Sizing motor circuit Q & A

[JES2727]

I'm looking for some clarification on a motor. A customer is having a problem with a vacuum pump. The pump is occasionally started without first relieving the vacuum in the chamber, so it's being started under load. This causes the breaker to trip. It is a 2hp motor supplied by a 1 pole 20 amp breaker, type BL, HACR. It is connected to a 20 amp twist-lock receptacle by a length of 14-3 SOW. The nameplate FLA is 18.4, so we've got a bit of a problem.
Using 24 amps FLA (Table 430.248) and multiplying by 125% (430.22A), I need conductors rated for 30 amps, but I can protect them with a 60 amp breaker (Table 430.52).
Here's what I'm unsure about:
I'm planning on replacing the 20 amp receptacle and attachment plug. When the wire size does not correspond with the breaker size do I size the devices according to the conductor size or breaker size? I think I need 30 amp devices. Likewise, I think I need to replace the 14-3 SOW with #10, but that seems like overkill. Do I size this cable according to the nameplate, Table 430.248, or the 30 amp attachment plug?
Can I use the 90 degree column in T310.16 and re-use the existing #12 THHN? (I think the answer to this one is "No")
Am I correct in using the "Inverse Time Breaker" column in T430.52 when considering the ITE type BL breaker?
I think I've got this mostly figured out, but I'd like someone to doublecheck me and set me straight if need be. Thanks.

 

[Jraef]

The nameplate FLA is 18.4, so we've got a bit of a problem.
Using 24 amps FLA (Table 430.248) and multiplying by 125% (430.22A), I need conductors rated for 30 amps, but I can protect them with a 60 amp breaker (Table 430.52).

Correction. You can protect the MOTOR with a 60A breaker, but the CONDUCTORS must be protected at their rating. So if you use 10ga wire, you need a 30A breaker. You can use a 60A breaker on the motor circuit, but you would need probably #6 awg cord for it (I don't have an SJO ampacity table handy). By the way, for a 120V load you don't need SO, SJ (300V) would work fine.

Table 430.52 is for the short circuit protection of the motor, but it is assuming you also have an overload relay in the circuit as well which is sized for the motor FLA and therefore protecting the conductors from long time overloads. In most 1 phase motors, there is no separate OL relay because the motor will likely have an internal OL protective device set for the nameplate FLA, in your case 18.4A. So you don't really need to overdo the cable size that much, unless a 30A breaker trips on you as well. I would see if I could capture what the starting current surges to on a loaded start and go from there. Better yet, figure out why it sometimes tries to start loaded and fix that problem. Most vacuum pumps are not designed for that kind of operation on a regular basis.

But to answer your question, size the wire for the load, size the breaker and connectors for the wire. But you are right to question the sizing they have already used on everything there. That 14-3 was undersized to begin with for an 18.4A load, and even 12ga cord would have been too small after derating. You didn't need to use the FLA table (Table 430.248) because you already have a nameplate FLA to work with. But even then, 18.4 x 1.25 = 23A, which would exceed even a 12-3 SO cord's rating (20A). It's 10-3 or nothing. In fact, that undersized cord may be a contributing factor to your breaker tripping on loaded start; you are likely getting a voltage drop due to undersized wire when the current goes high. You might try finding a 10-3 extension cord somewhere and see if using that solves the problem before changing out anything else.

And yes, you can use the THHN tables for sizing your feeder wire (assuming that's what you meant), but read the footnotes. #12 THHN can carry 30A, but it cannot be protected by anything larger than a 20A breaker anyway. So you will need to replace that with 10ga wire as well if you determine that the 20A breaker cannot hold in and you have to go to 30A.

 

[Jraef]

Double posted, couldn't figure out how to delete the 2nd post.

 

[GilbeSpark]

240.4(D) which says 14awg shall be breakered on a 15a breaker unless specified by 240.4(G). On 240.4(G) "Motor and motor-control circuit conductors" are both listed. So 20a CB would be acceptable, unless it's considered continuous duty.

You cannot size your wire based on the 90? table, you can only use the 90? table for derating purposes if your wire has a 90? insulation, which thhn does.

 

[augie47]

not so sure

not so sure

Jaref,
Sorry, but I disagree with the blanket statement that the "conductor must be protected at it's rating" (amactity).
In my opinion, 240.4 (G) would allow the motor conductor to be protected at 250% of the FLA. Although I would not personally do so, I beleive it would be code to wire the motor with a #12 using the 75 o ampacity of 25 amps.
This is assuming, of course, as you state there is overload protection. I do not read where that must be an overload realy..can you enlighten me ?
Woulkd the same rules not apply if the motor has internal OL protection ??That said, I really would like opinions if this is going to be a cord and plug
connected. I would think there would be problem in that sitaution with a OCP device greater than the condcutor ampacity with the possibilty that something other than this motor be plugged in.

 

[JES2727]

Correction. You can protect the MOTOR with a 60A breaker, but the CONDUCTORS must be protected at their rating. So if you use 10ga wire, you need a 30A breaker. You can use a 60A breaker on the motor circuit, but you would need probably #6 awg cord for it (I don't have an SJO ampacity table handy). By the way, for a 120V load you don't need SO, SJ (300V) would work fine.

Table 430.52 is for the short circuit protection of the motor, but it is assuming you also have an overload relay in the circuit as well which is sized for the motor FLA and therefore protecting the conductors from long time overloads. In most 1 phase motors, there is no separate OL relay because the motor will likely have an internal OL protective device set for the nameplate FLA, in your case 18.4A. So you don't really need to overdo the cable size that much, unless a 30A breaker trips on you as well. I would see if I could capture what the starting current surges to on a loaded start and go from there. Better yet, figure out why it sometimes tries to start loaded and fix that problem. Most vacuum pumps are not designed for that kind of operation on a regular basis.

But to answer your question, size the wire for the load, size the breaker and connectors for the wire. But you are right to question the sizing they have already used on everything there. That 14-3 was undersized to begin with for an 18.4A load, and even 12ga cord would have been too small after derating. You didn't need to use the FLA table (Table 430.248) because you already have a nameplate FLA to work with. But even then, 18.4 x 1.25 = 23A, which would exceed even a 12-3 SO cord's rating (20A). It's 10-3 or nothing. In fact, that undersized cord may be a contributing factor to your breaker tripping on loaded start; you are likely getting a voltage drop due to undersized wire when the current goes high. You might try finding a 10-3 extension cord somewhere and see if using that solves the problem before changing out anything else.

And yes, you can use the THHN tables for sizing your feeder wire (assuming that's what you meant), but read the footnotes. #12 THHN can carry 30A, but it cannot be protected by anything larger than a 20A breaker anyway. So you will need to replace that with 10ga wire as well if you determine that the 20A breaker cannot hold in and you have to go to 30A.

Thanks very much for the replies.
I used Table 430.248 because 430.6(A)(1) says I have to, " instead of the actual current rating marked on the motor nameplate", and 430.22 says I have to use 430.6(A)(1).
I also agree that 240.4(G) allows me to increase the breaker size without increasing the wire size. In fact, the exceptions to Table 430.52 allow me to increase the breaker up to 400%. Without 240.4(G) I would need to replace the wire every time I increased the breaker size. I could have #2awg supplying a 2hp motor! But I agree with Augie's concern about this being cord and plug connected, and something other than this motor being plugged into the receptacle protected by a breaker with a higher rating. I'm not sure about the right way to do this.
I'll size everything for 30 amps, but I still think I can protect the circuit with a 60 amp breaker if I need to.