Elevator motor breaker and fuse

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Electromatic

Electromatic

Senior Member
Location
Virginia
Occupation
Master Electrician
I'm wiring a machine room-less elevator whose overall controller nameplate states 65 running amps, 20HP. I believe the conductor ampacity needs to be at least 125% = 81A. I plan on using #4Cu on a 90A breaker. When it comes to the required fused disconnect, it looks like I can go to a maximum of 175% TD fuse = 114A. Is there any reason to size the fuse and breaker in relation to one another? I could still use a 90A or 100A fuse, right?

Also, if it looks like I'm calculating something wrong in general, please let me know.
Thanks as always
 
I think I'd use table 430.52 to select a 2.5 * 65 = 162.5 (150-amp mcb) with a 3 * 65 ~ 200-amp fuse. The overloads will protect the cable at ampacity. I'm not sure about the roundup for the fuse, but it's in that section somewhere. If your fault current is less than 10kA you might want to consider an unfused switch.
 
Will I be okay at 90A and #4, though? I know that's kind of the bare minimum, but I'm trying to save some cost. The specs call for a fused disconnect, and we were planning on one with the shunt trip built in. Going over 100A upsizes not just the conductors but the gear too.
The 65A nameplate is for the overall controller which looks to have a 55A VFD (its nameplate is a bit confusing) plus other circuitry.
 
Electromatic said:
Will I be okay at 90A and #4, though? I know that's kind of the bare minimum, but I'm trying to save some cost. The specs call for a fused disconnect, and we were planning on one with the shunt trip built in. Going over 100A upsizes not just the conductors but the gear too.
The 65A nameplate is for the overall controller which looks to have a 55A VFD (its nameplate is a bit confusing) plus other circuitry.
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The only reason I went to 250% is to allow for tolerance of starting current, but if you have a VFD then the inrush will be limited and your 90-amp breaker should be okay.
 
With motor circuits including VFDs the overload protection relay (built into the VFD) is your overload (thermal) protection. All you are providing is short circuit protection. So the sizing may look crazy but it’s just fine.

The 125% factor is there to account for temporary surges that occur. I’ve never seen any kind of Neher-McGrath modeling behind it…it just works.
 
Be very careful with elevators. The elevator inspectors are a pain. Also, you need to read the elevator manual first. In my experience elevator feeders are often sized larger than code because that is the manufacturer's recommendation.
 
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