I looked up the manual for that thing on line, its a beast.
Its rated to use up to 75 and 90 amps depending on the AC versus DC.

I would say you should be able to get up to about 200 AC amps out of it on a 50amp circuit as long as you don't stick the rod to the work.
Unless you bought it for the nearly continuous duty cycle at lower settings you are going to need a bigger circuit if you want to get any where near 300amps. To get it connected to a 90 amp supply it might need to be hard wired straight to a breaker, I don't know what kind of plug you would use for 90 amps.

 

How many amps do you reckon I could get out of it in DC mode?

Most of my work is under 200 amps AC.

I will probably put in a 80 or 100amp breaker and drop a four foot wire direct from the breaker box to a 6-50 outlet.

Thanks for your help!

 

I haven't messed with DC welders on limited power inputs enough to know for sure.
Looked at the manual some more and think the DC mode could flip the 50 amp breaker at as little as 130 amps.
I also noticed it runs at a really nasty power factor of .5 to .65

Remember the 6-50 outlet is only rated for 50 amps.

 

Wow, that really decreases the amperage rating.

Yeah that .5 power factor ain't very efficient.

I can't seem to find any 220v outlets/plugs rated above 50amps; it looks like the 6-50 has a certain "duty-cycle" above it's specified amperage.

You're starting to talk me into getting the thermal arc 252i which has a .99% power ratio and is quite a bit lighter because it's inverter based.

I haven't bought the shopmaster 300 yet, so I'm on the fence.

 

I think you would be fine with a 50 amp breaker. Does ths manul have a recommendation?
You should be fine with 50 amp recepticle and 6/3 or 8/3 IMO. Welders are not a continuous load.

You could add pfc to that beast. Should be pretty easy to find the values of the capacitors and put them in. Sorce them from a non welding rekated source to keep the cost down.

 

What is a pfc?

 

Power Factor Correction.

Page 13 has the wire and recommended breaker size: http://yabe.chudov.com/Miller-Shopmaster-300-AC-DC/Miller-Shopmaster-300-AC-DC-Manual.pdf

 

Where did you find the duty cycle for the over drawn plugs?

I too have never had to look for anything bigger than a 50 amp plug. My craftsman stick calls for 55 amp connection on a 60 amp breaker but I never run it higher than 160 to 175 amps (230 is the max setting that would need the 55 amps).

The manual does not have any recommendations for wire size. I guess miller figures you will have an electrician install it for you.
The wire chart I have says you should use 4ga for a continuous 80 to 100 amp load on a 30 foot cord. My craftsman recommends 8ga inputs wires for its 55 amp max power requirement and 20% duty cycle, my wire size chart says 8ga is good for continuous 40amps for up to 30 feet.
I think you should use 6ga if you plan to use it for much over 200amps.

If you are going to use the nuclear settings with 3/16'' rods do heavy repairs on excavator or loader buckets use 4ga input power lines.

PFC is a tricky beast. I have the math worked out for my arc and mig welder just need to find PFC caps that don't blow. The craftsman already claimed one 100uf motor run cap.

 

After some googling last night I found the following thread http://weldingweb.com/archive/index.php/t-93401.html .

Look at the 6th post for the "downsized multiplier".

That's where I can figure receptacle duty cycle.

 

I found the electrical code reference, I always figured there was some kind of guide like that but had never seen it until now.

Low Power Factor is when you have current draw that lags behind voltage as it cycles from 0v to 170 to 0 or 0 to 300 to 0 depending if you are on a 120 or 220 volt circuit, this is created by heavy inductive loads and fixed by adding capacitance.
In a purely resistive load the current flow perfectly follows voltage in proportion. In a highly inductive load the current flow shifts out of sync with the voltage, you don't want a lot of this.

 

http://www.millerwelds.com/om/o322a_mil.pdf

the manual does have the wire size: section 2-9