60 Hz hum and neutral bonding

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  #1  
Old 12-31-16, 09:14 AM
J
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60 Hz hum and neutral bonding

My service panel emits a barely audible continuous 60 Hz hum, that seems pretty common and I donít think that in and of itself is a problem. I noticed that when there is a higher unbalanced load (e.g. the 110 VAC microwave oven running) the hum gets noticeably louder that when there is a heavy balanced load (e.g. the 220 VAC AC compressor running). Even though the hum is louder, itís still hardly noticeable unless listening for it.

Iím an electrical engineer and mainly asking out of professional curiosity. Since the unbalanced load has a louder hum this made me think it had something to do with the neutral path. I took a look at the panel and all neutral connections are tight. I did notice that the neutral is bonded to the cabinet in two different locations. The two locations are about a foot apart, physically before and after the meter. The connections look original from when the panel was installed in 1972 (upgrading it is a whole different subject).

So the neutral current has a parallel path through the enclosure for about a foot. This appears to be wrong as my understanding is that the neutral should only be bonded at a single point, the cabinet is not intended to carry current except in a ground fault.

Iím starting to think the hum is caused by this condition as the cabinet vibrates at 60 Hz from the neutral current. Has anyone come across a situation like this, particularly in older installations? Thanks in advance.
 
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Old 12-31-16, 10:09 AM
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The neutral current is not trying to go to ground so there is no need for it to flow on the enclosure.

Ground is bonded to neutral to allow a fault to trip a breaker.
 
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Old 12-31-16, 10:19 AM
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Understood, and thanks for replying. I might not have been clear, the neutral is bonded to the cabinet in two different locations over a foot apart. It's not going to ground I'm concerned about but that the neutral current has a secondary, parallel path through the cabinet itself.

It's probably not a big deal but it seems like it violates the intent of the NEC's single neutral bond requirements.
 
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Old 12-31-16, 10:32 AM
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I would like to see this. Can you post a picture ?
 
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Old 12-31-16, 10:42 AM
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I would like to see this too.

The grounded conductor (neutral) also bonds the meter enclosure via the lug in the meter socket.
 
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Old 12-31-16, 10:51 AM
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I'll try and get one and post it later today. The cabinet is outside and it's been raining this morning.
 
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Old 12-31-16, 05:48 PM
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In most standard installations, the incoming service entry has 3 wires, H-H-N. In the meter pan, the neutral is bonded to the pan for safety (grounding). Then 3 wires continue to the main panel, where the neutral is again bonded to the panel, and all the neutrals and grounds are tied in this panel.

Sometimes the hum can come from older circuit breakers. The breakers are electromechanical and have an electromagnet in them that help trip the breaker when needed. It's not usually an issue, but something to keep an eye on if it becomes louder and louder.
 
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Old 12-31-16, 06:02 PM
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It's been raining all day and so I've passed on pulling the dead panel to take another look. After looking at installation photos online I think Zorfdt and Tolyn have it exactly right, the two bonds are at the meter pan and service disconnect. I guess this is pretty standard, I just never realized it.

So since they are separated a short distance, doesn't that put the cabinet in parallel with the neutral circuit? Sort of like a much shorter version of erroneously bonding a subpanel neutral?

I'm probably overthinking this. Anyways, the hum is faint and it's just something I happened to notice when I was working next to the panel. As long as it doesn't get louder I think its fine. Thanks.
 
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Old 12-31-16, 08:31 PM
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The hum is likely from a breaker, or breakers. I have heard many breakers hum with and without a load on them.
 
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Old 01-01-17, 06:31 AM
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Are you sure it's not a door bell transformer mounted to the panel?
Geo
 
 

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