OCPD selection for Welder

[Sparky2791]

Hello!

Is anyone able to offer some thoughts on this cut sheet for a welder a client wants to use.......


OCP is based on the primary input current, using that method because will not get the I1eff. The cut sheet shows several different currents based on Duty Cycle (DC). How does one size the OCP size if the input current is different based on DC and it is able to be used at any of those DC's?

Does this mean when ordering the welder they need to select the duty cycle they want the welder to have? Or the client needs to tell me the duty cycle they will use it at?

Thanks for the replies.

 

[zbang]

What they're saying it that the welder can do 100% duty cycle but only at 300amp output. It the user wants to weld at 425amp, it'll draw more and they can only use it at 30% DC.

The way I read it the max current for the OCPD is 23.1 amps because it can draw that much.

 

[Jraef]

You are debating the difference between 10ga and 12ga here…

I would just go with 10ga and a 30A breaker.

 

[kwired]

Probably size your supply for the highest draw, that lets the user have the ability to use it at that duty cycle. If they happen to exceed any mentioned duty cycle there likely is internal protection that will cut out. Some are auto reset after cooling, some may be manual reset.

 

[suemarkp]

There are three input currents and duty cycles listed on the nameplate. If you use table 630.11(A) you get the following conductor ampacity requirements:
(100%) 14.6 * 1 = 14.6A
(60%) 18.1 * .78 = 14.1A
(30%) 23.1 * .55 = 12.7A

So your worst case ampacity is 14.6A so you could use #14 wire. Breaker can be 30A. I'd probably use #12 wire on a 20A or 25A breaker. Just because you can use double the nameplate amps for a breaker doesn't mean you have to. In some cases it may trip, just like some motors won't start with a breaker at or just above 125% of their FLA rating. I thought most welders enforced their duty cycle limits with internal thermal mechanisms which prevent you from using it at 100% at those higher amp levels.

 

[Sparky2791]

The way I read it the max current for the OCPD is 23.1 amps because it can draw that much.

In the end how do you determine the size of the OCP unless you know the intended use (Duty Cycle)

Nothing guarantees someone cannot use it at the 100% duty cycle. To me this means you need to just size it for the maximum current. But why bother to list the other input currents in that case. Plus, if OCP is sized for 100% DC and welder is used used as a 60% DC the OCP would not be correct.

100% DC requires a 30A-3P Breaker
60% DC requires a 40A-3P breaker
30% DC requires a 50A-3P breaker

OCP cannot be sized any larger than 200% The breaker protecting the welder for 100% DC will not be properly sized for 60% DC or 30%.

I must be missing something. I have reached out to the vendor to discuss this.

Probably size your supply for the highest draw, that lets the user have the ability to use it at that duty cycle. If they happen to exceed any mentioned duty cycle there likely is internal protection that will cut out. Some are auto reset after cooling, some may be manual reset.

That is what I was thinking as I over think this!

There are three input currents and duty cycles listed on the nameplate. If you use table 630.11(A) you get the following conductor ampacity requirements:
(100%) 14.6 * 1 = 14.6A
(60%) 18.1 * .78 = 14.1A
(30%) 23.1 * .55 = 12.7A

So your worst case ampacity is 14.6A so you could use #14 wire. Breaker can be 30A. I'd probably use #12 wire on a 20A or 25A breaker. Just because you can use double the nameplate amps for a breaker doesn't mean you have to. In some cases it may trip, just like some motors won't start with a breaker at or just above 125% of their FLA rating. I thought most welders enforced their duty cycle limits with internal thermal mechanisms which prevent you from using it at 100% at those higher amp levels.

Really not worried about the conductors. I'll size them as equal to the ampacity of the C/B to be covered. My concern was OCP size. Seems to me based on the replies to this post I get to pick the one I want to use.

 

[Sparky2791]

I would just go with 10ga and a 30A breaker.

Why that size breaker?

 

[wwhitney]

The 630 text on OCPD size (sorry I don't have it in front of me right now) refers to I_max. So does that depend on duty cycle, or is that just the highest rated input current across all choices of duty cycle?

