I have a 3-wire 12 gauge cord (black,white,green) with no ground, a three wire plug and receptacle, I keep tripping the breaker, (2 poles,20 amp each) I wired the green to the neutral on the receptacle, is the breaker too light? I have an older 2-wire breaker 220 with no ground other thab a hole in the side of the breaker with 40 degrees C printed.
I'm stuck

Wow. You really should not be playing with this stuff.
All I got from this is that you have a cord with a black, white and green, that you think has not ground, and you wired the green (ground) to a neutral on a receptacle, all while saying this is 240v.
There is so much wrong here it's not funny.

Your cord DOES have a ground. It's the green wire.
240v does not use a neutral. It is two hots and a ground. You use the white and black as the two hots.

Where is the cord (hopefully it is actually cable) going to and coming from?
What receptacle are you using?
What is being plugged in?
What kind of breaker is it?
Breakers do NOT have a ground connection on them. Never did. What hole in the side are you talking about?

Do you know anyone who knows about this stuff that can come and help you out? Or, can you take pics of everything so we can get a better picture?

I can't tell what you are trying to do but I get the impression that you are trying to wire a 240V receptacle.

Quote:

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I have a 3-wire 12 gauge cord (black,white,green) with no ground

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The green is the ground. A 3-wire cable is a 2 conductor with ground cable. The ground is not a conductor and cannot be used as such.

I can't tell from your description whether the receptacle is a 120V or a 240V. A 240V receptacle does not have a neutral.

If you wired the ground to the neutral, where did you put the second conductor.

Do not use a 120V receptacle on 240V line

A breaker does not have a place for connecting a ground wire. The ground wire in the panel is connected to the neutral/ground buss.