# Running dedicated circuit for welder, a bit confused.



## BBQ (Nov 16, 2010)

Welders are treated differently due to the 'duty cycle', please check out NEC article 630.

But basically I am willing to bet an 80 amp breaker with 8 AWG will be fine for the 57 amp load.


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## sayn3ver (Sep 13, 2011)

630.11 (A) *Individual Welders*. The ampacity of the supply conductors shall not be less than the I 1eff value on the rating plate...continues on if that value is not given.

630.12 (A) Each welder shall not have over current protection rated or set at not more than 200% of I1max.....overcurrent device not required if supply conductors are protected by an OC device rated or set at not more than 200% of I1max or at the rated primary current of the welder.

(B) *For Conductors.* For conductors that supply one or more welders shall be protected by an OC device rated or set at not more than 200% of the conductor ampacity.



I1eff= √Ii^2X*I0^2(1-X)





1)With the equation the NEC explains that Isub0 is supply current at no load. What does that mean? How can there be current with no load?
2) #8 THWN @ 75C is 50amps. 200% = 100amps so the 80amp breaker works there.
3)I take Imax (according to the NEC definition in that section) to be the 57amps @ 30% duty cycle...so 200% of that is 114amps...so the 80amp breaker is legal. 

4) How do I determine which duty cycle to use? Or does the duty cycle only determine which multiplier to apply?

To me using table 630.11(a) 57amps @ 30% duty * .55 = 31.35 ampcity required.


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## FanelliBT (Dec 14, 2012)

I have never worked with welders so that part I wouldn't know... But how much draw is already on that 100 amp sub... I could see it being a problem in an attached garage if that sub is feeding other parts of the house. Might not be a bad idea to run some stuff on that panel and amp probe it.


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## mbednarik (Oct 10, 2011)

sayn3ver said:


> 630.11 (A) *Individual Welders*. The ampacity of the supply conductors shall not be less than the I 1eff value on the rating plate...continues on if that value is not given.
> 
> 630.12 (A) Each welder shall not have over current protection rated or set at not more than 200% of I1max.....overcurrent device not required if supply conductors are protected by an OC device rated or set at not more than 200% of I1max or at the rated primary current of the welder.
> 
> ...


With the 31.35 is where they come up with the #8. Look at 210.21 B 1 ex2 for the plug sizing.


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## garfield (Jul 30, 2009)

Our Lincoln buzz box is on a 40 amp breaker with #8 and had never tripped the breaker.


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## sayn3ver (Sep 13, 2011)

Subpanel is new with only 2x 20amp receptacle circuits for the work bench, etc. being fed from a 200amp main with only 20spaces filled. House originally had a 150amp service and nothing new was added to the main after the upgrade. 

The welding circuit needs to pass inspection so it needs to be right.


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## BBQ (Nov 16, 2010)

As far as the duty cycle I would go with the 35%, there is no way he will exceed that.

I actually would just do exactly what the instructions said to. 80 amp breaker with 8 AWG.


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## sayn3ver (Sep 13, 2011)

mbednarik said:


> With the 31.35 is where they come up with the #8. Look at 210.21 B 1 ex2 for the plug sizing.


I checked out that section and will have to remember those exceptions in the future. 

With 8awg wire a 40amp plug and receptacle is legal as far as my understanding of that exception goes. 

I suppose my last question is this. Am I interpreting correctly that the 57A @ 35% duty cycle on the plate of the welder as the primary current draw which is used with table 630.11(a)? Or is the 57A @ 35% the calculated draw with the multiplier/correction applied? The plate doesn't list primary current like the other welder so that is my last bit of confusion.

I would like to thank all for directing me to the appropriate sections of the NEC. I have much to learn.


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