Rooftop Unit Circuit

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Sparky1981

Sparky1981

PE
Location
Denver, CO
Occupation
Electrical/Lighting Engineer
I have an RTU unit that calls for MOCP 70A. This is an existing building. After 30 day metering a few panels we do not have capacity in any of them for this unit. Our only option is the old Westinghouse service gear. Not easy to get parts for. There is an existing 800A breaker that we can put a 400A digitrip unit in. Lowest we could find. What I am proposing is a 400A feeder from that breaker in the main gear to a 200A fused disconnect that has 70A fuses in it. As close to the gear as possible to limit the 400A feeder. Then do a 70A feeder from the fused disconnect to the unit. Anybody see any issues? I dont think this is a tap because the 400A feeder is protected.
 
You would need to address terminating 400 amp conductors on a 200 amp switch..... the normal lugs would not accommodate.
Will "as close as possible" not be within 25' allowing a tap rule ?
 
augie47 said:
You would need to address terminating 400 amp conductors on a 200 amp switch..... the normal lugs would not accommodate.
Will "as close as possible" not be within 25' allowing a tap rule ?
Click to expand...
Good point. If the wire is reduced to fit the lugs of a 200A we would need to make sure its within 25'. So I need to make sure that we are within 25' of the 400A breaker and that the wire between the breaker and fused disconnect is at least (1/3) 400A. Probably just round that to 150A and use 1/O copper.
 
Sparky1981 said:
I have an RTU unit that calls for MOCP 70A. This is an existing building. After 30 day metering a few panels we do not have capacity in any of them for this unit.
Click to expand...
What is the MCA? That is the figure you need to use for load calculations and feeder sizing, not the MOCP.

Cheers, Wayne
 
wwhitney said:
What is the MCA? That is the figure you need to use for load calculations and feeder sizing, not the MOCP.

Cheers, Wayne
Click to expand...
Right. I misspoke. I did the calculations a while back. The 70A is MOCP per manufacturer. The wire and loads were calculated per NEC. So its not a 70A feeder. The question here is more in regards of getting from a 400A breaker to a MOCP of 70A.
 
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