Incident Energy - Your Experience

busman

Senior Member
Location
Northern Virginia
Occupation
Master Electrician / Electrical Engineer
I know you won't be able to give specifics. However, was working in a plant with the following configuration:

500 kVA Transformer at 13,800 primary / 480V secondary
30' bus duct to switchgear room
400A Square-D I-line circuit breaker feeding 150' 500MCM (not paralleled) - for comparison, this circuit breaker enclosure is listed at 38 cal/cm2.

The question: Panelboard at load end of the feeder is labeled as 2 cal/cm2. This seems low to me. My experience of 480V equipment is that it is not usually this low. Any comments based on your experience would be greatly appreciated. I do much more residential than anything else.

Thanks,

Mark
 

ron

Senior Member
I'm confused. Where is the 2 and where is the 38?

Are they the same feeder relying on the same upstream 400A breaker or is one on the line side of the 400A CB and the other 150' downstream of the 400A CB?
 

busman

Senior Member
Location
Northern Virginia
Occupation
Master Electrician / Electrical Engineer
I'm confused. Where is the 2 and where is the 38?

Are they the same feeder relying on the same upstream 400A breaker or is one on the line side of the 400A CB and the other 150' downstream of the 400A CB?
I'm sorry. I should have been more clear. There is a 400A enclosed circuit breaker mounted on the end wall of the switchgear. I don't know if it is tapped off of the bus ahead of the switchgear disconnect or not. The enclosure for the circuit breaker is labeled at 38, presumably because it has line side conductors that are either protected by the switchgear disconnect or no line side protection before the transformer. The 2 cal/cm2 is labeled 150' downstream of that circuit breaker.

I guess my point was that I'm used to seeing 480V 400A panelboards within 200' of a large transformer be more in the 10-20 cal/cm^2.

Thanks,

Mark
 

mayanees

Senior Member
Location
Westminster, MD
I'm sorry. I should have been more clear. There is a 400A enclosed circuit breaker mounted on the end wall of the switchgear. I don't know if it is tapped off of the bus ahead of the switchgear disconnect or not. The enclosure for the circuit breaker is labeled at 38, presumably because it has line side conductors that are either protected by the switchgear disconnect or no line side protection before the transformer. The 2 cal/cm2 is labeled 150' downstream of that circuit breaker.

I guess my point was that I'm used to seeing 480V 400A panelboards within 200' of a large transformer be more in the 10-20 cal/cm^2.

Thanks,

Mark
I just modelled it and using a Square D thermal mag LA400 breaker that's not unreasonable to see 2 calories at the end of a 150 foot run.
 

ron

Senior Member
When you open the cover of the 400A enclosed breaker, you are exposed to the line side and the energy only protected/controlled by a larger breaker upstream, whereas the downstream 150' run is protected by the 400A breaker which will provide tighter and lower resulting incident energy.
 

steve66

Senior Member
The incident energy (IE) has a lot to do with how fast the breaker will trip. In this case, the 500 KVA xformer and the higher 480V feeder could actually make for a fairly high fault current, which would trip the breaker pretty fast. This results in a low IE.

Its great as long as the breaker works like it should.
 
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