My boss at work the other day told a guy to rob a neutral from the living room outlet to the 12-2 hr in livingroom for fireplace.The fireplace drawed the same at 4 different voltages but my boss said at 240 this model could do something it couldn't do at 120.............Any thoughts?

300.3(B)?

I would get more information on what he was trying to accomplish. Robbing or borrowing a neutral alone sounds fishy to me. If someone turns off the power for the fire-place, you are still going to have current present inside the connection box of the fireplace (from the livingroom rececptacles. Sounds like a short-cut or a quick fix based on the information provided.

Find a new employer.
This guy will make you screw up. Then your screwed.

Better still, Tell the Boss "NO".
Your the PRO. you live with your actions, Regardless of direction.

Thats what they will fall back on.

CYA---CYA-----CYA.

Yeah,seems to me you have at least two breakers on same phase sharing neutral nomatter what.Does that matter??

If you had two breakers from the same phase, you would not be able to get 240volts. You would get a meter reading of 0 between both hots in the connection box of the fireplace. You will only get 120volts. Now if both breakers are using the same neutral (same phase), you can have excessive current on that neutral. Also, if one of the breakers trips, there will still be voltage present from the other circuit in the connection box. I would not borrow or rig anything. I would homerun a 12/3 cable back to the panel as a dedicated line if it needs a 240volt branch circuit.