Feeding Disconnect with Motor Starter?

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PhaseShift

Senior Member
I have to install an 100A 480V disconnect to feed some temporary loads in the field. Near where the disconnect is I have an old 150hp motor that is not in use anymore. Can I use this motor starter and cables to feed this disconnect. All lods off of disconnect will be self protected.

The motor starter has adjustable electronic overloads. Can these overloads and starer be used to protect these cables and feed the disconnect? I would think that they already protected the cables when connected for the motor?

This will be a temporary installation.
 

iMuse97

Senior Member
Location
Chicagoland
should "work" OK. But, is there a breaker at the source of the motor feeders? And is the breaker sized for motor starting, or for wire size?
 

iMuse97

Senior Member
Location
Chicagoland
I would hook it up, if it were mine. Is this a farm? My brother has done things like what you are describing (with my tacit approval, or disapproval). You've got "self-protected" loads, also. You're good to go.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
is your inquiry, "will it work" or is it "does this meet NEC" ?
I think you already know it will work.
I see no way it would be Code compliant unless the protection on the circuit suppling the motor starter is sized to protect the cables, your panel, etc.
Neither an instantaneous breaker nor motor overloads are designed for your installation.
 
The "self protected loads" (whatever self protected means), are they individual loads with small conductor/cables than the conductors protected by the motor starter?
I think Augie stated it pretty well. I wonder, are you getting an inspection? ;):D
 

PhaseShift

Senior Member
The "self protected loads" (whatever self protected means), are they individual loads with small conductor/cables than the conductors protected by the motor starter?
I think Augie stated it pretty well. I wonder, are you getting an inspection? ;):D

The loads are essentially skids with motors and heaters on them. The contractor is supplying these units and says that each unit has protection for the motor and heater on the skid. I belive this is in the form of a breaker on the skid. The skids have cables that are going to hook into our disconnect.

I guess the point I'm wondering, is wont the cables between the motor starter and disconnect be protected by the motor overloads? If these overloads provided protection of the motor and cables while the motor was in service why would it not provide protection with a disconnect connected instead of a motor?
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
You can twist it whichever way you want, but 240.9 specifies that thermal relays and other devices that are not designed for SCGF protection shall not be used as such. In addition 430.52(C)(3) specifies that Instantaneous Trip breakers are designed to be used as a component of a listed combination stater. I doubt your starter is listed for the purpose you describe.
For the above listed reasons, if I were called to inspect it, it would fail.
 
Heaters protect the motor, not the conductors.
The conductors/cables supplying the motor loads may be able to be qualified as tap conductors, because you mention the skid mounted motors have protection. If the protection mentioned is the overload (heaters) protection, then this sounds like an issue.

Temporary installations do not permit the absence of proper protection. See 590.2(A).
 

PhaseShift

Senior Member
Heaters protect both the motor and the conductors against over current.

But they will not protect against short circuits and ground faults.

I understand this, that is what the instantaneous breaker in the starter bucket is for correct?
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
I understand this, that is what the instantaneous breaker in the starter bucket is for correct?

problem is that the instantaneous breaker is a part of a listed combination starter designed for specific applications and not for general use short-circuit ground fault protection.
 

iwire

Moderator
Staff member
Location
Massachusetts
problem is that the instantaneous breaker is a part of a listed combination starter designed for specific applications and not for general use short-circuit ground fault protection.

I agree 100%.

PhaseShift I sugest tapping the feeder and placing a fused disco right there to suppky your loads.
 

eric9822

Senior Member
Location
Camarillo, CA
Occupation
Electrical and Instrumentation Tech
I have to install an 100A 480V disconnect to feed some temporary loads in the field. Near where the disconnect is I have an old 150hp motor that is not in use anymore. Can I use this motor starter and cables to feed this disconnect. All lods off of disconnect will be self protected.

The motor starter has adjustable electronic overloads. Can these overloads and starer be used to protect these cables and feed the disconnect? I would think that they already protected the cables when connected for the motor?

This will be a temporary installation.

I have done this but I have bypassed the starter and fed the load directly from the disconnect. I also swapped out the fuses or the CB for the ampacity required for the load.
 

PhaseShift

Senior Member
I agree 100%.

PhaseShift I sugest tapping the feeder and placing a fused disco right there to suppky your loads.

So if at the end of the feeder cable I put a fused disconnect to service the loads, then this will be acceptable? The starter overloads will protect the feeder cable from overload, and the starter breaker will protect against short circuits. The fused disconnect will then protect branch circuits and loads?
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
...The starter overloads will protect the feeder cable from overload, and the starter breaker will protect against short circuits. ...
Here is the problem with your statement. The combination motor starter is NOT listed for this purpose. It "gets away with" a non-standard circuit protection scheme because its intended purpose is very specific as a single feeder to a single motor. It may work just fine, but if it doesn't, and something bad happens, you are going to be in big trouble. It's also unlikely you will find any lawyer willing to defend your actions as reasonable and proper without shelling out big bucks up front... So is it worth your house and your kid's college tuition?

Again, "will it work" is different from "can I do this".

Were it my job, I would either just replace the MCP with an identical TM breaker, or cut in a properly sized set of fuses down stream of the MCP breaker in front of the contactor, rendering it essentially a disconnect switch. You can leave the OL relay in, you can even set it to trip before the fuses if you like so they will not blow, but the OL relay and MCP combo is NOT a "legal" substitute for fuses or a TM breaker.
 
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