Resistance Spot Welder Conductor and OCP sizing


Senior Member
We are moving 3 old spot welders. These are old Stancos with a heavy base and long copper weld arms. Two are 50 KVA and the 3rd is 30 KVA. They operate at 208v single phase.
Currently they are tied into 80 amp breakers with short runs of #6 THHN. The 30 KVA has a 60 amp breaker and #6. The runs are 15' or less.

They want them moved to a location that will require ceiling drops, so the runs will be much longer.... longest being about 80'.

I am having a hard time nailing down a solid answer on conductor and OCP sizing. To make matters worse, I don't have a code book in front of me to read what exactly 630. says about it.
What I want to avoid is spec'ing #6 in 3/4 EMT (cuz that's what's there) only to change my tune tomorrow.

Just because it has been working well for years doesn't mean it was done correctly!
Any insight on installing medium sized resistance spot welders?


Master Electrician Electric Contractor Richmond VA
Henrico County, VA
Electrical Contractor
Breaker size doesn't need to change with voltage-drop compensation.

Do the welders use their rated power all the time, or is the usage adjustable?

Without checking the NEC for welder specifics, the wire seems undersized now.

Looking at your numbers again, 208 x 80 is only 16.6kva, not 50kva.


Senior Member
D'oh! Forgot my code book again today. They weld thin sheets with small spots, so the welders are not normally used at full capacity. Yes, the power is adjustable. Duty cycle is fairly low, but I have not tried to determine what it is yet.
Yes, 50 KVA is about 240 amps @ 208 which is why I started to question the existing install. They are old machines that seem to be hooked up with the whip that was attached at auction. Again, they have been using these for years and years in the current configuration.
I think (have not checked yet) 630 says we can uprate conductors 3x... if so then #6 is good for 225 amps. #4 would then allow 285 amps, but I would then need 1" pipe. This is the reason I need to find out so I can place a material order.
*** D'oh again! Forgot to hit POST earlier this morning. I'm batting a thousand today!***