Disconnect switch size

Date 01-11-2005 04:41

I just got a Miller Syncrowave 250 and would like to install a disconnect near the machine.I am using 230 V single phase.The welder draws 77 amps at rated output and spec sheet recommends 125 amp fuse or breaker and a maximum of 153' of 6 guage conductor.I am using 125 amp breaker and are close to the 153' length of conductor.
Since I spent all my money on the welder,I'm trying to save with a used disconnect.I can't find a 230 V 125 amp disconnect,so heres my question.
Would a 600 V, 60 amp work or is 125 amp capacity necessary regardless of voltage rateing.This is for farm use so I just need it to be safe.These 600 V disconnects seem to be a dime a dozen used.
Sure hope someone can help.Thanks for listening.

By Sand Man (**)

Date 01-11-2005 05:16

Your breaker is your disconnect. I am wondering why you would need an additional disconnect .
As far as I recall you would need the 125A switch. Single phase components are almost always more expensive than 3 phase.
I am sure someone will step in if I am wrong. Jeff

By burninbriar (*)

Date 01-11-2005 05:27

I want the disconnct because my breaker is at the other end of the 150' of line and I want something close to the welder.The breaker is in another building.

By billvanderhoof (****)

Date 01-11-2005 06:24

I make no claim that anything I am about to say is code compliant.

A rating of 600V 60A generally means that the switch will interupt that current at that voltage without continuing to arc and without allowing any leakage to ground or across the line. Most switches will carry higher currents than they will interrupt. At lower voltages the arc when the switch opens is easier to extinguish so lowering the voltage should allow some uprating of the switch. Careful reading of the label may indicate this. If the disconnect has fuses in it you should be sure that you can get 125 amp fuses in that size. One suspects that that is a bit more than the expected draw with the machine set at max and the electrode shorted to the work. Most likely the disconnect would not be operated while the machine is welding so one would expect the draw to be the idle current, probably a few amps, and easy for the disconnect to handle.

Now the dark side- if that building burns down for any reason, and the insurance investigator finds that that switch is underrated you may well wish that you had gone the extra bucks and gotten something bigger.

Bill

By burninbriar (*)

Date 01-11-2005 07:24

THanks Bill,for the advice and for reminding me about the insurance.(I feel kind of dumb now) Since I'm already on a breaker at the other end & the the disconnect is more a convenience I think I will use it as is & add the propper disconect later when I can aford it.
Thanks again
pete

By Arcandflash (**)

Date 01-11-2005 15:56

Like Bill says have a good look at the specs on various switches and you may find one rated at more than enough current at 230V even though the 600V rating is lower.

You have to make your own decision about insurance issues but a possible improvement would be to put 60A fuses in the switch thus limiting its maximum current. If you run the welder flat out a lot then this might not work for you. I have a stick welder that specs call for 56A input at 240V. I run it off a 40A breaker with no problems and I stick the rod down from time-to-time. I have run it flat out but not often and for short periods of time.