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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?

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    Are these 10awg wires protected by the 30 amp breaker the only wires on the conduit? If this is the case I would verify the insulation as 10 AWG wiring has the ability to Handel much more than this the NEC has large safety margins for residential and the only times I have found warm conduit or 60 Hz noise available is when there is leakage to ground.and that could have happened when pulling the wire. A megger would be needed to verify this a Meggers is a high voltage ohm meter and I usually test at a minimum of 2x the supply voltage. – Ed Beal Sep 22 '18 at 23:14
  • There are also three 120V 15 amp circuits running through the conduit. It's a 1" conduit. – Kevin Krueger Sep 22 '18 at 23:33
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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.

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    He's up to 8 CCCs in that conduit (3 120V hots, 3 120V neutrals, and the 2 240V conductors for the EVSE), putting him at 70% for the derate -- which means that his 10AWG conductors are definitely undersize for a 24A continuous load in this case. – ThreePhaseEel Sep 23 '18 at 14:26
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    @ThreePhaseEel - That's where it is tricky. We're only assuming he has 8 CCCs, but we really don't have a verified count. Assuming worst case is probably best. You could say the same about "continuous" is it correct to assume 2 hour operation? I kind of went to the middle. – Retired Master Electrician Sep 23 '18 at 14:45
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    NEC 625.41 says that all EVSE is a continuous load, anyway. Agreed that 8 CCCs is a worst-case assumption (having 2 of the 120V circuits as a MWBC would knock it down to 6, but that's the best-case for the circuit combo in question on a split-phase system) – ThreePhaseEel Sep 23 '18 at 15:07
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    Thanks for the info. Very enlightening. There are 5 hot wires, 3 neutral, and 1 ground in the conduit. I can set my car to draw fewer than 24 amps. Would 21 amps satisfy the derate? – Kevin Krueger Sep 23 '18 at 22:30
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    Also, in terms of a long term solution, would I be better off running a parallel conduit just for the car charger circuit, or running a lower gauge wire for the car charger through the same conduit? Is the 12awg wire serving the 15 amp circuits out of compliance due to the number of conductors present? – Kevin Krueger Sep 23 '18 at 22:36
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We have a large (probably 200+ gal) compressor at work on maybe a 50amp breaker. It has a dedicated 1.5" metal conduit. When that compressor starts, the conduit buzzes very loudly. I asked the electrician and he said it was normal given how much electricity it was pulling.

I would say your buzzing is normal.

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4 circuits in a conduit is usually fine

With a 120/240 split phase circuit, the neutral isn't counted for conduit fill derate. That means every possible circuit in residential split-phase service has 2 wires. Four circuits have 8 wires.

Now we look on table 310.15(B)3a. We see that 7-9 wires calls for a 70% derate. WAIT.

For THHN, the 70% derate comes off the 90C column of the ampacity table. This is because of section 310.15(B):

the temperature correction and adjustment shall be permitted to be applied to the ampacity for the temperature rating of the conductor...

So we arrive at numbers as follows:

  • 14 AWG: 25A x 70% = 17.5A
  • 12 AWG: 30A x 70% = 21A
  • 10 AWG: 40A x 70% = 28A

You said your draw is 24A.

Now if there were 4-6 wires, the derate would be 80%. That would give you 32A on your #10 wire, and you'd be unquestionably in the clear.

10 AWG is statutorially limited to 30A

Your 10 AWG wire is rated at 35A assuming the two wire terminations are rated at 75C, which is likely. (Less likely: 90C terminations in which case THHN or THWN-2 wire are good for 40A).

However, a separate rule in Code, 240.4(D), arbitrarily limits 10AWG to 30A max regardless. ThreePhaseEel explains why in a comment. It also limits #12 to 20A and #14 to 15A.

Continuous load derates to 125%

625.21 Overcurrent Protection. Overcurrent protection for feeders and branch circuits supplying electric vehicle supply equipment shall be sized for continuous duty and shall have a rating of not less than 125 percent of the maximum load of the electric vehicle supply equipment.

NEC 625.21 designates your EV charger a continuous load. You need to provision cable good for 125% of its 24A load, or 30A.

Do the derates stack?

That is the remaining question. Obviously some of these derates do not stack, for instance the 70% conduit fill derate does not stack with the statutory 240.4 derate. So do we take 125% of 35A, 30A or only 28A?

I have to tell you, my Code fu isn't quite there to answer that question. But I sure know it's a question worth asking, and not just assuming.

It's a wobbler, to be sure, but only a statutory one.

Other solutions

And are you sure that’s a 24A charger and not a 22A charger? Can the software be patched/configured to make it a 22A charger? May be worth hitting up the manufacturer and seeing what they can tell you.

If you can eliminate one of the three other circuits in the conduit, so you have 3 circuits in total, that will reduce the 310.15(B) derate to 80%, which will change that "28A" number to "32A" and again moot the question.

Another way to do that same thing without losing any circuits is to convert two of the three other circuits into a multi-wire branch circuit for the length of its run in the conduit. The entire MWBC counts as 2 wires for 310.15(B) purposes, so again your derate comes up to 80%/32A.

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    The issue with the lowered ratings for 14/12/10AWG has to do with the fusing current of smaller gauge wires AIUI... – ThreePhaseEel Sep 24 '18 at 3:37
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    Your all good except NEC Article 110.14 (C) states that the temperature rating associated with the ampacity of a conductor shall be selected and coordinated so as not to exceed the lowest temperature rating termination, conductor or device. Which means you have to use the 75 C rated conductors in 310.16 instead of the 90 C. – Retired Master Electrician Sep 24 '18 at 15:09
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    @RetiredMasterElectrician except, the terminations are not inside the conduit or raceway, so out of scope for the application of 310.16 to that conduit or raceway. – Harper Sep 24 '18 at 16:24
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    @Harper - Except they are a part of the circuit. – Retired Master Electrician Sep 24 '18 at 21:26
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    Would replacing the #10 wire with #8 also solve the problem? I can't change the amp rating of the charger, although I can configure the car to draw fewer amps. – Kevin Krueger Sep 25 '18 at 19:37

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