Motor Disconnect rated below C.B. rating?

[batman2010]

Hi all,

when specifying a motor disconnect switch according to art.(430.110) of NEC it shall not be less than 115% of the FLA of the motor; sticking with this formula will lead to a disconnect rating less than the rating of the circuit breaker protecting the motor which means the inrush starting current of the motor will be more than the rating of the disconnect switch; I always wondered doesn't this affect the disconnect switch after a period of time? isn't better to specify a disconnect switch with the same rating or more than the circuit breaker?

your inputs are appreciated.

Thank you

 

[X49]

There is no need to use a switch with the same rating as the breaker, for the same reason that motor cables do not need to be sized for the breaker rating. The motor cable and disconenct switch are thermally protected by the overloads in the motor starter, not by the breaker. The breaker is there only to provide a trip in the case of a short circuit. Generally, a breaker will be able to protect a cable with a significantly lower ampacity than the breaker's rating in the event of a short circuit (but not a thermal overload).

 

[waross]

DO NOT OPEN THE DISCONNECT SWITCH WHILE THE MOTOR IS STARTING and you will be fine.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

Actually, if you use a UL98 rated disconnect switch with a HP rating equal to or greater than the connected motor, you can even open it when the motor is starting, although I would not recommend that in other than an emergency.

By the way, when the NEC says "not less than", that does NOT mean "maximum" or even "recommended", it means "minimum". There is nothing stopping you from putting in a 1200A disconnect on a 1HP motor, other than common sense and economics.

"Will work for (the memory of) salami"

 

[advidana]

I think you are referring to NEC article 430.110 (A). It said disconnecting means not a switch. A disconnecting means can be a circuit breaker or a switch. Article 430, I found was confusing at first. But after using it a few years and a lot of code classes the intent of the code makes a little more sense. You need to look at every single word carefully in the NEC code. Like it said "not less than"- It's hard to find a 1 hp motor or larger that will run and not blow out the fuse using 115 percent protection.

First of all the Horse Power (HP) rating an un-fused disconnect switch (safety switch) is very small. Usually a dual element fuse is needed to up the HP rating of the switch to a practical range of HP sizes.

I would be more concerned that the fuse or breaker would not open when sizing protection to a motor as well as protecting the motor itself. I have always used the recommended sizes for protection by using the free Square-D Company card board slide rule for old school methods. I have used this method for over 40 years and no problems in the field. SKM has a default for motor protection too for people using software. These fuses and breaker sizes are larger than 115 percent more like 150 to 200 percent in the 1 HP to 200 HP range.

I would also check to see if sizing the motor's dual element fuse to the same size as the upstream Circuit breaker would cause a coordination problem, if protective coordination is important to your client (Health care facilities).

I find that knowing the about the 115 percent motor disconnect article in the NEC code is good for passing code tests and for sizing tiny motors.

Oh well, I hope I helped and not made it worse.
Good luck