New service for Machine Tool installation.

[mvokes]

We have a CNC mill that has a machine nameplate of 87 amps FLA three phase 480volts. Initially, I specked the following:

wire size = 1.25 x 87 = 108.75 amps = 2AWG copper
EGC = 6AWG copper
Busplug with DE fuses = 1.75 x 87 = 152.25 amps = 200 amp busplug with 175amp (rounded up to next standard size) DE fuses
1 1/2" EMT from machine to busplug (less than 50 feet)

I'm told that the busplug and dual element fuse size is incorrect by our maintenance electrician and that we can use a 100 amp busplug with 100 amp dual element fuses.

His reasoning is that the fuse size is a "maximum" at 1.75 and this should be calculated at 1.25 which still gives us 108.75 amps but we round down in size to the lower standard fuse size of 100 amps.

I have attended two code update classes and we have always used the 1.75 in the calculations for a dual element fuse.

My questions are, can we start at 1.25 and go to the maximum of 1.75 for a dual element fuse and do we round up or down for the standard fuse size?

 

[wirenut1980]

What is the locked rotor amps of the machine? The 175% fuse size is a maximum per Table 430.52 although not the be all, end all maximum (see Exception 2(b) for your situation), and can be lowered if it will allow the machine to start without nuisance blowing.

The 1.25 times the FLA is more for conductor ampacity sizing and not for OCPD.

Remember to calculate the conductor ampacity (factoring in adjustments) and then choose an OCPD that will protect it.

 

[hmspe]

wire size = 1.25 x 87 = 108.75 amps = 2AWG copper

75degC terminals throughout? If not, 110.14 would have you use the 60degC column, and #1 copper would be required at 108.75A.

Also, most CNC machines that I've worked with are multi-motor. If so you'd want to take the largest motor's FLA at 125% then add the remainder and the controls at 100%, not take the entire load at 125%.

If the equipment manufacturer's requirements are available I'd use those requirements rather than trying to calculate anything.

 

[mvokes]

All our machine tools are self contained and all OCPDs are in the machine cabinet and taken into consideration when they figure out the total FLA and as such it is included on the machine nameplate.
Our job is to run power from the bus bar over to the machine disconnect. We just need to know what size busplug with fuses to put on the bus bar and the conduit and wire size to run to the machine.
Let me re-phrase my two questions, can we use the smaller 125% for the short circuit protection of the wires going from the machine to the busplug? and do we round up or down for the next closest fuse size?

 

[davidr43229]

My questions are, can we start at 1.25 and go to the maximum of 1.75 for a dual element fuse and do we round up or down for the standard fuse size

You round up....
I would caution that depending upon the Class of the fuse ( RK-5 vs RK-1) an RK-1 may not hold at 100amp. You have just sized that at 115%, not 125%.
For RK-5, I would start at 125% to 175% and for RK-1, I would start at 130% to 175%.
Dual Element- time delay fuses have many applications. whoever told you to size them at 175%, whoever the instructor was, told you wrong. 175% is the max that you can go, with few exceptions.
With your bussplug at 100amps and your disco is at 100 amps, it will be difficult to predict which OCPD activates during a shortcircuit.
Just my $.02