Cheers, Wayne

 

[wwhitney]

Table 630.11(A) . . . thermal mechanisms

Note that the factors from that table are just the square root of the duty cycle. That's because for a square wave current, the average power dissipated through a resistance R (eg a conductor) is P=D*I²*R, where D is the duty cycle and I is the peak current.

Cheers, Wayne

 

[Sparky2791]

The 630 text on OCPD size (sorry I don't have it in front of me right now) refers to Imax. So does that depend on duty cycle, or is that just the highest rated input current across all choices of duty cycle?

Good question.

 

[wwhitney]

Good question.

Ah, now that I have the text in front of me, the informational note after 630.12(B) says "I_max is the maximum value of the rated
supply current at maximum rated output." To me that says that if the duty cycle options in the spec in the OP are user selectable, then for 480V 3-phase, I_max = 23.1A.

Cheers, Wayne

 

[Sparky2791]

Ah, now that I have the text in front of me, the informational note after 630.12(B) says "I_max is the maximum value of the rated
supply current at maximum rated output." To me that says that if the duty cycle options in the spec in the OP are user selectable, then for 480V 3-phase, I_max = 23.1A.

Cheers, Wayne

Thanks Wayne - I reached out to the vendor to ask that very question 'user selectable' that is. This one had me stumped based on the different values listed.

 

[topgone]

What they're saying it that the welder can do 100% duty cycle but only at 300amp output. It the user wants to weld at 425amp, it'll draw more and they can only use it at 30% DC.

The way I read it the max current for the OCPD is 23.1 amps because it can draw that much.

I remembered this problem when we were computing for the AC input amps of our welder units in my green days. Since this is a DC welder unit, the multiplying factor to come up with an AC input requirement for the welder with a 3-phase full-wave bridge rectifier would be 1.06. The multiplying factor for single-phase is also 1.23 (full-wave bridge rectifier).
I.e. @ 100% duty cycle, an output amps of 300ADC and a 29ADC, the DC output power will be 8.7 kVA multiplied by 1.06 and divided by a PF of say 0.79 will give the input amps of 14.6A AC as listed.
The same goes at 30% duty cycle when the output is 425A DC @ 35.25A DC voltage and power factor of 0.86. AC Input amps = (425A X 35.25V) X 1.06/(1.732 x 460 x0.86) = 23.1A AC, as listed! Per code, the answer to the OPs concern is to use a 50A CB (200%). Rated input amps is the I_1max being mentioned in 630.12.
On the I_1eff problem, the code states (630.11 (A) and (B)) that the value can be computed using their formula. But if you consider the no-load amps of the welder transformer to be negligible, the formula simplifies = rated input ac amps multiplied by the square root of the duty cycle. In this 30% duty cycle, the I_1eff will be = 23.1A x square root of 0.30 = 12.7A! Any conductor that can carry this amount of effective current will be enough, IMO.
The simplest thing to do is to refer to Table 630.11(A) Duty Cycle Multiplication Factors for Arc Welders.

 

[LarryFine]

It almost seems to be informational, like for an existing circuit, what duty cycle it can support.

 

[Sparky2791]

To all following this post: After all of this, the vendor finally got back to me and said to just refer to the operator’s manual for the manufacturer’s recommended OCP size. I always follow the manufacturer’s recommendations. Using a 50A, which is ultimately what I would have done same as Topgone lists, is incorrect according to their recommendations. We are dealing with several different types of welders from several different manufacturers , and for everyone, the manufacturer’s operator’s manual listed this for me. They also list the correct wire size. Their recommendation does make me 'scratch my head' though. Really not sure how they come up with that based on the input they list.
Hope this info helps another from going down a rabbit hole. Just look up the manual, if it is available.

 

[topgone]

To all following this post: After all of this, the vendor finally got back to me and said to just refer to the operator’s manual for the manufacturer’s recommended OCP size. I always follow the manufacturer’s recommendations. Using a 50A, which is ultimately what I would have done same as Topgone lists, is incorrect according to their recommendations. We are dealing with several different types of welders from several different manufacturers , and for everyone, the manufacturer’s operator’s manual listed this for me. They also list the correct wire size. Their recommendation does make me 'scratch my head' though. Really not sure how they come up with that based on the input they list.
Hope this info helps another from going down a rabbit hole. Just look up the manual, if it is available.

