A few years ago, I had a 20A 240V circuit for an electric car charger professionally installed. The circuit runs from the service panel through metal conduit to the hard-wired charger (EVSE) in the attached garage. A couple of years after the initial install, some additional circuits were added to the conduit to serve a newly finished basement.
When the car charger circuit was originally installed, I had the electrician pull 10 AWG wire instead of 12 AWG for a bit of future proofing.
I recently upgraded the car charger to a 30 amp (24 amp continuous) unit with a 30 amp breaker.
Now, when the charger is operating I can hear and feel a buzzing noise coming from the conduit. It's loud enough that I can hear it from a few feet away. The sounds seems to resonate into the drywall, amplifying the noise.
If I lower the number of amps the car draws, the noise diminishes. It seems that prior to upgrading to a 30 amp charger, the buzzing was quiet enough that I never noticed it. At a full 24 amp load though, it's definitely noticeable.
The car seems to be charging fine and while the conduit does get warm where I hear the buzzing, it doesn't seem to get hot.
Is this normal or is it something I should have checked out further?
Best Answer
This gets a little tricky. First NEC Table 310.15 (B) (3) (a) says 4-6 conductors in a conduit need to be adjusted to 80% of their normal current carrying capacity. So 30*.8 = 24A which is the continuous load of your charger. In NEC speak the definition of continuous is a load lasting over 2 hours. If that is true then you have to add 25% load to your charge for conductor sizing. If all of what I just stated is true, then your #10 conductors are under sized and this might be why your conductors are buzzing.
Buzzing is caused by the conductors emf (electromotive force) opposing each conductor and causing it to vibrate at 60 hertz per second. This causes two things to happen first they generate heat, which we know heat + electricity = bad, and second your conductors are moving causing splices and connection points to loosen up (also not good). Start up buzzing is common in industrial sites and sometimes allowed, but these sites have skilled maintenance crews who are supposed to have scheduled routines to maintain these circuits. In a dwelling this is not so common.
It might be a good idea for you to invest in an infrared thermometer which are fairly inexpensive nowadays, so you can scan your entire circuit while you are charging to see if it is heating up above average room temperature. It might turn out you will need to either up grade your conductor size or downsize your charger.
Remember our main concern is to prevent a burn or shock hazard so stay safe.