If you're putting in a new plug just use a 15 ampe one.

Are you asking if you can tap off of the 220 volt/30 amp circuit that's already in the receptacle box to add a 220 volt/15 amp receptacle? And you'll only use one piece of equipment at a time?

Mike

Problem is ...
When you over fuse a 15 amp plug , wire , and receptical ...
It could over heat and not trip the 30 amp breaker ...potential fire risk ..
Fuse the circuit at the maximum circuit ampacity and NOT beyond ..

I have to at a min add a new outlet, should it be a 15 amp to match the machine or just change the machine to 30 and add a 30 outlet.

Sounds like you already have three receptacles on one circuit. You are only using two receptacles now and want to plug a machine into the third, unused receptacle. Right?

Close but no. I have 2 receptacles on the circuit and the wires for a 3rd lacking only the receptacle. My question is the new machine has a 15amp male plug. should I finish the 3rd line with a 15 amp female to fit the new machines 15 amp male plug or should I just cut the male plug off the new machine and replace it with a 30amp male plug and wire in a matching 30amp female.



If I got it right, your 30A breaker will protect devices (receptacles) that are rated at least 30A. If you use a 15A receptacle for your new 15A machine, and that machine fails drawing 25A, your breaker won't trip but your receptacle might melt, or fail in some way - since it's only designed to carry 15A.

But...if your new 15A machine fails drawing 25A, the new machine might also burst into flames, or do something else bad because it's internal wiring is only rated for 15A.

How do you you account for any item that is sufficiently lower in amperage being plugged into a much higher circuit, like a 2 amp appliance being plugged into a 20amp typical house circuit? It happens all the time. There is a risk, yes, but its not feasable to have a separate circuit for every thing.

I don't always do it the right way. In my shop, I cheat a little once in a while. But the right way would be a new 15A circuit with a 15A breaker, wiring rated for 15A, and a receptacle rated for 15A.

How do you you account for any item that is sufficiently lower in amperage being plugged into a much higher circuit, like a 2 amp appliance being plugged into a 20amp typical house circuit?

Appliances in the US are required to be "UL listed". Part of the testing makes sure that an appliance designed to be used on a 20A circuit is safe plugged into a 20A circuit, even if the actual amp draw is less than 20A.