Sizing of feeder conductors and transformer, with consideration of future electrical loads.

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cjarvis64

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Hello everyone,

I am sizing an electrical panel (240/120v rated 125A) for a new industrial process building. The panel is only serving building-related loads, i.e. lighting, receptacles, exhaust fans, heater fans, and a few control panels. So far I only have around 22 amps of load calculated, and 11 of 24 circuits are occupied. With consideration of possible future loads, how would you guys size the feeder conductors and the transformer (480:240/120) feeding the panelboard?

If you size it off of the load I'd be looking at 10 AWG feeder conductors with a 7.5kVA transformer. However this feels short-sighted to me and the right thing to do feels like to size the feeders and transformer to max out the panelboard capacity of 125A, giving you feeder conductors of 1/0 w/ #6 ground and a transformer of 25 kVA.

Somewhere in the middle seems reasonable, like installing a 100 MCB in the panelboard, and feeding it with #2 (#8 Ground) but keep the 25 kVA transformer in case the MCB needs to be upsized.

This is a perennial quandary for me as I am more of a controls guy, an opinion from someone who typically installs/sizes equipment like this would be of great help.

Thanks
 
As a practical matter, you will need to buy a transformer you can actually get. I think you can get a 15 KVA off the shelf transformer.

But it might be more cost effective to buy a larger transformer than absolutely needed and get a panelboard with a standard size MCB so you can use the MCB as both the secondary protection, and secondary conductor protection, and PB protection. Otherwise you might end up with seperate OCPD for the secondary/secondary conductors, which will add cost and also another place for something to go wrong.
 
petersonra said:
As a practical matter, you will need to buy a transformer you can actually get. I think you can get a 15 KVA off the shelf transformer.

But it might be more cost effective to buy a larger transformer than absolutely needed and get a panelboard with a standard size MCB so you can use the MCB as both the secondary protection, and secondary conductor protection, and PB protection. Otherwise you might end up with seperate OCPD for the secondary/secondary conductors, which will add cost and also another place for something to go wrong.
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I was planning on the MCB to be the secondary OCPD, and the MCC breaker feeding the transformer to be the primary.
 
I'm not a big fan of installing 1Ø panels when 3Ø is available. Do you really need 120/240 or will 208Y/120 work?
 
infinity said:
I'm not a big fan of installing 1Ø panels when 3Ø is available. Do you really need 120/240 or will 208Y/120 work?
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Our design requires 240/120. My question is really about what considerations are made for future loads on an electrical panel. How much spare is typical to install beyond the minimum size requirements on a new build?
 
cjarvis64 said:
Our design requires 240/120. My question is really about what considerations are made for future loads on an electrical panel. How much spare is typical to install beyond the minimum size requirements on a new build?
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Good question, I don't know of a simple rule of thumb because there can be so many variables down the road. How does 25% extra capacity sound?
 
infinity said:
Good question, I don't know of a simple rule of thumb because there can be so many variables down the road. How does 25% extra capacity sound?
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Sounds like it is purely a matter of design choice then. I'll talk with the customer more on it and see how much expandability he wants.
 
cjarvis64 said:
Hello everyone,

I am sizing an electrical panel (240/120v rated 125A) for a new industrial process building. The panel is only serving building-related loads, i.e. lighting, receptacles, exhaust fans, heater fans, and a few control panels. So far I only have around 22 amps of load calculated, and 11 of 24 circuits are occupied. With consideration of possible future loads, how would you guys size the feeder conductors and the transformer (480:240/120) feeding the panelboard?
Click to expand...
15kva then size the feeder and panelboard for the transformer. i wouldn't consider anything smaller. I would consider up-sizing the transformer at around 50-70% of the demand load and there were sufficient circuits.
cjarvis64 said:
If you size it off of the load I'd be looking at 10 AWG feeder conductors with a 7.5kVA transformer. However this feels short-sighted to me and the right thing to do feels like to size the feeders and transformer to max out the panelboard capacity of 125A, giving you feeder conductors of 1/0 w/ #6 ground and a transformer of 25 kVA.
Click to expand...
25kva is overkill for the load you describe. if in the future they want to install something that needs 20 kva worth of single-phase load, let the next designer worry about it.

it feels short-sighted because it is. talking about 7.5 VS 15 kva isn't worth anyone's time unless you are building and selling machines that need transformers, in which case you get exactly the smallest transformer you need. most designers in industrial facilities start at 15 kVA and don't go above 45 kVA because, in most industrial settings, single-phase loads aren't concentrated enough to justify that much power. in fact, often times you will see 2-3 transformers, one 15 kVA for process instruments and control panels, then a 30-45 kVA or two for building loads-lights, receptacles, HVAC, etc.
cjarvis64 said:
Somewhere in the middle seems reasonable, like installing a 100 MCB in the panelboard, and feeding it with #2 (#8 Ground) but keep the 25 kVA transformer in case the MCB needs to be upsized.
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provided the xfmr and panelboard aren't adjacent to one another, be sure to make the conduit large enough for the "future" conductors.
cjarvis64 said:
This is a perennial quandary for me as I am more of a controls guy, an opinion from someone who typically installs/sizes equipment like this would be of great help.

Thanks
Click to expand...
good luck.
 
I wired a lot of small water pump stations, 50 hp pumps, 10 Kw heat all at 480V and a 5kw step down transformer. Many locations had chlorination equipment added later requiring larger transformers and lighting panels.
So allow space for larger panels, transformers etc for future unkown loads
 
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