Understanding Branch and Feeder Sizing

[dgmellin]

Hello, I'd like to double check I understand and am applying the NEC correctly for my situation after searching through some threads here.

I have an existing 480V/50A breaker that feeds an existing distribution system:

Transformer (no info)

120/208/3ph Panel​

Subfed 120/208/3ph LP on 2 pole 30A breaker​

30A Fused Disconnect feeding 480V/30A/3ph panel

Client wants to add 4x 480V/18.1A/15KW (not sure how they got those amps from the KW rating, that's the info I was provided) space heaters on a new 480V panel. Those are continuous per 424.3(B), so I'm using 3#8+1#10G for each on a 60A non-fused disconnect, fed from individual 480V/40A breakers based on 125% of the 15KW/480V.

From there it seems pretty obvious the 50A breaker feeding the system needs to be upsized if I had to have a 160A panel, but it's a 100A frame breaker. The existing system was not designed this way though, and they used 30A breakers for 7.5KW heaters (125% checks out) in a 30A panel.

Am I missing something that would let me size the new panel lower than a multiple >1 of the branch circuits' rating? Existing panel was done in 1996.

 

[augie47]

can you provide a on-line diagram ?

 

[dgmellin]

can you provide a on-line diagram ?

https://imgur.com/a/Latoqf7

 

[augie47]

The amperage on your new heaters is correct, 18.1 amps (15kw/480X1.732) however 424.3 relates to the branch circuit and not to the feeder so your feeder load would be 72 amps for those heaters,.
If the 7.5 kw heaters are space heaters also and 3 phase, that;'s an added feeder load of 18 amps + 1 amp for the motor which brings you to 91 amps.
You could verify the transformer requirement and possibly get by with the 100 amp breaker

 

[dgmellin]

Thanks Augie. Part of what I messed up here was not multiplying 480 by 1.732 to double check that amperage so I ended up with 31.25 amps per heater before sizing the breaker.

The 7.5KW heaters are also space heaters and 3 phase, so continuous, but existing. Luckily the site is close enough that visiting is not an issue so I'm planning to go verify the transformer info and other questions I have. This one line doesn't have all the info as there's a second subfed panel on that transformer and they also want to add more loads on the 120 side. At any rate, feels like it's going to be very close to that 100A breaker still but not as crazy as my incorrect calculations were leading me to believe.