waross]

does the kWHr meter have a demand reading? If it is an electronic meter it probably does. It will show the peak kW demand since the last time that it was reset. This is a more important figure than monthly kWHr numbers. If your customer pays demand charges the monthly demand will be on the power bill.
The AHJ may accept either the demend on the power bills or a demand reading on the meter if the demand reset on the meter is sealed when deciding whether to allow added load on an existing service.
You may have to have demand recording equipment installed to justify installing more load. If the loads are seasonal, you may have to record formost of a year.
OR
You may do a survey and recalculate the entire installation to determine whether added load may be allowed.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[EEJaime]

nguyenvd1,
Code says that for an existing service added loads must be calculated per Article 220-12 and 220-14, (see 220-16 2005 NEC). Article 220-42 has other methods for calculating the loads, but for an existing installation we find that most AHJ's will accept the installation if you have the highest DEMAND load from the previous 12 month's billings, or a recorded load reading of the peak kW over a period of time, AHJ's vary as to the mimimum length. We've had some accept 30 days, 7 days, even 24 hours if it's in a peak period of activity.

Taking 125% of the highest recorded demand PLUS the new connected load, must be less than the size of the overcurrent protection serving the facility. NOT 80% of the service size, 100% of the service size. So in your case if you can get the peak demand in that peak month, (note that the peak demand may not occur in the same month as the peak usage), and say the peak demand was 65 kVA. The load on your new 100A panel is 48 Amps, (17.3kVA). Then the minimum service size that would be needed would be 65kVA x 1.25 = 81.25 kVA + 17.3kVA = 98.55 kVA or 273.75 Amps at 208Y/120V,3ph,4w.

But this whole discussion is moot if you don't have the peak demand load. You can estimate it, or spot measure it, but those methods are not very accurate and I doubt your plan checker will accept them.

Best of luck.

EEJaime