How do you make an extension cord that would be a four-prong male plug on one end and a three-prong outlet on the other for 240V 50A that would plug into a generator to power a welder?

Answer home depot or a electical supply store has what you need..just put it together you need your cord and your and your plugs but you would have a problem with the ground... you wouldn't have one.... very dangerous I do this often to use the generator on my motorhome to power my house or more often the reverse when I want to plug the shore power line on my motorhome (which is a 4 prong male connector) into my 50 amp 230 volt welder outlet in the garage (which is a 3 prong female outlet). To do the latter is easy and not dangerous. The wire you will need to purchase is a four conductor number 6 guage copper wire to support 50 amp service and meet National Electrical Code.

The colors of the wires in this conductor are black, green, red, and white. Since you are making an extension cord you will be using a flexible cable which can be purchased from a home center or electrical supply house but it should be rate as junior hard service or hard service and will have a marking on the thermo plastic cover of SJO, SJOO, SJT or SO, or SOO for hard service. These cable types are rated for outdoor use in wet and oily applications such as garages, construction sites, theater stages and the like.

Type SO is quite common and more detailed discussion on these and others are covered in article 400 of the NEC Now for the wiring connection. From the four conductor cable connect the green and white wires together to make the ground connection on the three prong welder recepticle. Do not cut either of these wires as they are needed to carry the amps of the load to ground.

Connect the Black wire to one of the spade connectors on the three prong recepticle and the red wire to the other. The four prong plug is mearly a two 120 volt circuit connetion that has a black and white wire connected for load and ground on one, and the red and green connected for load and ground on the other. Since each of these two circuit are from different breakers in the generator or house circuit breaker panel.

They add together to give 240 volts at the three prong recepticle You can make this all easier, however, by going to Miller Electric who make this adapters for their welder generators for $59.00, or going to an RV supplier and buying the 50 amp 50 foot cable with the four prong connector already attached and made of the right rated cable. Attach the four remaining leads to a weather proof outlet box with a snap lid much like those on the outside of your house but made for a 240 volt three prong female recepticle. Hope this helps.

From Grant.

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