Estimating Load Calculations for Industrial Panelboards

[mull982]

I am in the process of verifying a load calculation for a 480V substation in an industrial facility and was having trouble estimating the existing loading on several 480V and 120/208V panelboards that are located in the facility. The substation feeds several downstream MCC's which are easy to calculate the load on since the motor loads on the MCC's are identified and I can use NEC 430.24 to calculate the load on each MCC.

Several of these MCC's feed 480V panelbaords and 120/208V panelbaords (via transformers) that do not have really good documentation on what the existing loading on the panelbaords are. Obviously with several branch circuits on each of these panelbaords it would take a long time to trace out each circuit and identify the connected load kVA rating (Unlike MCC's where you can easily identify motor loads). We could obviously meter the load on each of these panels but that would also take some time.

Is there a reasonable assumption that others have used in the past for loading on existing panelbaords when branch circuit loads are not known? Can you use a certain percentage of the panel or transformer rating? Any suggestions would be appreciated.

 

[ADub]

Put a pq on the suspect loads and get some actual demand data


Sent from my iPad using Tapatalk

 

[donaldelectrician]

FWIW - With all on , different times of day , AMP CLAMP the feeds to such varied panels . Get an Idea , go from there . Will also recorde highest load if clamp left on for awhile .



Don

 

[petersonra]

I am in the process of verifying a load calculation for a 480V substation in an industrial facility and was having trouble estimating the existing loading on several 480V and 120/208V panelboards that are located in the facility.

It seems to me that a load calculation has to be done in accordance with the way the NEC says it has to be done. Otherwise it is not a load calculation. Art 220 is pretty clear about this procedure.

220.87 allows you to measure the actual load in a specified way and use the measured maximum demand as your load calculation.

So really, your only code legal options are to do a load calculation as specified in art 220, or use 220.87.

I don't see how any "estimating" is involved in either.

 

[jglavin427]

I am in the process of verifying a load calculation for a 480V substation in an industrial facility and was having trouble estimating the existing loading on several 480V and 120/208V panelboards that are located in the facility. The substation feeds several downstream MCC's which are easy to calculate the load on since the motor loads on the MCC's are identified and I can use NEC 430.24 to calculate the load on each MCC.

Several of these MCC's feed 480V panelbaords and 120/208V panelbaords (via transformers) that do not have really good documentation on what the existing loading on the panelbaords are. Obviously with several branch circuits on each of these panelbaords it would take a long time to trace out each circuit and identify the connected load kVA rating (Unlike MCC's where you can easily identify motor loads). We could obviously meter the load on each of these panels but that would also take some time.

Is there a reasonable assumption that others have used in the past for loading on existing panelbaords when branch circuit loads are not known? Can you use a certain percentage of the panel or transformer rating? Any suggestions would be appreciated.

Sorry but I don't think there's a quick answer there. The least amount of effort solution with a meaningful result is probably a metering study. Barring that, I would gather info on the panels. Get breaker sizes and try to find some circuit directories. Once you have all the breaker sizes and any load information you can get, you can start to make educated guesses on loads for each branch circuit. Then simply perform the panel load calculation as you normally would.

Here's a few things I would try when estimating branch circuit loads: For motor loads you could work backwards from the breaker size (example: 30A, 3-pole, 208V breaker feeding unspecified motor load, I would assume the motor is around 5HP). For heating you might estimate 70% of breaker rating as the load. For things like lighting you could estimate the lighting power density for the area served, use the square footage to determine lighting load and divide by the number of circuits (for example, estimate approximately 1.5W/SF, 10000 SF, total 15kVA, divide by number of circuits). Receptacle loads estimate the number of recepts on each circuit based on the descriptions and multiply by 180VA.

 

[Cow]

We put a Fluke logger on unknown loads with a 15 minute sample rate at a duration of your choice. Sometimes just a week or so, other times a month if it's needed to go through the permit process.