[dbltrbl1978]

replaced wiring in 30 year old storage building with 12/2 yellow .keeps trippping 40 amp breaker.do i need bigger breaker or orginal white wire?

 

[Old Thomas]

You have 12/2 wire on a 40 amp breaker? 12 gauge wire can only handle 20 amps.

 

[roughneck]

The color of the outer wire jacket isn’t causing your breaker to trip.
But the 12 wire on a 40 amp breaker isn’t safe. You’ve overloaded the wiring for that circuit.
You need a smaller breaker or bigger wire.
What is this 40 amp circuit serving?

 

[kjones1829]

40 amp on a 12 gauge wire is not ideal.

 

[joed]

You need a smaller breaker. You probably need all new cables because 40 amps has probably melted the #12.