Branch circuit protection for motor groups

[ntr]

Hi all,
In a control panel for air cond. and refr. equipment as per UL508A I have to provide branch circuit protection and switching on/off for 6 of these water pumps (see image below). They have to be switched on and off simultaneously. Is it possible to use one inv. time circuit breaker and contactor for all these pumps as per 31.4 a) branch protection for motor groups? The UL508A is not very instructive. As far as I understand you have to comply with NEC (2014) article 430.53 (D) and 430.53 (A). In 430.53 (A) there is a reference to 430.32 (B) (4) Impedance-Protected. But i'm not sure what is meant by "if the motor is part of an approved assembly...". Can someone clarify if one CB is sufficient?
Thanks.


Nico

 

[kwired]

Whether you have one or six of those - the next higher standard size circuit breaker per 240.6 is 15 amps. If using time delay fuses you likely would need a 3 amp fuse for a single unit or a 10 amp fuse for six units.

 

[Jraef]

"Impedance Protected" means it is designed to not need additional protection, i.e. the way the motor is designed is such that even if you lock the rotor completely, it will increase the impedance such that the current is inherently limited to safe levels (in this case, 0.97A) almost indefinitely. For them to get it UL listed like that, it means a minimum of 15 days with a locked rotor. Bottom line, you can't make it catch on fire by itself. Generally, this means it is a Shaded Pole motor, the same thing you see in little muffin fans.

So yes, you can use one breaker for as many of those as will fit behind it, basically 80% of the breaker rating.

 

[kwired]

"Impedance Protected" means it is designed to not need additional protection, i.e. the way the motor is designed is such that even if you lock the rotor completely, it will increase the impedance such that the current is inherently limited to safe levels (in this case, 0.97A) almost indefinitely. For them to get it UL listed like that, it means a minimum of 15 days with a locked rotor. Bottom line, you can't make it catch on fire by itself. Generally, this means it is a Shaded Pole motor, the same thing you see in little muffin fans.

So yes, you can use one breaker for as many of those as will fit behind it, basically 80% of the breaker rating.

But can you still comply with NEC by putting say 300 of them on a 200 amp multiwire branch circuit?:blink:

 

[GoldDigger]

But can you still comply with NEC by putting say 300 of them on a 200 amp multiwire branch circuit?:blink:

As long as the wires are all large enough to be protected by the 200A breaker.
AFAIK using an impedance protected motor does not get you the same permission to use smaller wires compared to the breaker size that having overloads in the circuit gets you.
But at a minimum the wires would have to be large enough that the 200A breaker provided adequate short circuit and ground fault protection.

 

[kwired]

As long as the wires are all large enough to be protected by the 200A breaker.
AFAIK using an impedance protected motor does not get you the same permission to use smaller wires compared to the breaker size that having overloads in the circuit gets you.
But at a minimum the wires would have to be large enough that the 200A breaker provided adequate short circuit and ground fault protection.

I didn't think there was anything prohibiting such a thing as long as proper size conductors were run. Since this is feeding motors - the conductor should be able to be less then 200 amps but still at least 125% of the FLA of the motors - I think. I was going to say 125% of largest plus all others but think that is just for a feeder, this example would be a branch circuit. But either way with motors you possibly have larger breaker just to allow starting as long as conductor is sized to the load.

Practicality of doing such a thing maybe isn't all that great though.

 

[Jraef]

The limiting factor would be the size of the conductors and the ability to terminate them in the motor connection box. Most likely you would be limited to 12ga wire on a tiny little motor like that.

 

[ntr]

Thanks all for the explanation. So in short, if I use a 10A or 15A circuit breaker and 14AWG wires for six motors i'll be safe.

 

[Jraef]

Thanks all for the explanation. So in short, if I use a 10A or 15A circuit breaker and 14AWG wires for six motors i'll be safe.

Safe? That depends more on your skill set. But legal? Yes.