# 150HP motor 200A disconnect



## MDB1988 (Nov 13, 2017)

I've come across a 150HP 480V motor driving a centrifugal pump that's being manually line started and stopped with a 200A Eaton disconnect rated for 125HP with 200A time delay fuses in it. Next to it is another pump with the same setup except it is started with an autotransformer starter controlled by floats and the disconnect is only used as a maintenance lockout. I believe the first pump had a starter at one point as well but it seems to have failed and been removed/bypassed.


Obviously the first pump has a number of issues; undersized disconnect, no thermal overloads, dangerous method of starting/stopping. But is the second pump ok since the disconnect is after the starter and shouldn't see full starting current?


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## Cow (Jan 16, 2008)

I've replaced several disconnects such as this over the years that just didn't last as long as they should have in my opinion.


Being that the motor in your situation is rated higher than disconnect, automatically means you need to move up to the 400 amp disconnect category. A 400 should last a long time with a 150HP on it.


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## gpop (May 14, 2018)

I wouldn't be to concerned with the second as a 150hp name plates around 180 amps. (if you are into the 15% service factor then its over 200amps so it should be replaced).

The first is a problem as fuses being used as overloads which means that when one pops it has to single phase for a while before it takes out the second fuse. 

If the disconnect is load break rated then technically it should be able to break the load to the motor but in real life its not expected to do this very many times. 

Looks like it due for a complete rebuild.


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## 460 Delta (May 9, 2018)

That’s a lot sketchy but I’ve seen it done many a time and I cringe every time. The best fix is a nema 5 combination starter, one unit and done.


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## MDB1988 (Nov 13, 2017)

The starters are in another building 250' away. The disconnects are on the outside wall of the little pump house as there's no room for them inside.


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## paulengr (Oct 8, 2017)

NEC is pretty clear so let's step back a minute. You need a lockable disconnecting device. Starters can and do weld themselves shut. Second you need short circuit protection which is fuses or a circuit breaker or a motor circuit protector (MCP), which is really a breaker with magnetic (instantaneous trip) only, which is what some companies call them. Third you need overload protection which cannot be done with fuses. Finally somewhere in there you need a start/stop mechanism.

Traditionally in the NEMA general purpose system the order is:
MCP or fused disconnect
Starter
Overload relay
Optional secondary disconnects
Motor

In the IEC approach the order is:
MCP and overload combined into a manual motor starter
Starter
Optional secondary disconnects
Motor

In medium voltage it's usually:
Fused disconnect, disconnect is not load breaking
Starter interlocked to disconnect
Overload relay and ground fault relay, also does short circuit, often faster than fuses
Motor

The overload relays are 3 phase. Phase loss increases current to about twice FLA which rapidly trips the overloads. Also fuse and MCP protection is short circuit only, not overl9ads. So the motor will burn up and light on fire. Mersen has a great handbook that explains all this. If they could sell fuses that do double duty covering overload protection they would but it's simply impossible.

Keep in mind most safety disconnects have overhang cams so they rapidly open but are not designed to close into a load. It can and will eventually explode when it fails. I've seen lots of secondary disconnects abused this way.

Sent from my SM-T350 using Tapatalk


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## micromind (Aug 11, 2007)

There's a fair chance the disconnect has a label that states 'not to be used as a controller over 100HP'.


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## frenchelectrican (Mar 15, 2007)

micromind said:


> There's a fair chance the disconnect has a label that states 'not to be used as a controller over 100HP'.


Also some will state " DO NOT DISCONNECT UNDER LOAD "

this especially true with 400 volts and above system., the reason is arc flash issue when if something go wrong it will do it in hurry and I have see few disconnect switch blowen up and it is a mess to clean up and fix it.


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## gpop (May 14, 2018)

MDB1988 said:


> The starters are in another building 250' away. The disconnects are on the outside wall of the little pump house as there's no room for them inside.


So there are starters they are just located 250' away. 

So all you need is a start-stop buttons and control wire or you need to teach the operators that they need to walk 250' to shut the pumps down before opening/closing the local disconnects. (also nice to add a aux to the disconnect just in case pushing the red button is to difficult to teach them)


Disconnects really should be visible from the pumps.


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## MDB1988 (Nov 13, 2017)

So does this sound like the proper solution here?


NEMA 5 starter for pump #1 in the same place as pump #2's.
New 400A (unfused?) disconnects for both pumps with aux contacts to disable the starters when open.
Better controls at the pump house.


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## paulengr (Oct 8, 2017)

MDB1988 said:


> So does this sound like the proper solution here?
> 
> 
> NEMA 5 starter for pump #1 in the same place as pump #2's.
> ...


Still need either MCPs or fused disconnects ahead of the starter.

Yes aux contacts on the secondary disconnects are nice. If you're already pulling wire for that just pull the extra wire for start and stop buttons and mount on the same panel since the cost of pulling extra control wires is just the wire. Don't forget that with 250+ feet do NOT put the control wires in with the power conductors. Inductance will be an issue if you do unless you use a big machine tool relay as a load for the start circuit or Go DC.

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## 460 Delta (May 9, 2018)

Is there control wiring that's been abandoned from the old setup that you can use/repurpose for a control station?


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