Understanding the power requirements of a welder

PixMan · Mar 28, 2010

I might like to buy a Miller XMT350 at some time in the not-too-distant future, but I know next to nothing about welding right now. I know this is a good, versatile unit that with the right add-ons will allow me to do MIG, TIG and stick. I just want to be able to handle most small jobs that fall in the door, nothing heavy.

We have a 15HP RPC right now, and it's never seen much load with the 5HP lathe, 2HP grinder and 1.5HP Bridgeport. We never run any of the machines together anyhow.

My question is this: We have 100amp 240V single-phase service coming in, that's all. The RPC has a main breaker of 60amp. What would be need to do differently to run that kind of a welder? Would we need to go to 200amp (or higher) service, get a bigger RPC or what? I understand this welder can run on 3 phase or single, so what difference does it make and which would be better?

Any help is much appreciated.

motion guru · Mar 28, 2010

A quick look on Google yielded the following:

Rated Output

* 350A at 34VDC, 60% Duty Cycle, 3- Phase
* 300A at 32VDC, 60% Duty Cycle, 1- Phase

It would seem to me that power is power with an inverter based welder. If you want to run using the 3-phase specs - that welder needs 350A x 34VDC / .95% efficiency (guess) . . . = 12.5kW of power

This is approximately the same as running a 17HP load - which in the single phase world would be 240V at 68Amps . . . just over your breaker size feeding your RPC

So you could run the welder - but you couldn't run it at full current / voltage without tripping your incoming breaker.

The Welder does have the option for powering it on single phase - at 300A x 32V / .95 = essentially 10kW . . . ~ 13HP load which is about 55Amps single phase.

You should take this analysis with a grain of salt - this might be a better topic for the welding forum to take up?

PixMan · Mar 28, 2010

Thanks for your help. I had seen those 350A specs when I was on the Miller site looking at the XMT350 family of welders, and it scared me. I just had no idea how 350 amps output can come from 100 amp (max) input.

I went here with the question because it seemed more like a question for electrical-type people than welder-tech type people.

This is definitely back-burner anyhow, as we're still tooling up the new (used) surface grinder. I may defer a little longer on the welder as I might want to change the 6x14 permanent magnet chuck for a 6x18 electromag one. I've still buying wheels, mounting arbors and need to build a balance fixture.

I just thought I'd like to know what I've have to do to prepare for having a welder, because if it requires going to 200 amp service, there's much expense involved in that too and it only pushes back a welder buy even further.

DaveKamp · Mar 28, 2010

Because...

How to get 350A output from a machine with 65A input?

Easy... current is only HALF the equation.

You're getting 350A at 35vdc (12,500w)

...from a machine that's getting 65A @ 240vac (15,600w).

You're exchanging volts for ampres.

You can RUN the machine on your current service, but when you run it HARD, you'll be tripping the breaker. Really- how big a welding job will you do? If you're using 0.035" wire, you won't push that welder hard enough to worry about your service, because the wire, even in spray-mode, won't carry enough weld current to trip a 50A 240v outlet's breaker.

If you're doing high-power work, as well as running big machines under heavy load, then you'll need a bigger service regardless.

PixMan · Mar 28, 2010

I would anticipate having only .035" wire since that's so readily available and good for thin gage metals. We'd do some TIG no & then, but stick would be very limited. I assume that's the one that would use the most power.

I would anticipate upgrading the service at some point, but it'll probably wait until we plan out bringing in CNC equipment. Right now, that's not on the radar. When it does appear, I'd be looking at machines that aren't a converted flat bed lathe or knee mill.

wippin' boy · Mar 29, 2010

I'd buy 2 machines
a 252 MIG or Lincolns similar
and then a purpose built tig (readily available used) later

S_W_Bausch · Mar 29, 2010

Ohm's Law is what explains the relationship between volts, ohm, amps, and watts.

As far as stick welding is concerned, 100 amps (out of a possible 300, for that unit) will draw 1/3 of 50 amps single phase.

