Time- delay fuses and Motor Load Calculations 1

[InnovationT]

We Recently have a disagremment with the enginers of Ingersoll-Rand when in a installation of a new compressor, our calculations take place for a 25 HP/3ph/208 V Design B Motor; need Time-Delay Fuses of 70 A and #4 THHN AWG Copper Conductor to feed this compresor; when they came with #6 AWG size conductors. What do you think about it?

 

[peebee]

I'm confused. NEC Table 430.150 indicates a 25 hp 3-ph 208v motor has an FLA of 74.8 amps. It seems to me all the sizes you listed are too small. Am I missing something? What's the nameplate FLA?

 

[bhwtam]

InnovationT,

Did you take into the account of allowable voltage drop and other derating factors when you reach your cable sizing?

It is difficult to comment who is correct as information given is limited. What is the cable length and cable installation method?

 

[Crowbar1]

Seems to me your 25H 208V motor would need #2AWG Copper conductors and 125A Time-Delay fuses. Conductors sized for 125% FLA and Fuses sized at 175%.

 

[bhwtam]

Crowbar1,

If your fuse ratings are larger than the capacity of your conductors, then the fuses won't be able to protect your conductors.

 

[peebee]

bhwtam, crowbar is dead on. The fuses provide short-circuit protection only, and they'll do that adequately for both the motor and motor feeders sized as crowbar indicates. The downstream motor overload relay provides overload protection for both the motor and feeders. Crowbars sizes are per NEC. The text of the NEC admittedly does not make this as clear as it could be, but the NEC handbook eliminates any doubt.

 

[peebee]

Despite my last post, keep in mind that the motor feeder may still need to be increased in size to address voltage drop (as previously mentioned by bhwtam).

 

[jbartos]

Suggestion: If one uses fuses the conductor insulation temperature Tc may be 90 deg C instead of 60 deg C. No 4 AWG copper could be sufficient if the motor feeder is short and upstream condutors are sufficiently large. However, the voltage drop calc could be performed to verify the voltage drop for the motor steady state operation and for the motor starting condition. Aslo, the motor LRA needs to be taken into consideration.

 

[peebee]

I disagree with your statement regarding the 90C conductors, jbartos, or perhaps I misunderstand. The way I see it, 90C ampacity can not be used in this installation as the terminals at which the wires land are not rated for that temperature. 90C wiring can be used, but it cannot be loaded to a 90C ampacity.

 

[jbartos]

Suggestion: 90 deg C conductors may be used with fuses but not with certain other devices, e.g. circuit breakers (see UL stds). Fuses and fuseholders often become hot; therefore, 90 deg conductor insulation is more suitable. This is not the case for the circuit breakers by UL standards.

 

[dpc]

The use of 90 deg C conductors depends on the rating of the terminations at **both* ends of the circuit, regardless of the overcurrent protection used.

If this is a "package system" with motor starter and controls provided by I-R then you will have to use whatever overcurrent protection they have obtained their UL listing based on. This is a common "feature" of HVAC equipment where the UL listing is based on the use of an upstream fuse. In these cases, a fuse MUST be used to meet NEC/UL requirements, regardless of any technical issues. There are thousands of HVAC packages fed from a circuit breaker in a switchboard that also have a *fused* disconnect at the equipment.