Shop welding circuit 220V....blow one fuse?

I'm in the process of adding a 50amp 220V circuit to the shed for a welder/air compressor (in the future). The shed has a SquareD breaker box, with a 50 amp breaker and 50 amp receptacle to go on the other end. The wiring is some 4 gauge three phase wiring that happened to be avaliable (I'm leaving off the neutral)

I was given an old 3 phase disconnect that is also fused, and have been considering using as a disconnect for the receptacle. I just have a worry...is it possible to blow the fuse on one hot leg and not the other? I have a TIG that is pretty computerized and I'd hate to know what would happen if one leg of the 220 circuit dropped out.

Thanks
Brandon
 
Sure it's possible (and likely) only one fuse will blow if there's ever a problem.

If the welder doesn't require a neutral, I wouldn't think blowing one fuse would kill it???

On the other hand, if you update to breakers and use a typical double pole breaker with common trip, disconnecting just one "leg" of the 240 Volts to the welder will never happen.
 
I would go with a hundred amp. Boss has his shop wired for that. We are always tripping breakers if we try to use a bunch of electric at once. He is too cheap to update to a hundred amp.
 
Oops let me say things properly.

The shed has a 100 amp Square D breaker box.

There is going to be a 50 amp circuit out of that, using a breaker for the welder.

I was thinking of having the disconnect just for a nice and easy shut off in that 50 amp circuit, but wondered if I really should, since it's fused.

Brandon
 
(quoted from post at 01:03:59 12/01/14) Oops let me say things properly.

The shed has a 100 amp Square D breaker box.

There is going to be a 50 amp circuit out of that, using a breaker for the welder.

I was thinking of having the disconnect just for a nice and easy shut off in that 50 amp circuit, but wondered if I really should, since it's fused.

Brandon

Why do you want a shut off? The breaker is all that is needed.

Dusty
 
Good question, here's my take:

From your post it appears the Welder is straight 240 volts, NO 120 VAC right???

I say that because you mentioned NOT running a Neutral (240 volt only, NOT 120) which is fine PROVIDED the Welder doesn't require 120 volts.

YOUR QUESTIONS:

"is it possible to blow the fuse on one hot leg and not the other?"


That's "possible" and if that happens, there's NO 240 volts to the welder. Sure, 120 volts exists between an unbroken Ungrounded (Hot) and the bare/green safety equipment GroundING Conductor, but if the welder doesn't use any 120 that shouldn't be an issue provided all is well and correct.

"I have a TIG that is pretty computerized and I'd hate to know what would happen if one leg of the 220 circuit dropped out."

If one leg drops out, there's no 240 to the welder, see the above. (See Notes below)


Now to using that 3 phase fused disconnect.

Having and using two of its three poles as a convenient disconnect will work and the fuses would be matched to the ampacity of the wires feeding it. The breaker in the panel is there to protect the feeders also, but additional fuses downstream (though not required) will not prevent the welder from working.

NOTES: 1) The two pole breaker will open BOTH conductors if there's an overload, and that will prevent the situation you mentioned of ONLY one leg being open to the welder.

2) If you want to possibly prevent or lessen the chances of only one fuse opening if theres an overload in a case where the breaker did NOT trip.............You might want to consider using dual element time delay (aka Slow Blow) fuses in the disconnect. HOWEVER all this depends on the time current curves of the fuses and breaker, as those change the chances of a fuse blowing but NOT the breaker or vice versa will change. Without getting into all the exact specific time current curves and calculations, I'm ONLY saying you may want to consider the use of a "Slow Blow" fuse IF YOURE WORRIED ABOUT ONLY ONE FUSE LEG OPENING. Again, if the welder is 240 ONLY and no 120 is used that shouldn't be a problem, but its just one possible way to insure the breaker opens which opens BOTH legs.

3) From the disconnect to the welder Id consider a section of flexible conduit.


SUMMARY if the welder is 240 ONLY (NO 120), no Neutral is required,,,,,,,,,,,,Its okay to use the disconnect although its not required to protect the feeders, the breaker already does that,,,,,,,,,,,,,If youre worried about opening only one leg (though shouldn't be a problem) consider slow blow fuses in the disconnect.

FINAL consult local authority and trained qualified professional electricians if in doubt and dont hang your hat or risk a hazard on my or any advice here lay or professional, NO WARRANTY ON ANYTHING I SAID I'm long retired and rusty on the latest codes.

Electrical and Legal Questions draw a lot of responses, some good, some perhaps possibly not so good, your choice and decision, so read and consider and do your homework and keep safe now.

John T Long retired EE
 
The ONLY times I recall using dual element time delay (Class K5 if I recall correctly??) fuses was for motor supply branch circuits where I didn't want to risk an initial higher surge current a certain motor application might require to blow the fuses. Otherwise, I used regular fuses. To determine when a breaker or a certain class of fuses will open you would have to study their curves if you're concerned about when say ONLY one fuse opens versus when a two pole breaker (BOTH legs are then open) might open.

John T
 
Another thing you could do if you since your are not going to depend on that disconnect as a protection point, and just use it for a convenient disconnect point, is to replace the fuses with a piece of copper pipe, or jump around the fuse holder with no 6 awg stranded copper wire. number 6 will carry 60 amps on your 50 amp double pole breaker, if one leg faults, it will open the breaker to both legs. I have seen this several times on different farms over the last 35 years. I have also seen flattened no 6 solid wire used to jump across an idle meter socket that is fed by another meter on property.
 
I looked up one site that says the machine has Smart Logic -prevents damage from being connectoed to the wrong voltage. That same protection would probably work if it was single phasing also. I am confused though ; Does Miller and Lincoln both make a Thermalarc 185 ?
 
Thermal Arc is its own brand, or at least it used to be. I think that either Thermal Arc or Thermal Dynamics was bought by somebody else in the last few years, but Thermal Arc welders are currently still being sold under their own name.

Stan
 
They now have the Victor name on them and changed the color to green. FYI, Smith torches are sold with the Miller name on them now. The consolidations are hard to keep up with.
 

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