combo starter/disconnect - can the "starter" component be ignored?

[malachi constant]

I am a consulting engineer and am admittedly ignorant on the finer points of motor controls. Got a question from a contractor this week. My electrical plans called for a combo starter/disconnect for a 5HP 208V/3P motor. The EC has the combo on site, but the motor came with a control cabinet with an integral starter. The EC is suggesting they need to toss the combo and purchase a straight up 30A/3P non-fused disconnect.

My instinct is the combo can still be used, just the starter put in "hand" mode or whatever. This has been a good contractor to work with but they've been "uncertain" when facing the occasional motor schedule oddities. Not in a greedy way, I just get the feeling most contractors know more than I do about these things, and that may not be the case here.

Your collective thoughts are appreciated!

 

[David Castor]

I would never use a non-fused disconnect switch for motor disconnect, regardless. Maximum SC rating is 10 kA.

If the combination starter you bought includes a magnetic trip only circuit breaker, then you can't use it upstream, IMO. If the starter provided with the equipment includes short circuit protection (CB or fuse disconnect) and it is in sight of the motor, you don't really need anything upstream except a feeder breaker at the source.

 

[augie47]

Is the breaker inverse-time or a MCP ??

 

[ptonsparky]

The motor probably came with installation instruction. What do they say?
I have removed the factory supplied starter and provided my own combination starters, Pump Panels, on occasion.

 

[petersonra]

I would just forget the combo starter and run a circuit direct to the control panel it came with, if the control panel has some kind of disconnecting means.

If it does not maybe the unfused switch is in order if as another poster mentioned it meets the sccr required. Otherwise fused

 

[Jraef]

One thing to be aware of and careful about. A LOT of OEM equipment is arriving with control panels that have been “lazily” given an SCCR of only 5kA, which UL allows with no testing or investigation. It is REALLY difficult to find an industrial or commercial installation with an Available Fault Current that low. But the OEM panel MIGHT have a label on it that says a higher SCCR if you use a specific fuse class and rating ahead of it. So the outcome of that investigation would determine what you need to put ahead of this equipment.

 

[kwired]

5 HP 3 phase motor @ 208 volts only needs 10 AWG conductors for supply (before any adjustments or voltage drop considerations).

Unless your circuit length is pretty short distance, you may very well have enough conductor impedance that the available fault current will be below 10 kA or even below 5 kA at the load end of the circuit.

Still should consider this but may not be that much of an issue in many cases.

Some(maybe even most) non fused safety switches have same rating as their fused versions if there happens to be fuses in the circuit upstream from them. Say a fuse at the combination starter and a non fused disconnect at the motor would be one place you may use that allowance for increasing AIC to same as a fused version of same switch.

 

[Eddie702]

If the control panel contains a starter then the one in the combo is not needed. What we don't know is the combo fused or does it have a circuit breaker the correct size to protect the control panel and motor? If it does just do not use the starter in the combo just use it as a switch.

 

[petersonra]

One thing to be aware of and careful about. A LOT of OEM equipment is arriving with control panels that have been “lazily” given an SCCR of only 5kA, which UL allows with no testing or investigation. It is REALLY difficult to find an industrial or commercial installation with an Available Fault Current that low. But the OEM panel MIGHT have a label on it that says a higher SCCR if you use a specific fuse class and rating ahead of it. So the outcome of that investigation would determine what you need to put ahead of this equipment.

That's not really true. You still have to go thru the process of determining the sccr. It's just that there are very few power components with a rating of less than 5 ka so it is a pretty easy process. You still have to make sure all the components used are in the list of components ul says are acceptable for that particular use.

I don't recall what power components are less than 5ka other than gfcis and mostly they are in control circuits where the sccr is not required. Although I have done a few where there are things plugged into a GFCI like pumps.