Size of srvice for Shopmaster 300

[conteco]

I have an opportunity to get a Miller Shopmaster 300, 1994 vintage, inexpensively. However, the manual suggests a 125 amp service at 230 Volt, with actual consumption at 75-80 amp. Since I'd probably only use this in my garage, and most house services are 200 amps or so, is the shopmaster just too big for a garage set up? Or can a service be added to accomodate this? My current house service is a 3 wire 110 V, and I already have a 230 V receptacle at 30 amps.

Any ideas?

[Mark Lindquist]

The same thing happened to me. I got a 1990 syncrowave 250. After testing the welder, i had a #6 - 3 wire service to the garage. The welder input was 85 amps at 250 amps output. I installed power factor correction capacitors which I would not recommend, But i installed a control switch , a contactor and internal circut breaker at 60 amps to protect the capacitors. At 250 output it lowered the amps to 38 input amps. They will tell you with a welder you can run smaller wire with a larger breaker and that might be ok as long as no one ever uses it for any thing else. I got #4 three wire for 2.30 a linear foot and installed a 70 amp breaker in the main and ran to a new subpanel in the garrage. Then installed a fused disconnect, If i tig weld at 175 output I use the power factor. The only problem i have with all the experiance i have at arc welding, my tig welds are not acceptable.

[Bodybagger]

They will tell you with a welder you can run smaller wire with a larger breaker

If by "they," you mean the fine folks at NFPA via their publication National Electric Code, you are 100% correct.

The Shopmaster 300 has a highest input current rating of 91 amps at 50% duty cycle in CC mode.

NEC Article 630 Table 630.11(A) lists a factor of 0.71 for 50% duty cycle non-motor driven welders. So you need to feed the welder with cable sufficient to carry 71% of the rated 91A, which is 64.6 amps. You should use #4-2 WG if you are using NM cable, which is rated to 70 amps.

You can use up to a 140 amp breaker for the welder because NEC allows the use of a breaker up to 200% of the rated ampacity of the cable it feeds.

You will find it difficult to get a 140 amp breaker. A 100A will probably do you, but it may periodically nuisance trip.

Note that this is just for your welder. If you are planning on installing a separate subpanel in your garage which will subsequently feed the welder, you should consider installing a new 200A residential service dedicated to the garage.

[Mark Lindquist]

I know and understand the nec code for a welder, I just worry about people placing a larger breaker than can protect the home wiring. I think I worry more than most.

[Sberry]

When I use a wire smallere than the mfg lists I reduce the breaker. I run my 300 on a 6 but limit it to 60 or 70. Welders are allowed to use de-ratings for the wire but one cant derate from those.

[Bodybagger]

That's a good point.

The critical thing that everyone reading this should understand is that these exceptions apply only to dedicated circuits for welders. The second you tap into that circuit for anything else, the exception no longer applies, and doing so can create a hazardous situation. See NEC Article 630 or a licensed electrician.

Unfortunately, there are tons of people who will read into what you type however they want and extract hidden, or even opposite meaning from what you say. If I had these people in mind when I typed, I'd have to fill the post with legal disclaimers and dumb down the original content so as to make alternative interpretations impossible.

[conteco]

Thanks, I think I have enough info to go to the next step. I think I'd get an electrician to install a receptacle that large, but it sounds doable. Conteco.

[Sberry]

Those machines run fine from 100A service and breakers, most of the time you can run then from a 60 at home current levels but my 300 synch calls for number 2. I think in all these yrs, 6 wire with 60 brkr I tripped it once really smokin on it, not paying any attn. I checked, the wire wasn't even warm