Welders & Article 630

[Sparky2791]

Looking for some back up on OCP & conductor calculations for a welder. Working on renovations to a welding lab. The new welders cut sheet shows the MIG welder @ 460V is 17.1A 60% Duty Cycle. Highlighted by me on page 4 of the cut sheet attached with this post

I will not get the I1eff for the welder & this is a non motor generated welder. So…..

Overcurrent

17.1 x 200% = 34.2A - 35A is OCP size.

Conductor

17.1 x .78 = 13.338A…..#12 (20A) I am using 75deg. column since C/B/disconnect and receptacle will all be rated for 75deg. Although with #12 this does not matter.

This also works with 630.12 (B) #12’s rated for 20A x 200% max = 40A Max C/B size permitted. Per above we are @ 35A.

Design question

The shop currently has a 277/480V switchboard with 60A C/B’s feeding existing welders. I would like to reuse these C/B’s to feed the disconnect switch (required per 630.13) with 40A time delay fuses in the disconnect switch. This disconnect switch would feed the welding receptacle.

Any thoughts on this design consideration?

Thanks for the replies

 

[LarryFine]

I believe the local fused switch would make the supply circuit a feeder; I'm not sure if that matters.

Even if compliant, #12 just seems so small to me. What is the total circuit length?

 

[Sparky2791]

Even if compliant, #12 just seems so small to me. What is the total circuit length?

There are several welders so circuit lengths vary but none over 100' and @ 480V no VD concern on length. I agree on the #12's seems so odd but Article 630 spells it out and is one of the articles listed in 240.4 (G) under specific conductor applications listing 630.12.

I'll likely run #8's and be done with it to avoid any 'arguments' with AHJ.

 

[Sparky2791]

I believe the local fused switch would make the supply circuit a feeder; I'm not sure if that matters.

Yes that would make it a feeder, I agree would not really matter. There are about 30 of these and looking to save the cost of replacing the 60A breakers with 40A breakers.

 

[suemarkp]

Since you calculated the max breaker at 34.2A and rounded up to 35, I don't think you can round up again to 40A. Welders are like motors in that you can have a much larger breaker to handle start or other surges and be larger than the wire ampacity. Many times you don't need to go to the limit allowed. Sometimes, the limit allowed isn't enough, so you are allowed to go even larger if the motor won't start or the welder keeps tripping it.

I'm not sure where all the fear is for a #12 conductor. The nameplate is 17.1A. You get to cut it down even more (.78 multiplier) because of the 60% duty cycle. So you could even wire this with a #14 wire if you wanted to. #12 handles the full nameplate amps and has plenty of margin if using 75C conductors because 240.4(D) is exempted for welders. Even has margin with 60C conductors.

I would think this would be fine using #12 on a 30A or 35A fuse/breaker.

If you are using receptacles and not hard wired welders, 210.21(B)(1) exception 2 allows for a smaller (e.g. 20A) receptacle to be on a larger (e.g. 40A) welder circuit as long as the receptacle and branch circuit conductors are rated for the welders 630.11 ampacity. The inspector may want a "for welder use only" sign on those receptacles.

 

[Sparky2791]

Since you calculated the max breaker at 34.2A and rounded up to 35, I don't think you can round up again to 40A. Welders are like motors in that you can have a much larger breaker to handle start or other surges and be larger than the wire ampacity. Many times you don't need to go to the limit allowed. Sometimes, the limit allowed isn't enough, so you are allowed to go even larger if the motor won't start or the welder keeps tripping it.

Sorry - That was a typo on my part. I meant to say 35A breaker , not 40A. But thanks for pointing that out.

 

[Sparky2791]

I'm not sure where all the fear is for a #12 conductor. The nameplate is 17.1A. You get to cut it down even more (.78 multiplier) because of the 60% duty cycle. So you could even wire this with a #14 wire if you wanted to. #12 handles the full nameplate amps and has plenty of margin if using 75C conductors because 240.4(D) is exempted for welders. Even has margin with 60C conductors

No fear per say. Just concern about an inspector sees a 35A C/B with #12's on it. Which prompts a comment during plan review or during inspection that have to write a letter explaining why its permitted. Also using #10' or #8's would allow the school district to use a larger welder in the future, provided the OCP is OK and if not its just a fuse change in the 60A disconnect. Thanks so much for your comments!

 

[Sparky2791]

If you are using receptacles and not hard wired welders, 210.21(B)(1) exception 2 allows for a smaller (e.g. 20A) receptacle to be on a larger (e.g. 40A) welder circuit as long as the receptacle and branch circuit conductors are rated for the welders 630.11 ampacity. The inspector may want a "for welder use only" sign on those receptacles.

I need to find out if they are hard wired or cord/plug, the cut sheet does not say. Thanks for the tip on exception 2 under 210.21 (B) (1). Good to know in the cases where EC writes an RFI about it or if the inspector questions it