Good afternoon good neighbor George, GREAT TOPIC for us sparkies, thanks for posting.
While this subject may be a bit confusing for some (Not all) non electricians and non engineers, FWIW I consider it very basic and easy so will explain how I was taught in school and several NEC Seminars and Workshops and practiced for many years. NOTE that's NO guarantee it's still correct !!!! codes change but it is how I was taught by the NEC experts and actual board members at the time who conducted those seminars, a couple that come to mind as I best recall were Joe McPartland (EC & M Magazine) and Mile Holt, here goes:
To size the conductors per the NEC when I last practiced power distribution, I:
1) Computed the Maximum Continuous Current the load can draw..... 2) Specified the Conductors to have a MINIMUM AMPACITY of at least 125% of the MCC
EXAMPLES if the MCC was 16 Amps, the minimum wire ampacity would be 1.25 x 16 = 20 Amps = 12 Gauge if the MCC was OVER 16 Amps, I specified the next wire size or 10 Gauge
THAT BEING SAID
1) What current an AC Buzz Box Welder draws from its Primary input circuit DEPENDS ON WHAT THE SECONDARY WELDING CURRENT IS Well Duh If youre welding at 50 amps you dont draw near as much primary input current as if you're welding at 225 Amps EVERYBODY HERE KNOWS THAT IM SURE..
2) As far as overcurrent protection on the primary input wiring, its common to use a THERMAL MAGNETIC CIRCUIT BREAKER and those can trip EVEN AT CURRENT LESS THAN LABELED ON THE THERMAL SIDE GIVEN LONG ENOUGH TIME. Thats where DUTY CYCLE COMES INTO PLAY. EXAMPLE thats one reason why on a 20 amp circuit the MCC being 80% or 16 Amps WILL NOT CAUSE THE BREAKER TO TRIP OUT ON THERMAL However if you were to draw 18 or 19 amps long enough it could over time still trip ...I can not say what breakers designed specific for certain welders may now be ????????????
FWIW When and where I practiced per the NEC as I recall at the time (may have since changed) if I were to install a 50 Amp Receptacle I SPECIFIED 50 AMP WIRE AND A 50 AMP BREAKER. If the actual load was less that was not a problem BUT NEVER MORE. If I designed a branch circuit that terminated in a 50 Amp receptacle, I at that time (maybe changed since???) would NOT have wired it with 20 amp wire but 50 amp wire AGAIN MAYBE ITS SINCE CHANGED NO WARRANTY check it out for yourself, don't necessarily do what I did back then lol
While some may think they know more than the NEC board of experts (maybe they do, if so take it up with them not me lol) and want to do different and have got by fine for years or want to take shortcuts or do it half or how they think best YOU GET NO ARGUMENT FROM ME, GO FOR IT its YOUR business YOUR risk YOUR choice, wire it as you well please, I support a persons free choice on their own property yayyyyyyyyy
REGARDLESS I still advise, suggest and politely would ask everyone here when sizing conductors for welders or anything else to consider USE WIRE THAT HAS A MINIMUM AMPACITY OF 125% OF THE LOADS MAXIMUM CONTINUOUS CURENT Do NOT take MY word for it, if in doubt consult the NEC and professional current practicing electricians and engineers, doing so could save a life.
PS Yessssssssssss Duty Cycle is relevant and can affect how a Thermal Magnetic Circuit Breaker may be specified and operate......
Best wishes to all here, be wise be safe now, take care
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