fuse and wire sizing for multiple motors and other loads on 1 circuit

[Tainted]

If I understand how to size wire and overcurrent protection if 1 circuit contains only motors. But what if it contains other loads like a heater for example.


Suppose I have one 25HP motor (74.8A FLC), one 10HP motor (30.8 FLC) and one 5kW heater (13.9A). All loads are continuous at 208V 3phase.

minimum wire size would be:

74.8(1.25) +30.8 + 13.9(1.25) = 141.7 amps -> 1/0 copper

Fuse size would be:

74.8(1.75) + 30.8 + 13.9(1.25) = 179amp ->175 amp fuse?

Since a heater is associated with the circuit, can we use 1/0 copper for the circuit or must it be 2/0?

is the calculation correct?

 

[ptonsparky]

Feeder is 125% of largest plus 100% of others
93.5+44.7=138.2

Don't add the 25% of the heat load to either calculation.
Doesn't make any difference in the wire size or the fusing.
I believe you could increase the fuse size to 225 if needed. 430.53(C) Exception No. 2, (2)

Expect corrections

 

[infinity]

Fuse size would be:

74.8(1.75) + 30.8 + 13.9(1.25) = 179amp ->175 amp fuse?

Is this going to be a feeder? How are you providing overcurrent protection for each individual load? Are there going to be taps at each piece of equipment?

 

[Tainted]

Is this going to be a feeder? How are you providing overcurrent protection for each individual load? Are there going to be taps at each piece of equipment?

Yes it’s a feeder. Each equipment will tap the feeder

 

[wwhitney]

Feeder is 125% of largest plus 100% of others
93.5+44.7=138.2

Don't add the 25% of the heat load to either calculation.

430.24 on conductor sizing says to use a 125% factor for continuous loads, and the heater was specified as a continuous load. Likewise 215.3 specifies a 125% factor on the OCPD for a continuous load (unless the OCPD is 100% rated).

Cheers, Wayne

 

[wwhitney]

Suppose I have one 25HP motor (74.8A FLC), one 10HP motor (30.8 FLC) and one 5kW heater (13.9A). All loads are continuous at 208V 3phase.

minimum wire size would be:

74.8(1.25) +30.8 + 13.9(1.25) = 141.7 amps -> 1/0 copper

Yes.

Fuse size would be:

74.8(1.75) + 30.8 + 13.9(1.25) = 179amp ->175 amp fuse?

That method seems rational, but it's not what 430.63 says, which is a bit weird.

Cheers, Wayne

 

[kwired]

Yes.


That method seems rational, but it's not what 430.63 says, which is a bit weird.

Cheers, Wayne

full rating of each load plus extra for the largest starting current is all that is needed. The water heater doesn't have a starting current that is more than running current and only needs to be taken at 100% when determining overcurrent protection.

 

[wwhitney]

full rating of each load plus extra for the largest starting current is all that is needed.

That's makes sense, but that's not exactly what 430.62 and 430.63 say. They say, in effect, to take the largest motor, determine the OCPD size permitted for it (which includes going up to the next standard size per 430.52(C)(1) exception 1), and then add to that the rating of the other loads (including 125% for continuous loads, since 215.3 specifies that). Then if the load is motors only, 430.62 says the OCPD shall not exceed the calculated value if the conductors are sized per 430.24. And if the load is mixed, 430.63 says that the OCPD shall be "not less than" the calculated value, without mention of how the conductors were sized. [That "not less than" has always struck me as off, it should be "not more than".]

The water heater doesn't have a starting current that is more than running current and only needs to be taken at 100% when determining overcurrent protection.

It is not in evidence that the load is a water heater. It was described in the OP as a 5kW heater, and as a continuous load. I am taking that at face value, and not second guessing whether it is a continuous load.

Cheers, Wayne