I have a heat trace around an outside exit door on a freezer. The voltage to the trace should be 115-120volt. Iam only getting 80volts and the trace does not work.

Usually I have found the connection where the building wire, 120 volts, connects to the heat leads can corroded from frost/water.

If your checking there and not getting 120 volts, then start at the panelboard, circuit breaker, and check at every accessible splice point to find where the volts have dropped off, again, usually due to a poor connection.

This is assuming someone had not done something stupid, like makes connectiosn so the circuit is in series with another heat section, or something like that.

I am also assuming this is the heat built into the door frame/jamb of the walking box, and not an added string of heat tape.

The trace is built into the door. I took voltage reading at junction box above door which is outside. The main line comes into box and is wire connected to wires to trace. Box is dry and connections are tight. Took voltage check from black to white and black to green 80 volts. Will work my way to breaker box like you suggest. Thanks

If there is a built in light fixture, switch, or any junction box on or close the reefer, moisture collects at these points, due to not sealing conduits between atmospheres, great for condensation.