# Disconnects, hp ratings, another one goes poof!



## gpop (May 14, 2018)

What's the amp rating on the disconnect


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## Breakfasteatre (Sep 8, 2009)

It is 100a, 100hp @ 600v.

Tbh, 100a, 100hp disconnect rating is unusual, but in terms of the CEC here in Canada, it requires that the disconnect is rated for the horsepower of the motor, no mention of the amperage. 

I thought it was safe to say if it's rated for 600v 100hp that you can use it with a 100hp 600v rated motor...

Fla of motor is 92a


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## emtnut (Mar 1, 2015)

What is the Model # ?

Some of those Siemens 100A are 100hp, others are 100A 75hp


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## gpop (May 14, 2018)

What kind of starting. Across the line or softstart as this will affect the disconnect size.


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## joe-nwt (Mar 28, 2019)

I'm no expert but that doesn't look like a starting current issue? Looks like a prolonged overload/failure?


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## FaultCurrent (May 13, 2014)

When Siemens stopped using the old Clampmatic design in their switches and went to this new cheapo design I believe the trouble started. I won't use this new version.


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## Breakfasteatre (Sep 8, 2009)

It's on an AB 100hp HD 755 VFD. startup is real slow, like minutes to get up to full speed.

It is 100hp rated, marked on the disconnect itself. That is hp in the pic, not amperage. Although it is also rated 100amp which as I said before seems strange


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## gpop (May 14, 2018)

Vfd drops the voltage and ramps the amps until it's at speed. Disconnect is way to small for that


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## Breakfasteatre (Sep 8, 2009)

That's not what's happening when we observed it. I had an amp-probe on the leads after I made the temp fix and Amps slowly increased from around 25amps to 72 amps over the course of a couple minutes.

It was pumping water and it usually sees milk but there wasnt a discernible jump in amperage on start up


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## gpop (May 14, 2018)

Is this a pump or a homidginizer


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## just the cowboy (Sep 4, 2013)

Is that disconnect on the Primary of the drive or on the load side.

If load side could it be harmonics heating?
If load side is someone opening/closing disconnect as start/stop. I see interlock contacts on disconnect are they hooked up to stop drive?

Keep us posted
Cowboy


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## Breakfasteatre (Sep 8, 2009)

Yeah, aux is a break before main, to shut the VFD output off before disconnect, but as speaking to the operator, they don't use the disconnect to turn the motor off. Everything is done by hmi. Disconnect is for safety if they have to service it so they can lock out locally.

Tbh I don't know if it's a homogenizer. All I know is there is a huge cylindrical filter that it pushes the Milk through downstream.


Disc is located on load side of disconnect 

VFD cable was used, shield is isolated and continued through disconnect and bonded at motor terminal box and of course at the panel


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## gpop (May 14, 2018)

On the drive it has a parameter that tell you max amps which will be around 150 percent of fla amps. Disconnect needs to bigger than that especially if you are controlling the speed


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## Cow (Jan 16, 2008)

I've seen this several times, if you are around this long enough, you will too.

You need to bump your disconnect up a size. 

In my opinion, never max the HP rating of a disconnect out. On top of that, if it's an across the line motor, you may even need more cushion than. It all comes down to how often you want to replace disconnect guts? For instance, if it says max 75hp and you know you have a hard starting 50-60hp, I would strongly recommend installing a 200 amp. Or if you have a load that is consistently running near the top end of the disconnect rating, you need a larger disconnect. I don't care what the manufacturers say the disconnect is rated for. They are trying to sell disconnects that only have to survive through the warranty period.

I had a deal probably 10 years ago, I still remember it. It was a nonfused 100 amp Siemens disco feeding a 480v 60HP roller mill that was started across the line. That is a fair amount of rotational mass to get up to speed in those roller mills, obviously a lot of amps at startup. There was another identical 60HP mill and disco next to this one. 

The disconnect had the guts burned out of it. I read the label inside the door, just like you did, max 75hp. Hmm. Changed the guts. About a year later, the other disconnect for the adjacent mill did the same thing.

Wanting to stop this problem once and for all, I visited with the mill manager and talked him into replacing both mill disconnects with 200 amp. I haven't touched those disconnects since.

This is a common problem with disconnects, but nothing that can't be overcome by just going up a size. 

It may be worth your time to do some prevantative thermal imaging. I found a Sq D 200 amp disconnect last year running a 125HP VFD well pump with a similar issue. I only found it because I was doing some other work and happened to put my hand on the disconnect cover and it felt fairly warm. I opened the disco and used an infrared thermometer and saw one lug next to the knife blades was at 168 degrees F, the other lugs were around 110 F. I was lucky I caught that one before it went down and killed the water supply to his cows.


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## gpop (May 14, 2018)

Just to add to what cow has already said.

If you look at a disconnect you will hardly notice a difference in the contract points all the way from 30 amps to 100 amps. The box gets bigger but the contacts all seem around the same size. Ive often wondered if they just build a one size of guts and 100 is the max it can handle (very little safety margin left at that point).


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## wcord (Jan 23, 2011)

Looks like the load side of C phase was the start of the fire.
I've always sized my disconnects to handle both the HP and the starting amps. (250%).


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## Breakfasteatre (Sep 8, 2009)

i know this thread is almost 1 year old, but we ended up replacing both disconnects with 200a units. Luckily the enclosures were very similar in size as they are recessed within some framing on the skid.

no issues since


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## Norcal (Mar 22, 2007)

FaultCurrent said:


> When Siemens stopped using the old Clampmatic design in their switches and went to this new cheapo design I believe the trouble started. I won't use this new version.


The design is a lot like GE which is the problem.


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## CMP (Oct 30, 2019)

GE design has been junk for decades.


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## Norcal (Mar 22, 2007)

CMP said:


> GE design has been junk for decades.


Eaton & Siemens have made their products similar to GE.


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## JRaef (Mar 23, 2009)

Breakfasteatre said:


> i know this thread is almost 1 year old, but we ended up replacing both disconnects with 200a units. Luckily the enclosures were very similar in size as they are recessed within some framing on the skid.
> 
> no issues since


What kind of wire is that? If it is high strand count "extra flexible", like DLO or RHH/RWW cable, that is a possible issue. I see that you use crimp lugs, but for Class K stranded conductors, you need specific lugs and crimp dies.


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## Breakfasteatre (Sep 8, 2009)

JRaef said:


> What kind of wire is that? If it is high strand count "extra flexible", like DLO or RHH/RWW cable, that is a possible issue. I see that you use crimp lugs, but for Class K stranded conductors, you need specific lugs and crimp dies.


It is not extra flexible, but it is fine strand. 

I ordered the right lugs for fine strand and borrowed the supply house's crimper


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