Heating equipment calc 424.3b

Status
Not open for further replies.
J

jimhairston

Member
In-house electrical engineer at large national a/c manufacturer is not applying "continuous duty" requirement found in 424.3b. The situation is as follows:

Separate feeder to - 117kw heater at 460v
(117000/360=146.85 1/0 or x 1.25=182a 3/0)
note: the equipment only has terminals for 2/0
1. What is the size of the branch ckt conductor & ocp?

Other issues:
424.22b max 48amp load/ocp60a
2. How many subdivided loads are required?
3. Does the NEC apply to the internal conductors inside
of the heater unit?
4. When can 310.16 column three 90 degree be used?
 
I am not sure why they only have terminals for 2/0 unless this unit can be fed by more than one circuit. The circuit does need 3/0 if it is a one feed unit. The 90 C column can only be used for derating unless the terminals and breakers along the entire circuit are label 90C. I doubt you will find that to be true. 75C column is what we are usually looking at.

I don't believe the NEC would be applicable inside the unit.
 
Follow Up To 424.3b

Follow Up To 424.3b

Does Anyone Know Of A Circumstance Where The "contunuous Load" Requirment is not applied?

When A Heater Is Greater Than 50kw Can The Branch Circuit Conductor Be Sized At 100% Of The Rated Amperage
 
It seems as if 422.22(D) applies if the installation can meet all 3 conditions. My guess is the (D)(3) would keep it from ever being a continuous load.
 
117kw/(.48x1.73)= 141 amps x 1.25 = 176amps. 2/0 = 175 amps. Pretty close. I believe the code uses 480 volts. Is this a branch ckt or feeder?
The equipment should have the info on the nameplate.
 
bob said:
117kw/(.48x1.73)= 141 amps x 1.25 = 176amps. 2/0 = 175 amps. Pretty close. I believe the code uses 480 volts. Is this a branch ckt or feeder?
The equipment should have the info on the nameplate.
Click to expand...

I believe you don't need the 1.25 in the equation based on article 422.22(D). If you did 176amps is over the rating of 2/0. Pretty close doesn't get it.

141 amps means a 1/0 conductor may be run-- at least that's how I see it. 2/0 for voltage drop perhaps.
 
Dennis Alwon said:
I believe you don't need the 1.25 in the equation based on article 422.22(D). If you did 176amps is over the rating of 2/0. Pretty close doesn't get it.

141 amps means a 1/0 conductor may be run-- at least that's how I see it. 2/0 for voltage drop perhaps.
Click to expand...

I agree with you that 422.22(D) says 100%. I am aware that 176 amps "doesn't get it" but I was just making to the point since the MFG apparently supplied 2/0 cu treminals.
 
Status
Not open for further replies.
Top