Thread: CB size needed for spot welder

I have a spot welder that is marked 220 VAC and 20 K.V.A. at 50% duty cycle.

What is the recommended circuit breaker and how do you calculate the value?

How am I affected if I am on 208VAC?

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Originally Posted by Electrical Inquiry

I have a spot welder that is marked 220 VAC and 20 K.V.A. at 50% duty cycle.

What is the recommended circuit breaker and how do you calculate the value?

How am I affected if I am on 208VAC?

Look for a data plate on the unit or look up the manufacturers info on the web to get the min and max primary current. Wire and C/B for the max shown.

Is this a basic home workshop welder?

Think it would be rated at 30 amp. So looking at 50 amp breaker, I think.

Do you have the Make and model for it???

The unit is a production model spot welder. The manfacture is no longer in business.

The only placard information is as follows:

Rex Welder & Engineering, Kansas City

Type: FR-SW
Cycles: 60
Primary Voltage: 220
Short-circuit secondary current at Max Secondary voltage: 14, 500
Rated K.V.A. at 50% Duty Cycle: 20

Maximum Open circuit secondary voltage: 4.8
Minimum Open circuit secondary voltage: 2.4

The welder came with a 50 Amp plug on the cord. (probably for compatability with other equipment in the previous owners shop.

No other information on the system.

The 20% duty cycle means that you can weld for 2 minutes in a 10 minute period. The welders I have seen with that rating with a 50 amp plug need a 30 amp breaker. The breaker will kick out before you burn up the welder if you weld too long....hopefully!

Isn't the formula for calculating Amps when you have KVA and Voltage

Amps = (KVA x1000) / Voltage

So (20KVA x 1000)/ 220Vac = Amps

Therefore X= 90.9 Amps for a 100% Duty cycle?

You have a welder with a 20% duty cycle so that does not apply. Check the prices of welders with 100% duty cycles....

Originally Posted by Electrical Inquiry

Isn't the formula for calculating Amps when you have KVA and Voltage

Amps = (KVA x1000) / Voltage

So (20KVA x 1000)/ 220Vac = Amps

Therefore X= 90.9 Amps for a 100% Duty cycle?

There are different types of duty cycles.

For welding, the duty cycle is based on the number of minutes you can use it during any 10 minute period. That welder says 50% duty cycle, which means you can use it for 5 minutes, the you must let it cool for 5 minutes before you use it again.

For electronics, the duty cycle is based on a pulsed square wave operation. A 50% duty cycle means that the pulse is on for 50% of the time, and off for the other 50% of the time. Let's say you have a 60 cycle frequency with a 50% duty cycle, that means that the square wave is "on" for half the time and "off" for the other half of the time (one cycle) and that "cycle" repeats itself 60 times per second. It is not like a 60 cycle ac sine wave, though, as the 60 cycle ac sine wave is goes from '0' to '+ max' to '0' to '- max' to '0' (one cycle) 60 times a second, and that cycle is only 'off' at '0' and is only 'fully on' at 'max' (both + and - from the reference point. For a square wave, think of someone operating a switch which connects and disconnects a battery 'on' then 'off' (one cycle) and does that 60 times in one second. In this case the power is only 'on' when the switch is 'on' and is off when the switch is 'off', with an 'almost' instantaneous 'power on' and 'power off', but it is not truly "instantaneous", just "almost" instantaneous. A 60% duty cycle would be 60% 'on' and 40% 'off' (one cycle) at the frequency being operated at.

I believe you are thinking of the latter, whereas with a welder, it is the former, which means that it is 'fully on' the entire time you are welding with it.

(Anyone who can describe that better than I did is free to do so.)

One of the posts had this: Rated K.V.A. at 50% Duty Cycle: 20

Its easy to think the duty cycle is 20 when it actually says 50 due to the format.

If it has a 50 amp plug I would put it on a 50 amp circuit unless the cord looks like the original factory part and is smaller than what would be used for 50 amps.

http://www.lincolnelectric.com/asset...ln3/imt237.pdf

Here is the manual for a welder I used to own that had a 20% duty cycle. As you can see it is wired with #10 wire and fused at 50 amps. The welder has an internal breaker that kicks out before it overheats due to excessive use.

The one I had several years ago had a 50 amp plug and was wired with #10 and was fused at 30 amps. The one I had did not have a internal breaker and depended on the 30 amp breaker for protection. It looks like they changed the design some.

Originally Posted by Electrical Inquiry

Isn't the formula for calculating Amps when you have KVA and Voltage

Amps = (KVA x1000) / Voltage

So (20KVA x 1000)/ 220Vac = Amps

Therefore X= 90.9 Amps for a 100% Duty cycle?

Is this a portable gun welder or a stand (fixed position welder) ?

Originally Posted by Electrical Inquiry

I have a spot welder that is marked 220 VAC and 20 K.V.A. at 50% duty cycle.

What is the recommended circuit breaker and how do you calculate the value?

How am I affected if I am on 208VAC?

You'll need a power source for true 1~ 220 VAC 60 Hz. (180<, not 120<).


PF, performance and safety issues.

Consult with a Commercial electrician, power source suppliers, building management and the utility.

H.G.,
Is your pictured F R N the same as a FR-SW ?

Originally Posted by Garry Sorrells

H.G.,
Is your pictured F R N the same as a FR-SW ?

Garry,

Its a vintage Type F R... (resistance) Spot Welder, do you know what that means?

Apparently not, because you asked if the OP's spot welder was a fixed position or a gun.

Its a fixed position welder, no doubt. It has a horn (rocker arm) the R tells us this, and the F tells us it is operator's own force upon the (foot) peddle is what controls the pivot (manual "foot power" mechanical) of that arm.


In regards to the subject and the random speculations and comparisons to completely different welding methods, and types, and what the information (disinformation) and format errors regarding the plate information, and what it actually MEANS, yep, you betcha. Provided so as to STOP the speculation and guessing. Furthermore this is not modern equipment. It is the same brand, vintage, KVA, duty cycle etc. of a Type FR.. which is a (F)oot operated (that is to say operator's power (foot) on the pedal, not pnuematic to the horn), horn or (R)ocker arm type Spot Welder.

As far as the finer details, gaps, positioning, etc. It doesn't matter much at this point. The OP doesn't seem to know what he has, electrically speaking.

Seems everyone has ignored his topic posts' final sentance regarding this vintage welder and data plate information (flawed in format slightly) provided by the OP and have gone off on tangents, and don't seem to realize the title question is flawed.

Furthermore the random guessing as to the title question has more than a little concerned.

This isn't anything to screw around with.