Reason for Motor Circuit Protector with Upstream OCPD?

[djr3203]

I tried to find a previous post that answered this question but had no luck. Please feel free to steer me in the correct direction if there is a previous post that covers this question:

I am failing to see the reason for having a Motor Starter with a Motor Circuit Protector (MCP) when there is a single motor on a feeder and a CB in the upstream panel?

Does the MCCB in the upstream panel not have the sensitivity to protect the motor if there is a fault at the motor?

As always, thanks for the help all!

DJR

 

[dpc]

Couple of issues: The starter must have a disconnecting means WITHIN SIGHT of the starter. If the upstream MCCB is not within sight, a local disconnecting means is required.

If the upstream MCCB is within sight of the starter, then a second overcurrent device is not required by the NEC. However, an MCP (instantaneous trip only MCCB) does over some advantages over a standard thermal magnetic breaker for motor short circuit protection. The MCP has a wider range of trip ranges and adjustments than a thermal-mag breaker, so could provide better protection in some situations.

 

[jraef]

So yes, TECHNICALLY the MCP would be redundant in a circuit with a dedicated TM feeder breaker of the correct size in a remote distribution panel. The requirement for a local disconnect COULD have been satisfied by buying a non-combination starter and a separately mounted disconnect device, but long ago the concept of a "combination starter" proved out to be much more cost effective because of the added cost of mounting, running conduit and wiring between separate devices. So a combination starter with a molded case breaker (no trips) or a non-fused disconnect switch as the local LO/TO device.

Now go out and try to buy one... What you will discover is that nobody stocks them and even if they offer them at all, they are generally made to order so long lead times compared to an off-the-shelf combo starter with an MCP, and virtually no difference in price. Sometimes it's all about volume and convenience.

"Will work for (the memory of) salami"

 

[djr3203]

Thanks for the clarification all. Sorry for being dense here jraef... but what does TM stand for?

I have seen a lot where they have the non fusible disconnect switch, control power transformer, and non combination starter all in the same disconnect enclosure next to the motor.

Do the combination motors on the shelf come already with a disconnect switch/enclosure and the MCP, CPT, Contactors, and OL already mounted inside the disconnect? So you just purchase the whole unit off the shelf?

I guess my confusion started with the fact that I thought whether it was a combination motor starter or non fused disconnect switch, that they were both composed of discrete parts that were field installed and wired. I need more exposure in the field....

DJR

 

[djr3203]

Opps I meant to say, "Do the combination motor starters on the shelf come already with a disconnect switch/enclosure and the MCP, CPT, Contactors, and OL already mounted inside the disconnect? So you just purchase the whole unit off the shelf?"

 

[dpc]

TM = thermal magnetic in this context.

You should put the idea of using a NON-FUSED disconnect switch (for anything) out of your mind. They have very limited short circuit withstand capability unless protected by an upstream fuse. Always use fused disconnects at 480 V - it will save a lot of hassle.

The "combination" in combination motor starter means it includes both the motor starter and its short circuit protection/disconnecting means. This can be either a fused disconnect or an MCP. And yes you can purchase it as an assembly complete with all of the auxiliary devices such as CPT, indicating lights, pushbuttons, etc.

 

[djr3203]

Ah stupid me, I should have known that was what TM stood for. Makes perfect sense. Thank you for the clarifications and insight!