Electrics - Amperage draw on cable.

[Yer Man]

Not entirely sure where this should be posted.. so I'll appologise now if it's in the wrong place.

Question.. Is there any way of knowing what current (Amps) a welder is pulling down a cable when being used. For the purpose of this question.. use a 250 Amp Oxford as an example, being used at full tilt. (Is there a difference between Arc startup and welding mode?)

Reason for asking.. I want to run a cable from the house through the roofspace via gantry cable to an outbuilding. (approx 30 metre run). I only have a 110 Oxford which I'm 'playing with' at the minute, which works fine on a 13 amp plug, but if I wanted to upgrade to a larger machine in the future, I want to make sure that the wiring is capable of coping with the load.

I would intend having a large(ish) compressor (something wired to a 30 amp dedicated switch) as well as the usual sockets, lights etc.

As I would be working in the place on my own, in theory only one piece of equipment will be used at any time. It's just in case the compressor and welder would be running together, that I need to get the right size of cable worked out so as not to overload the circuit. (Hence the question)

Any advise or thoughts on the matter?

I don't get a chance to get on here very often, so if I don't acknowledge anything straight away I'm sorry, but will reply.

Thanks, Tony

 

[shenion]

Most larger welders have a table of recommended wire sizes and the maximum current draw. That should give you a atarting point.

 

[irondarren]

if i remember correctly a 250amp oxford draws around 45amps, so would need to be suitably fused, cable size i guess would be 10mm2. You may find if you have a 3hp compressor and are running the oxford flat out, the total consumption could well exceed 75amps, when the compressor starts due to the fact most single phase motors draw 3x power when starting.

 

[Yer Man]

Shenion , Irondarren

Thanks for the replies lads..

Shenion.. Problem is I haven't bought a bigger welder yet. I want to make sure that all possible problems with the loading are covered. I only want to have to do this job once, and not have to upgrade again.

Irondarren.. I've looked at Pickhills Engineers who supply Pic-Arc and Oxfords. They recommend a 45 amp fuse for the single phase cable run. ( which I have to use). Is the 45 amp draw constant? or is that only on startup, and current draw drops off? (Sorry, don't know anything about the current usage required).

______________________________

Have managed to find a site that quotes a 10mm sq cable will cope with 45 amp load(encased in an insulated wall) and up to 63 amp if clipped up in free air.(open for debate)

Can't ever seeing me needing anything bigger than a 250 amp.

I'm thinking it might be better to go bigger to make sure (poss 16mm?)

Do you (or anybody else) know if voltage drop will have to be taken into consideration, and if so....
all thoughts appreciated.

Thanks

 

[irondarren]

not sure on the power draw at arc up on the oxford type, inverters are generally smooth power consumption even at start up. The man that probaly would know is weldequip.

250amps is a serious size for arc. 2.5mm rods run about 70-90amps, 3.2mm run about 80amps - 140amps 4mm run 130-180amps 5mm run 160-250 and 6mm run about 190-300 amps. so you could run a 6mm rod no problem, and give a 5mm rod what for. happly weld 25mm thick plate easy, what sort of thickness do you want to weld?

The oxford 300amp will run on single phase but thats a 63amp draw, but will roast a 6mm rod.

 

[shenion]

Current draw varies considerably. There is a current spike when the arc is first started. Then it settles down to the set current.

My big Airco states 108A at full load (475 amps). I have a breaker at 50A/220V.

Idle it is something like 7 amps. It includes power-factor correction. This increases the idle amps.

I have run it at 200A with no problems. It still trips the no-name breaker sometimes when the contactor activates (MIG and TIG mode). Breakers here are almost instant. I did change to a name brand breaker and it fixed the trip issue. I should be able to run it at about 250A. I think it is either 6 or 4 ga wire, but it is only 20'.

If I had a separate building I'd run 60A to a 125A breaker panel. Above that, it gets real expensive.

Some here get a whole separate service from the pole to their workshops. Costs a bit more, but gives all the power you would ever need.

I'd look at the the biggest welder you need, the startup current for a compressor and any other tools/lights etc and see what the max current is. Then look the the wire size needed. I know there are online resources for the NEC standards for wire size given current length.

 

[bump]

Have managed to find a site that quotes a 10mm sq cable will cope with 45 amp load(encased in an insulated wall) and up to 63 amp if clipped up in free air.(open for debate)

That sounds perfectly normal to me, as the cable in free air can easily allow heat to escape, more can be run thought it than if it is encased in a insulating material.