Circuit breaker panel voltage drops when a load is added
It sounds like a bad connection to the main breaker or a bad hammer in the main breaker. Try making the measurement at the main prior to the breaker. If your voltage is normal above the breaker the hammer in the breaker is not making a good contact. If it is the breaker turn your small breakers all off. With all the small breakers turned off flip the main breaker 10-20 times on and off and see if this cleans the contact. Sometimes this fixes the problem with a bad contact. but be prepared to replace the main if it happens again. If your voltage is low prior to the main you need to call the power company and have them check the connections at the transformer / line fuses on the pole for a loose connection. I just realized you said this was a sub panel if this is a sub measure the voltage coming out of the main and verify the breaker feeding the panel is not the problem other than that same procedure for checking the main in a sub. The next thing to try is swap L1 &amp; L2 your 2 hots feeding the sub and see if the problem follows the wire.