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

Tainted

Tainted

Senior Member
Location
New York
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Engineer (PE)
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?
 
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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
 
Tainted said:
Fuse size would be:

74.8(1.75) + 30.8 + 13.9(1.25) = 179amp ->175 amp fuse?
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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?
 
ptonsparky said:
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.
Click to expand...
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
 
Tainted said:
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
Click to expand...
Yes.

Tainted said:
Fuse size would be:

74.8(1.75) + 30.8 + 13.9(1.25) = 179amp ->175 amp fuse?
Click to expand...
That method seems rational, but it's not what 430.63 says, which is a bit weird.

Cheers, Wayne
 
wwhitney said:
Yes.


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

Cheers, Wayne
Click to expand...
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.
 
kwired said:
full rating of each load plus extra for the largest starting current is all that is needed.
Click to expand...
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".]

kwired said:
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.
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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
 
Tainted said:
what would be the rational and correct way of doing it though. I am confused what the fuse size should be for the feeder
Click to expand...
Well, 430.63 says to take the minimum OCPD size for the non-motor loads, which in your example would be 13.9A * 1.25 for your 3-phase continuous heater load of 5 kW. And add to that the value determined by 430.62 for the two motors.

And 430.62 says to take the largest motor, determine the largest OCPD allowed per 430.52, and add to that the other motor. So if a 175% factor will permit motor starting, the largest fuse size per 430.52 for the 74.8 FLC motor would be 130.9A, but exception one says you may round up to the next standard size OCPD, which is 150A.

So our final value is 150A + 30.8 + 13.9*1.25 = 198A. And 430.63 says the OCPD shall be not less than this. So the smallest allowable standard size is 200A.

I guess you could take the point of view that the use of 430.52(C)(1) Exception 1 is optional. Then the final value would be your calculation of 179A, but still the smallest allowable standard size would be 200A.

I do not understand why 430.63 uses the "not less than language" rather than saying "not more than."

Cheers, Wayne
 
wwhitney said:
So our final value is 150A + 30.8 + 13.9*1.25 = 198A. And 430.63 says the OCPD shall be not less than this. So the smallest allowable standard size is 200A.
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Shouldn't the max allowed fuse size be 175 amps because 430.62 says this in bold:

"shall be provided with a protective device having a
rating or setting not greater than the largest rating or setting
of the branch-circuit short-circuit and ground-fault protective
device for any motor supplied by the feeder [based on
the maximum permitted value for the specific type of a
protective device in accordance with 430.52, or 440.22(A)
for hermetic refrigerant motor-compressors], plus the sum
of the full-load currents of the other motors of the group."
 
Tainted said:
Shouldn't the max allowed fuse size be 175 amps because 430.62 says this in bold:

"shall be provided with a protective device having a
rating or setting not greater than the largest rating or setting
of the branch-circuit short-circuit and ground-fault protective
device for any motor supplied by the feeder [based on
the maximum permitted value for the specific type of a
protective device in accordance with 430.52, or 440.22(A)
for hermetic refrigerant motor-compressors], plus the sum
of the full-load currents of the other motors of the group."
Click to expand...
Actually 430.63 says it should not be less than so nevermind.

So it would be ok to feed 1/0 copper rated 150 amps with a 200 amp fuse then?
 
My thinking would go along with Post #10 values but limit the OCP device to 175 amps.
 
I am so accustomed to 430.62 wording of "not greater than" I thought the same applied to 430.63 but I see your point.
That said, as written where is the OCP device limit in 430.63 since it only states "not less than" ?
 
augie47 said:
That said, as written where is the OCP device limit in 430.63 since it only states "not less than" ?
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Apparently there is none? Install as large an OCPD as you like?

Or another interpretation is that the language in 430.63 does not actually amend 240.4's basic requirement, while the language in 430.62 does. So that would mean that any mixed feeder with motors and other loads must have a conductor ampacity that matches the OCPD size (or complies with 240.4(B)).

Neither result seems reasonable.

Cheers, Wayne
 
wwhitney said:
Apparently there is none? Install as large an OCPD as you like?

Or another interpretation is that the language in 430.63 does not actually amend 240.4's basic requirement, while the language in 430.62 does. So that would mean that any mixed feeder with motors and other loads must have a conductor ampacity that matches the OCPD size (or complies with 240.4(B)).

Neither result seems reasonable.

Cheers, Wayne
Click to expand...
I haven't found any examples online at all either
 
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