I have a 800a 480v 3p MDP in a boiler room that feeds about 200 feet to a penthouse for hvac equipment. The wires are 500kcmil type TW 60degC in parallel, for a total of 640amps. 3 in each pipe. I don't want to replace the wires but the loads are one small pump moter that uses 20 amp ocpd ,and two rtu's that have a marked MCA of 338a. The total is now 696a.I don't think you can do this but i thought i would ask, can i use 240-3 to use 700a fuses to supply this and be legal?thanks, john
wire load
- Thread starter jociha
- Start date
- Status
- Location
- Illinois
- Occupation
- retired electrician
Re: wire load
The 700 amp OCPD would be fine to protect the wire, but the wire is too small for the load. See 215.2(A)(1).
Don
The 700 amp OCPD would be fine to protect the wire, but the wire is too small for the load. See 215.2(A)(1).
Don
Re: wire load
What if the MCA of each rtu was 310a, then the total load would be 640a. I believe this would than be legal. This wire would be feeding a 800a panelboard in the penthouse with 1 20a breaker for the small moter and 2 400a breakers for the rtu's. The units say 400a MOCP. This would than all be protected by the 700a fuses in the boiler room. The problem i have with this is it would imply that looking at the 700a fuses someone would think they could add another breaker to the panel and add another load to it. john
What if the MCA of each rtu was 310a, then the total load would be 640a. I believe this would than be legal. This wire would be feeding a 800a panelboard in the penthouse with 1 20a breaker for the small moter and 2 400a breakers for the rtu's. The units say 400a MOCP. This would than all be protected by the 700a fuses in the boiler room. The problem i have with this is it would imply that looking at the 700a fuses someone would think they could add another breaker to the panel and add another load to it. john
Re: wire load
Since this load is considered continuous, you must use a 1.25 multiplier. If MCA is the minimum circuit ampacity then the 1.25 factor is usually included. If the units are marked requiring a circuit capacity of 338 amps, then you must provide that capacity. Your existing conductor is only good for 320 amps. If each units FLA is
338 amps x 2 = 676 x 1.25 = 845 amps. As Don says you don't have enough wire even if you drop the pump. How do you reduce the load from the
338 amps to 310 amps? Using the 310 amps you have
310 x 2 x 1.25 = 775. You still do not have the proper wire size. Looks like you need another ckt
or increase the 2 you have. Are these new units?
A 400 amp breaker will not allow the units to start without tripping the breaker. You are allowed to install a breaker that is 1.75 times the FLA of the unit.
[ February 29, 2004, 04:34 PM: Message edited by: bob ]
Since this load is considered continuous, you must use a 1.25 multiplier. If MCA is the minimum circuit ampacity then the 1.25 factor is usually included. If the units are marked requiring a circuit capacity of 338 amps, then you must provide that capacity. Your existing conductor is only good for 320 amps. If each units FLA is
338 amps x 2 = 676 x 1.25 = 845 amps. As Don says you don't have enough wire even if you drop the pump. How do you reduce the load from the
338 amps to 310 amps? Using the 310 amps you have
310 x 2 x 1.25 = 775. You still do not have the proper wire size. Looks like you need another ckt
or increase the 2 you have. Are these new units?
A 400 amp breaker will not allow the units to start without tripping the breaker. You are allowed to install a breaker that is 1.75 times the FLA of the unit.
[ February 29, 2004, 04:34 PM: Message edited by: bob ]
Re: wire load
First of all the MCA i stated has the 125% included and i can drop the 338MCA down to 310MCA because each unit has a power exhaust that draws 28a and i am trying to explain to our mech. engineer to drop the power exhaust and how close we are to pulling new wire. john
First of all the MCA i stated has the 125% included and i can drop the 338MCA down to 310MCA because each unit has a power exhaust that draws 28a and i am trying to explain to our mech. engineer to drop the power exhaust and how close we are to pulling new wire. john
- Location
- Massachusetts
Re: wire load
Are they getting eliminated, fed from another source?
I do not think you are going to find a miracle to make this wire carry the load within the code requirements.
John why do you feel you can drop the two 28 A exhaust motors?
Are they getting eliminated, fed from another source?
I do not think you are going to find a miracle to make this wire carry the load within the code requirements.
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