Determining available amperage.

[benjamintylerr]

Hi all,

I am trying to figure out how to determine what amperage I have available in an existing service. I need to see if I have an additional 100A amps available but I am unsure on how to do so. Will I need to perform a load calcualtion or take a physical measurement of the circuit?

Thanks

 

[brantmacga]

You need to do a load calc. Annex D Examples in the back of your code book is a good place to start learning how to do this.

 

[benjamintylerr]

You need to do a load calc. Annex D Examples in the back of your code book is a good place to start learning how to do this.

Awesome, thank you. I've been scouring the rest of the chapters but haven't looked in the annex's yet.

 

[AC\DC]

call power company and get the peak usage.

 

[benjamintylerr]

call power company and get the peak usage.

I wish it were that easy. This is for a feeder circuit in a multistory building and is not residential.

 

[AC\DC]

? Well your trying to come off something for 100 amps, that has a meter!.
I don’t rember that it specify between different uses it’s the peak they are after.
I’ll have to look in the book

 

[benjamintylerr]

I'm not doubting that there is a meter somewhere but I don't have the luxury of looking at it. Basically what I am dealing with is the secondary side of a 75 kva transformer that goes to a 225A OCPD. This OCPD then feeds a busway that has (12) 100A fused disconnects on it. Not all are being used to their full potential. Meaning some feed service panels that only have (4) 20 A breakers and the rest are spares.

 

[AC\DC]

Well I don’t do that large of stuff around here so I’ll shut up

 

[brantmacga]

what are all of the connected loads? And what is the building type (warehouse, office, manufacturing, etc?)

 

[benjamintylerr]

what are all of the connected loads? And what is the building type (warehouse, office, manufacturing, etc?)

Most are "sub panels" that feed different rooms and some go to disconnects that run machines and a compressor. The building is office/lab space.

 

[brantmacga]

Most are "sub panels" that feed different rooms and some go to disconnects that run machines and a compressor. The building is office/lab space.

So the place to start would be get the sq/ft of the space to do a general lighting load calc. And then get specs on the equipment. You should be able to get some pretty accurate data on lighting/receptacle loads by just putting an ammeter on it. If its really just lighting & receptacles it will be a very straightforward load calc.

I'm guessing you don't have any HVAC loads? If you can give more details I can try to walk you through it.