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Old 11-04-2019, 03:10 PM   #1
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Default Safety switch rating

Question for the gear gurus:
A Seimens 100A heavy duty NF safety switch is rated for 75HP at 480v. Is it advisable to use one on a 75hp blower motor rated 85 amps? Are they built heavily enough that they will take that load and last for years or should I go up a size?
If it would be on a machine that the load varied a lot i probably wouldn't be concerned, but I'm assuming the blower manufacturer has built the blower to use all the power available from that motor.

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Old 11-04-2019, 07:01 PM   #2
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Originally Posted by Forge Boyz View Post
Question for the gear gurus:
A Seimens 100A heavy duty NF safety switch is rated for 75HP at 480v. Is it advisable to use one on a 75hp blower motor rated 85 amps? Are they built heavily enough that they will take that load and last for years or should I go up a size?
If it would be on a machine that the load varied a lot i probably wouldn't be concerned, but I'm assuming the blower manufacturer has built the blower to use all the power available from that motor.

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Let's be clear on one thing. It's a safety DISCONNECT, not a safety CONNECT. So you CAN open these things in an emergency to shut down the motor but don't expect to be load breaking and making all the time or it can and will burn up. It is NOT intended as a starter. I have had (stupid idiot production worker) friends burned that way.


So except for emergencies you will not be using it to stop or start a motor. It's just a "bus bar" for 99% of it's life. So realistically this is a heat argument not a "big enough" argument. In fact I think you are screwing with people and simply using the NEC tables, but using your example...



Second, if it's rated in HP, stop there. It is so it is rated up to 75 HP. The HP ratings that you see on starters and disconnects are tied to the tables in Article 430 which are the largest current ratings allowed by NEMA based on HP. See how it all ties together? NEMA specs out the motors, and NEC is simply copying the worst case numbers on those tables. And the disconnect is rated based on the NEC/NEMA tables when it gives a horsepower rating.



If you buy a cheap junk switch it might not have a horsepower rating. In this case 430.109 applies. I don't know your locked rotor code or the short circuit duty rating so can't go any further here. 430.110(A)(1) requires 115% rating on the disconnect which works out to 87 A. At 85 A it passes but if I want to make it "generic" and be able to substitute motors then looking at the NEC FLC tables, a 75 HP motor is rated for a maximum of 96 A and the disconnect would be under-rated. But since it is already rated at 75 HP we aren't going here.


There are some goofy things with disconnects in general. A nonfused disconnect is rated for 100% of the load whether it is continuous or noncontinuous duty. A fused disconnect is rated for only 80% of the continuous load and 100% of the noncontinuous load, just like fuses and breakers. This is the usual goofy 125% breaker rule rearing it's head again.
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Old 11-12-2019, 04:57 PM   #3
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Originally Posted by paulengr View Post
Let's be clear on one thing. It's a safety DISCONNECT, not a safety CONNECT. So you CAN open these things in an emergency to shut down the motor but don't expect to be load breaking and making all the time or it can and will burn up. It is NOT intended as a starter. I have had (stupid idiot production worker) friends burned that way.

...
Actually, not true if it is a UL98 listed NF disconnect, such as the Knife Switch type (in Siemens parlance, the VB-II series). The UL 98 testing protocol is to Make and Break 115% of the the motor FLC 6,000 times minimum, at a rate of 6 times per minute (10 seconds per cycle).

The cheaper little IEC rotary disconnects are only listed at 100% of the motor FLC, but are themselves also listed as Manual Motor Starters under UL 508, so they too are rated to open and close under motor load.

Bottom line Forge Boyz, it's totally legit to use a UL 98 listed NF fused disconnect as a way to disconnect a motor under load, they are rated for that, so long as you don't exceed the listed HP rating (which is required by the NEC anyway).
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Old 11-12-2019, 07:55 PM   #4
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My question wasn't about a lot of make/break events. It was simply a question of, in your experience, is running a disconnect at the its full horsepower rating a good idea or not. There is a soft starter on the motor so the disconnect will only be used in an emergency.

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Old 11-12-2019, 08:24 PM   #5
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Originally Posted by Forge Boyz View Post
My question wasn't about a lot of make/break events. It was simply a question of, in your experience, is running a disconnect at the its full horsepower rating a good idea or not. There is a soft starter on the motor so the disconnect will only be used in an emergency.

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Not a problem.

Most fail due to loose connections or contamination (that includes seized mechanisms or rusted springs)
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