ajhill
Member
why is it said that the circuit breaker for a welding circuit can be 200 percent of the rated ampacity of the feeders
welder
- Thread starter ajhill
- Start date
- Status
e57
Senior Member
- Location
- San Francisco, CA
I'm going to answer this from the prespective of an amature welder in this case. :grin:
Most welders are transformers - being abused....
And since they have a duty cycle, and you really got to be working hard or welding poorly to reach the duty cycle on most - so for the most part they are intermitant loads. And they have OCP on them as well.... But you also dont want to be tripping the branch circuit if you stick your electrode. Which draws very high current - but you should be welding better.... Since nothing is fool proof - eventually the electrode will act as a fuse if it sticks and re-open the welding circuit - the circuit breaker at the panel is just there to trip if you're dumb enough to keep doing it - and to protect the conductors feeding it.
Most welders are transformers - being abused....
And since they have a duty cycle, and you really got to be working hard or welding poorly to reach the duty cycle on most - so for the most part they are intermitant loads. And they have OCP on them as well.... But you also dont want to be tripping the branch circuit if you stick your electrode. Which draws very high current - but you should be welding better.... Since nothing is fool proof - eventually the electrode will act as a fuse if it sticks and re-open the welding circuit - the circuit breaker at the panel is just there to trip if you're dumb enough to keep doing it - and to protect the conductors feeding it.ajhill
Member
so why can I have 6 gauge feeders with a 100 amp breaker where is the protection
cadpoint
Senior Member
- Location
- Durham, NC
Ever have a hand pump top as a Kid? Pumping the top to make it go faster as it sits there in rotation. kinda the same thing.
With welding your asking for more work thus its a demand to increase RPM's on the Engine, adding a higher inrush - draw for amps that's a spike demand that needs a "momentary" higher setting (the breaker) to hold this spike.
I beleive the spike to work would be similar to a square root more to a division sign , but spike and falls off to a work level...
With welding your asking for more work thus its a demand to increase RPM's on the Engine, adding a higher inrush - draw for amps that's a spike demand that needs a "momentary" higher setting (the breaker) to hold this spike.
I beleive the spike to work would be similar to a square root more to a division sign , but spike and falls off to a work level...
Last edited:
e57
Senior Member
- Location
- San Francisco, CA
In the 100A breaker.... And the possible one on the other side at the unit too... It is short circuit protection. A welder will either shut off, trip it's own CB, or self-destruct in an over-load....
That said - if the recommended CB size is 100A then that is one hell of a welder.... You should be sizing the conductors feeding it by the nameplate amperage - see 630.11
And the possible one on the other side at the unit too... It is short circuit protection. A welder will either shut off, trip it's own CB, or self-destruct in an over-load....That said - if the recommended CB size is 100A then that is one hell of a welder.... You should be sizing the conductors feeding it by the nameplate amperage - see 630.11
dbuckley
Senior Member
- Location
- Canterbury, New Zealand
The reason that cables are rated for a particular current is due to thermal effects; a cable carrying a current gets warm, and the NEC tables assume a given load for a given temperature rise, and make assumptions about average loadings or derate for continuous loads.
Welders are generally only used for a small duty cycle, often just 20%, meaning something like 5 minutes welding, 15 minutes not-welding.
The upshot of this is that the conductors have a faster than expected rate of temoperature rise, as more current is flowing through a given conductor than normal calcs would allow. But... then the conductors get plenty of time to cool down. Thus the duty cycle of the application is factored in to the cable behaviour allowing you to use less heavy cable than you would normally for that size load and thus breaker size.
This leeway is not unique; POCOs allow terrible overloads to be placed on their transformers, particularly when there has been a contingent fault somewhere nearby, but they do this knowing that the tranny will only have to cook for a couple of hours, and then it'll be able to cool down again.
Welders are generally only used for a small duty cycle, often just 20%, meaning something like 5 minutes welding, 15 minutes not-welding.
The upshot of this is that the conductors have a faster than expected rate of temoperature rise, as more current is flowing through a given conductor than normal calcs would allow. But... then the conductors get plenty of time to cool down. Thus the duty cycle of the application is factored in to the cable behaviour allowing you to use less heavy cable than you would normally for that size load and thus breaker size.
This leeway is not unique; POCOs allow terrible overloads to be placed on their transformers, particularly when there has been a contingent fault somewhere nearby, but they do this knowing that the tranny will only have to cook for a couple of hours, and then it'll be able to cool down again.
- Status