Motor Disconnect

[Steelhead]

I need an unfused safety switch for a 50hp 3ph 480V motor. The FLC for a 50hp motor is 65A, but the disconnect I want to use is rated 60A at 600VAC, 50HP at 480VAC 3PH. Can I use this switch or do I need one with a higher ampere rating? The ampere rating and horsepower rating is what's confusing me.

 

[LarryFine]

I believe the HP rating trumps.

 

[Jraef]

Huh. Interesting. I would not have thunk it, but I looked up a Sq. D 60A 600V NF disconnect and sure enough, it's rated for 50HP at 480V, even though the FLC on a 50HP motor is 65A.

Line Rated Current
60 A

Enclosure Rating
NEMA 3R/12 galvannealed steel

Horsepower Rating
60 hp 600 V AC
50 hp 480 V AC

Disconnect Type
Non-fusible disconnect

Short Circuit Current Rating
10 kA

My guess is that they don't really make a 60A NF switch, it's actually a 100A switch mechanism, so it passed the test for a 50HP motor when unfused. It's called 60A though because if it were FUSIBLE, it will come with 60A fuse clips, in which case the max motor HP drops to 30HP.

 

[LarryFine]

I'm sure it means either/or; i.e., up to 60a (non-motor) or 50 HP (motor).

What's the FLC on a 60HP 600v motor?

 

[Cow]

I wouldn't do it.

Disconnect switches lead much shorter lives when they are loaded that heavy in my experience. Whether following manufacturer ratings or not.

 

[LarryFine]

Disconnect switches lead much shorter lives when they are loaded that heavy in my experience.

Doesn't that depend on whether and how often it's operated under load, all other things being equal?

 

[Cow]

Doesn't that depend on whether and how often it's operated under load, all other things being equal?

The disconnects I've replaced did not appear to be cycled very often, they were maintenance disconnects. Every one of them started getting hot right where the knife blades rock into their holders.

 

[Steelhead]

Based on your responses I'm going to assume that using the 60A switch rated at 50hp 480V is Code permissible, but obviously going larger wouldn't hurt.