Jim Ingram

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jimingram

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I will be installing a feeder for two identical roof top air conditioners. I obtained some data for the A/Cs from a salesman. It was not quite a nameplate, but it's useful. I want to make sure I'm properly crunching the numbers.

The data sheet lists minimum current amps (MCA) and Maximum Overcurrent Protection. I needed the rated load current (RLC). I worked backwards from the MCA to get the RLC.

EG. At 208 volts, 3P, the MCA is 145.5 amps. The maximum overcurrent protection (MOCP) is 250. Here's my math:
(MCA) 145.5 amps - 8 amp fan load= 138.5. 138.5/1.25=110 amps (RLC). The MOCP doesn't work out for that number.

The feeder for the two units would be 110X125% + 8(fans)= 145.5 (amps) + 110 (RLC) + 8 (fans)= 263 amps. Voltage drop will be calculated later.
Expressed another way: 1st unit 145.5amps
2nd unit 110 amps
fans 8 amps
total 263 amps

The feeder would be fused at 350 amps. (MOCP) 250 + 118 (RLC + fans) =368. Next size fuse down 350 amps.
MOCP 1st unit 250 amps
2nd unit RLC + fans 118 amps
total 368 amps
Let me know what you think of my math.

Jim
 
jimingram said:
I will be installing a feeder for two identical roof top air conditioners. I obtained some data for the A/Cs from a salesman. It was not quite a nameplate, but it's useful. I want to make sure I'm properly crunching the numbers.

The data sheet lists minimum current amps (MCA) and Maximum Overcurrent Protection. I needed the rated load current (RLC). I worked backwards from the MCA to get the RLC.

EG. At 208 volts, 3P, the MCA is 145.5 amps. The maximum overcurrent protection (MOCP) is 250. Here's my math:
(MCA) 145.5 amps - 8 amp fan load= 138.5. 138.5/1.25=110 amps (RLC). The MOCP doesn't work out for that number.

The feeder for the two units would be 110X125% + 8(fans)= 145.5 (amps) + 110 (RLC) + 8 (fans)= 263 amps. Voltage drop will be calculated later.
Expressed another way: 1st unit 145.5amps
2nd unit 110 amps
fans 8 amps
total 263 amps

The feeder would be fused at 350 amps. (MOCP) 250 + 118 (RLC + fans) =368. Next size fuse down 350 amps.
MOCP 1st unit 250 amps
2nd unit RLC + fans 118 amps
total 368 amps
Let me know what you think of my math.

Jim
Click to expand...

do you have 480 3 ph available?

145 amps on 208 3 ph ,would be 62amps on 480 volts 3ph
 
What size conductors are you figuring to use? 600 KCM or parallel 4/0?

Using the name plate, I figure separate feeders with a set of 3/0 for each unit due to the 145.5 min amps, and a 250-amp OCP device.
 
Fwiw

Fwiw

I would use the MCA from the units.
145A x 2

Another Journeyman I know always uses the Max OCP for his wire sizing.
250A x 2
 
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