That would be a safe position for the manufacturer. That OCPD specification holds true for the 100% duty cycle of the welder unit (I1max = 300 x 29 x 1.06/(1.732 x 460 x 0.79) = 14.6X X200% ~30A . On the extreme amp usage (though duty cycle is limited to 30%), specifying a lower amp protection ensures their unit don't get damaged and complies with NEC verbiage "not more than 200%". With that specification from the manufacturer's manual, you'll just have to reset CBs/ replace fuses more often when the welder is used for high amp welding jobs.
Honestly, I had been using 200% of the rated welder unit amps since I began my profession and it never failed me.

 

[Sparky2791]

That would be a safe position for the manufacturer. That OCPD specification holds true for the 100% duty cycle of the welder unit (I1max = 300 x 29 x 1.06/(1.732 x 460 x 0.79) = 14.6X X200% ~30A . On the extreme amp usage (though duty cycle is limited to 30%), specifying a lower amp protection ensures their unit don't get damaged and complies with NEC verbiage "not more than 200%". With that specification from the manufacturer's manual, you'll just have to reset CBs/ replace fuses more often when the welder is used for high amp welding jobs.
Honestly, I had been using 200% of the rated welder unit amps since I began my profession and it never failed me.

I suppose my understanding of the Duty Cycle is what confuses me. I thought 100% meant you could use the welder for the full 10 minutes as opposed to 30% which is only 3 minutes in a 10 minute period. But that seems to go against the currents listed. Lower Duty Cycle + higher amp draw

 

[Eddie702]

Had a job once in a building my brother in law rented with a 208/3/60 400 amp service. The customer went out and bought a welder that needed like 300 amps 3 phase. They called my BIL and he was all rattled as they told him he needed to install a new service.

I went over and took a look. All they were doing was welding 1/8" tubing this welder was 4x what they needed. I had put a 60A disconnect on the wall that they used to test the machines they built They had their own "electrician" on site not licensed but he built control panels and wired the new machines they were building so he knew his way around well enough.

I told him if I hooked this up I would have to follow the name plate. I said for what your welding this would be a rediculuous cost. I said after I leave wire it up to one of the 60 a disconnects and you can weld what you need.

Its just like a transformer if you don't load the output you don't need as much input

 

[zbang]

I thought 100% meant you could use the welder for the full 10 minutes as opposed to 30% which is only 3 minutes in a 10 minute period. But that seems to go against the currents listed. Lower Duty Cycle + higher amp draw

It's all about heat, a lot of the NEC is about heat (it's a fire protection code). Maybe you can draw 10 amps all day and the wire/etc never even feel warm, jump that up to 40 amps and it'll heat up quickly and maybe melt or start a fire, but if you only draw 40 amps for a couple of minutes then let it cool off everything is fine -- that's the duty cycle and how it matters.

Art 630 takes into account that many welding loads are high but short (low duty cycle) so it allows smaller wire sizes for a given OCPD assuming that it'll have a chance to cool off. We're not allowed that assumption for most circuits.

 

[topgone]

I suppose my understanding of the Duty Cycle is what confuses me. I thought 100% meant you could use the welder for the full 10 minutes as opposed to 30% which is only 3 minutes in a 10 minute period. But that seems to go against the currents listed. Lower Duty Cycle + higher amp draw

You limit the time of use of the welder unit in order not to exceed the build up of heat in the unit, if you intend to avail of higher amp output. The basic equation for heat in welders is Heat energy = I² x R x t x K, t in the equation is the welding time, R being the resistance of the welder and K a constant for the welder setup. Assuming R and K remains the same, the only variables left are t and I (amps) and that to keep the heat energy within a certain level, one can use a higher amp (I) while keeping the time t shorter, hence a smaller duty cycle. E.g. a 200A welder rated at 100% duty cycle could be set to deliver 200A x sqrt(1/0.30) = 365A @ 30% duty cycle! Hope that clears everything.