Motor Disconnects and types

[minesh21]

I always see these Eaton Heavy Duty Safety Switches that serve compressors or motors, but they only have the ampere rating and voltage (400A, 600V for example). I also always see the ampere rating on single line diagrams but they don't usually indicate the HP rating. Looking at 430.109(A)(1) it appears that the code requires the switch to be rated for the HP of the motor. But then I see 430.110(A) which only talks about Ampere Rating but doesn't really require the switch to be rated for the motor HP. Does the switch need to be rated in ampere and HP for a motor load?

Also, section 430.109(E) talks about an isolating switch for motors over 100HP, but what about motors below this but above 2HP? Also, 430.109(E) says that the switch should be marked "Do not operate under load." This got me looking into the purpose of the disconnect switches. I always thought they were required not only to lock-out tag-out, but also to turn off the motor and loads in the case of an emergency. But the code is explicitly saying do not use this switch when the load is live. So what should someone do in an emergency? They cannot use this switch right?

I am confused after reading this previous post on Mike Holt:

http://forums.mikeholt.com/showthread.php?t=146430

Then I read this link:

https://iaeimagazine.org/magazine/2...ans-for-motor-and-air-conditioning-equipment/

The first link says don't open a switch under load. The second link says, yes, do install and operate this switch under load. I am confused...

 

[Wrd4spd]

Per 430.110(A), the safety switch should be rated for min. 115% of the full load current (amperes) of the motor as defined by the tables at the end of Section 430. The nameplate current would be used for torque motors only. The exception to this section allows you to size the switch below 115% if you provide a motor circuit switch [430.109(A)(1)] which is rated for the HP. The switch only requires a HP rating if you intend to use the above exception.

 

[minesh21]

Per 430.110(A), the safety switch should be rated for min. 115% of the full load current (amperes) of the motor as defined by the tables at the end of Section 430. The nameplate current would be used for torque motors only. The exception to this section allows you to size the switch below 115% if you provide a motor circuit switch [430.109(A)(1)] which is rated for the HP. The switch only requires a HP rating if you intend to use the above exception.

Could you help answer the other parts of the question?

 

[minesh21]

So I think I found the answer to part of my question. NEC article 430.110(E) is referring specifically to isolating switches which the NEC defines as not having interrupting capability and typical only used when the circuit has been opened by other means.

I would assume if you wanted to actually open the circuit under load for a large motor it would need to be a HP rated motor-circuit switch, which is defined by the NEC as having interrupting capability of maximum overload of the motor.

Also I noticed that if you go to the brochure of Eaton heavy duty switches, the ampere rating corresponds to a comparable HP rating as shown in the NEC FLC tables. So by default, if they are ampere rated they are also HP rated. Therefore, Eaton safety switches are motor-circuit switches as defined by the NEC.

 

[rbalex]

So I think I found the answer to part of my question. NEC article 430.110(E) is referring specifically to isolating switches which the NEC defines as not having interrupting capability and typical only used when the circuit has been opened by other means.

I would assume if you wanted to actually open the circuit under load for a large motor it would need to be a HP rated motor-circuit switch, which is defined by the NEC as having interrupting capability of maximum overload of the motor.

Also I noticed that if you go to the brochure of Eaton heavy duty switches, the ampere rating corresponds to a comparable HP rating as shown in the NEC FLC tables. So by default, if they are ampere rated they are also HP rated. Therefore, Eaton safety switches are motor-circuit switches as defined by the NEC.

You are definitely on to something. I would suggest also you read Section 430.109(E) and the Informational Note to Section 430.102 (B) Exception to (1) and (2)(a) (I wrote about 4/5 of the IN while I was on the API SOEE) The underlying caution is there are no isolating switches rated above 40 hp dc or 100 hp ac.

Believe me, in a refinery situation (my basic experience) I would want to be as far from the motor as possible to open the circuit in an emergency.

 

[minesh21]

You are definitely on to something. I would suggest also you read Section 430.109(E) and the Informational Note to Section 430.102 (B) Exception to (1) and (2)(a) (I wrote about 4/5 of the IN while I was on the API SOEE) The underlying caution is there are no isolating switches rated above 40 hp dc or 100 hp ac.

Believe me, in a refinery situation (my basic experience) I would want to be as far from the motor as possible to open the circuit in an emergency.

I made an error in my NEC reference. I meant to reference 430.109(E) not 430.110(E).