NEC 220.87 Determining Existing Loads

[Calrissian45]

If I am adding a load to the end of a chain of panelboards then would I determine the existing loads, per section 220.87, for each feeder in the chain of panelboards, all the way back to the service, or do I just do it on the feeder for the panelboard that I am adding the new load to?

 

[petersonra]

If I am adding a load to the end of a chain of panelboards then would I determine the existing loads, per section 220.87, for each feeder in the chain of panelboards, all the way back to the service, or do I just do it on the feeder for the panelboard that I am adding the new load to?

IMO, you have to work your way back if you add load.

However, it is possible that there is some kind of load calculation you could build on and just update.

You could also record the actual loads over some period of time (I forget what period of time the code says you have to use) and use that for the load calculation.

As a practical matter, unless the AHJ forces one to do so, I doubt many people make new or updated load calculations for anything other than the feeder they are coming from unless they are adding a lot of load.

 

[jumper]

You could also record the actual loads over some period of time (I forget what period of time the code says you have to use) and use that for the load calculation.

You need POCO records for 1 year or you can monitor it continuously for 30 days with an average demand figure for every 15 mins.

Typical occupancy and seasonal fluctuation demand for HVAC must be accounted for.

 

[iwire]

You need POCO records for 1 year or you can monitor it continuously for 30 days with an average demand figure for every 15 mins.

I think the exception explains it.

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

 

[jumper]

I think the exception explains it.

You no like me paraphrase?

 

[Calrissian45]

I understand the method for determining existing loads. I just wasn't sure if that applies to every panelboard in a chain. For example, assuming I don't have a meter on every panelboard in a series of panelboards, then it seems that I would need to monitor usage on each panelboard for 30 days so that I can verify that the new load won't overload any of the panels in the chain. It seems like overkill for a new load that is small, so I wanted to know if other engineers do that on every job (no matter how small the new load is) or just when they think they are near the feeder's capacity.

 

[Smart $]

I understand the method for determining existing loads. I just wasn't sure if that applies to every panelboard in a chain. For example, assuming I don't have a meter on every panelboard in a series of panelboards, then it seems that I would need to monitor usage on each panelboard for 30 days so that I can verify that the new load won't overload any of the panels in the chain. It seems like overkill for a new load that is small, so I wanted to know if other engineers do that on every job (no matter how small the new load is) or just when they think they are near the feeder's capacity.

It's fairly simple. Either you monitor and are certain, or you take an educated guess and are not. :happyyes:

 

[mahaney03]

It's fairly simple. Either you monitor and are certain, or you take an educated guess and are not. :happyyes:

But Fairfax County Permits don't like it when you take "educated guesses" For a project I'm working on, I'm actually adding less load than before but they still want the metered loads for 30 days.

 

[Smart $]

But Fairfax County Permits don't like it when you take "educated guesses" For a project I'm working on, I'm actually adding less load than before but they still want the metered loads for 30 days.

NEC is quite specific... but the AHJ may be lax, may not. My preference is to be certain.