100 amps of stick welding is 3/32" electrode, perhaps 1/8" electrode. That's a lot of sparks.

Regarding Ohm's Law, the higher the voltage, the lesser the amps, and less amps takes less copper to distribute the power.

You might not see a need for 460 volts, and you might not need it, but as soon as you think "will never need it" some machine will present itself with an attractive price, most likely since it's 460 only.

Your local utility may only provide 230, but the proper transformer will allow a step-up of voltage to 460.

pistonskirt · Mar 29, 2010

PixMan

From my own experience with my Kemppi Mastertig 2500 ac/dc inverter set (250amp) running on 440v 3 phase with a 20amp breaker. I have TIG welded 1" thick alluminium without ever tripping the breaker, the pedal was occasionaly to the floor so I assume full power was used for short periods, can't see the panel whilst welding of course

but I am frequently surprised how little pedal is required once the metal is hot.

MIG & me were never meant to be so I have no experience but assume it unlikely to demand higher current loadings than TIG / alluminium.

"I just want to be able to handle most small jobs that fall in the door, nothing heavy"

Which is exactly the purpose my set serves & does so with aplomb, in 5 years of varied use I have never found myself lacking power.

regards

Brian

wippin' boy · Mar 29, 2010

unlikely to demand higher current loadings than TIG / alluminium.

that is my understanding also

PixMan · Mar 29, 2010

pistonskirt said:
PixMan

From my own experience with my Kemppi Mastertig 2500 ac/dc inverter set (250amp) running on 440v 3 phase with a 20amp breaker. I have TIG welded 1" thick alluminium without ever tripping the breaker....

Brian,

Can you tell me more about this unit? Where did you buy, how much do they go for and can they run on 230V 3-phase?

Thanks

pistonskirt · Mar 30, 2010

PixMan said:
Can you tell me more about this unit?

Two things I should mention...firstly, I am not a certified welder just self taught & been doing it for 35 years...secondly I am not an electrical engineer. Also this may all be somewhat academic as I dont see any listing for Kemppi distributors in the USA (just Mexico it seems :confused:

) though I am fairly sure I have seen mention of them being available more recently in the USA.
My post was aimed more to the point that my set runs perfectly from a 20amp breaker & that a 250amp set has served my every requirement under seemingly similar circumstances to your own.
I also know nothing about RPC's, never even seen one, as 440 3 phase supply is standard in almost any commercial property over here & runs down every residential street.

So if only for curiosity.....link to the operating manual below, mine has all the bells & whistles, minilog panel, cooler etc

http://www.kemppi.com/inet/kemppi/k...rtigACDC_2000_2500_2500W_3500W_0701_om_EN.pdf

PixMan said:
Where did you buy, how much do they go for

4 or 5 years ago the UK price was a gnats under £4,000, I was on the verge of ordering new when a 6 month old (as new condition) unit popped up via a Kemppi service engineer, final cost was about £2,750 all in, with which I was delighted

Kemppi are highly regarded over here but are generaly considered "high end" (expensive) I would say.

Welcome to Kemppi

PixMan said:
can they run on 230V 3-phase?

Yes, it appears so from the manual.

regards

Brian

PixMan · Mar 31, 2010

wippin' boy said:
I'd buy 2 machines
a 252 MIG or Lincolns similar
and then a purpose built tig (readily available used) later

Space limitations and the fact that dad & I are machinist/toolmakers, not welders, is why I'm interested in a compact yet versatile single-piece unit. We doubt we'd be using it very much. I just wanted to have something capable of quality welds for those times when we need it.

Dad had gotten a low-budget 120V wire mig unit a few years ago, and it's nearly useless. It may be hard to get a good-looking weld, but at least when you do it's weak.

Just not enough power and control.

