PhaseShift
Senior Member
I am calculating the size for a feeder circuit to feed (4) 5kW heaters at 240V. Each heater has a rated current of 21A.
We will have a feeder circuit fed from a disconnect with the feeder circuit feeding a 120/240V panel. The panel will then have individual branch circuit to feed each of the (4) heaters.
Section 424.3(B) states that heaters must be treated as a continuous load and thefore each branch circuit must be sized at 125%. So in my case the branch circuit for each heater will be 21A*1.25= 26.25A. Therefore each heater will have a #10AWG fed from a 30A OCPD.
For the feeder circuit to calculate the toal load I add up all (4) heaters and then take 125% of this value because they are treated as continous loads. Doing this I get 21A *4 * 1.25 = 105A. This tells me that the feeder circuit must have an ampacity of at least 105A. Is this correct? Is there anything in the code that allows this to be rounded down to 100A or must it be 105A at a minimum.
Assuming feeder circuit must have an ampacity of 105A I was going to use a disconnect rated larger than 100A. I'm not sure what the next size up is but I'll assume there is a disconnected rated for 125A. So I will use a 125A rated disconnect with 110A fuses since feeder cable will be existing #2AWG cable. Fuses in disconnect must be 100A in order to meet size of load as well a protect #2 cable rated at 115A. The 120/240V panel has a rating of 200A with a 200A MCB.
Does my above design sound correct?
We will have a feeder circuit fed from a disconnect with the feeder circuit feeding a 120/240V panel. The panel will then have individual branch circuit to feed each of the (4) heaters.
Section 424.3(B) states that heaters must be treated as a continuous load and thefore each branch circuit must be sized at 125%. So in my case the branch circuit for each heater will be 21A*1.25= 26.25A. Therefore each heater will have a #10AWG fed from a 30A OCPD.
For the feeder circuit to calculate the toal load I add up all (4) heaters and then take 125% of this value because they are treated as continous loads. Doing this I get 21A *4 * 1.25 = 105A. This tells me that the feeder circuit must have an ampacity of at least 105A. Is this correct? Is there anything in the code that allows this to be rounded down to 100A or must it be 105A at a minimum.
Assuming feeder circuit must have an ampacity of 105A I was going to use a disconnect rated larger than 100A. I'm not sure what the next size up is but I'll assume there is a disconnected rated for 125A. So I will use a 125A rated disconnect with 110A fuses since feeder cable will be existing #2AWG cable. Fuses in disconnect must be 100A in order to meet size of load as well a protect #2 cable rated at 115A. The 120/240V panel has a rating of 200A with a 200A MCB.
Does my above design sound correct?