Ad for Ohm's law, I do understand the voltage-amperage relationship when it comes to AC, it's very straightforward. I just don't know how things change when you mix AC and DC voltage. :confused:

wippin' boy · Mar 31, 2010

i don't disagree with your theory
but a 250 amp class machine is in no way similar to the 115v machines
you can weld submarines with .045 6 minutes out of 10 with a 252
by all means drop the 4k + tig setup if thats your desire
but ,,we digress
the website says that machine needs 54 amps, balls out, on 240 single phase
your setup should be fine as long as everything else in the shop ain't running all at once

breacher51 · Jan 4, 2021

What kind of power do I need to run for a multi purpose weld with this output

60% Duty Cycle @ 250A 380/415/440/480 VAC

DaveKamp · Jan 5, 2021

breacher51 said:
What kind of power do I need to run for a multi purpose weld with this output

60% Duty Cycle @ 250A 380/415/440/480 VAC

Welcome to the forum. It requires 'electric power'. Aside from that, the information you've posted is insufficient to provide any better answer.

Duty cycle suggests how long the machine will operate at any given load state prior to overheating. Is is therefore irrelevant to power requirements, save for the fact that incoming power must be high enough to produce output of the desired level.

While welding OUTPUT is somewhat proportional to it's input, it doesn't accurately reflect the incoming line load, especially on a multi-process welding supply. You've identified that it accepts several different input voltages, all of them are common to 3-phase input, but could be a single phase machine, you haven't specified. Based on experience, I could GUESS it as anywhere from 6 to 30kw, with anything under 10kw being entirely ineffective.

Rather than resurrecting a 20 year old thread... would be better to start a new thread specific to your circumstance.

Take a closer look at your welder, find the data plate, and look at what it's Full Load Amperes rating IS, what it's voltage input and current load ratings identify for each wiring configuration... and then tell us what type of power service you have, or are considering. From that, we can tell you what will and will not work, and suggest what would work best, vs. what would be most practical for your circumstances.

wippin' boy · Jan 7, 2021

Take a closer look at your welder, find the data plate, and look at what it's Full Load Amperes rating IS, what it's voltage input and current load ratings identify for each wiring configuration... and then tell us what type of power service you have, or are considering. From that, we can tell you what will and will not work, and suggest what would work best, vs. what would be most practical for your circumstances.

and then dave will tell you how to run it off two pug dogs and an old grocery cart

josephk · Jan 7, 2021

wippin' boy said:
i don't disagree with your theory
but a 250 amp class machine is in no way similar to the 115v machines
you can weld submarines with .045 6 minutes out of 10 with a 252
by all means drop the 4k + tig setup if thats your desire
but ,,we digress
the website says that machine needs 54 amps, balls out, on 240 single phase
your setup should be fine as long as everything else in the shop ain't running all at once

Just keep in mind, if you plan to spray transfer or weld thick aluminum the 252 won't be enough

wheels17 · Jan 7, 2021

Keep in mind that a 65A breaker does not trip at 65A. There is a characteristic curve that allows higher currents for shorter periods of time.

https://download.schneider-electric...File_Name=0600DB0105.pdf&p_Doc_Ref=0600DB0105

A breaker should hold rated current forever.

If you run a 65A breaker at 130A, it is rated to trip in 30 to 100 seconds. But that doesn't mean you should. A .03 second trip should occur with 20 !!! times rated current.

There's a whole special section on welders in the NEC. I haven't looked at it in a while, but there are a lot of exceptions that can be used for welders that cannot be used for other electrical devices that can really help out. It bears careful reading. Here's one example from StackPath

A non-motor/generator arc welder has a primary current rating of 40A with a duty cycle of 50%.
Calculated Load = Primary Rating x Multiplier [Table 630.11(A)]
Calculated Load = 40A x 0.71 = 28.40A

Therefore, the branch-circuit conductor for the welder must not be sized less than 30A. As per Table 310.15(B)(16), a 10 AWG conductor has an ampacity of 35A at 75ºC.

Understanding the power requirements of a welder

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