Transformer humming noise

I inspected a condo unit and the electrical room had this transformer in it. The transformer was humming a lot. Is this typical for a transformer of this type? Thanks.


Here is better pic…

The easiest way to reduce the transformer from humming is to teach it the words…sorry, I could not resist.

Transformers do hum, and for a multitude of reasons. Some of the reasons and humming are normal, and others may present a problem.

It is hard to say from your post whether or not the humming is an issue.

Was the room:
Overly Hot?

Was the hum really loud, almost sounding like the metal may rattle?

Did the transformer seem to be an old installation?
Were you able to see the panel/disconnect on the load side - if so was there a relatively large load on it?

Jeez… I hope the reds start piling up for that one.:smiley: I was wondering how long it would take for someone to say that.

As Pierre said, transformers hum…they all do. That’s a decent size step-down transformer (150kva). Since they work via induction, they’re going to hum, it’s just one of the effects of the magnetic field inside. Another effect is the dropping down of voltage to 240/120.


All transformers hum some, but there are at least 2-3 causes for abnormal, very load humming in dry type transofrmers - 1) insecure mouting to a resonant surface, 2) particularly in humid locations, the build-up of rust or other corrosion between the core laminations which causes noise when they vibrate against each other and 3) loose laminations caused by poor design, manufacturing or just old age.

The old one’s hum pretty loud. When they start to sound more like a fire alarm buzzer, then you worry.

If, as an electrician, I was called to look at a humming transformer, I really only have one choice. Recommend replacing it. Why? Because lacking a present visible or measurable condition, there’s no way to tell if any loud humming is an indicator of future failure. It almost never is, but my crystal ball sucks.

GFCI’s get 121 degrees. I don’t think you have anything thermally.

Do you have anything more than a 60 cycle hum?

I have made a lot of $$$$ changing humming transformers and contactors.

another theory…the shipping bolts might not have been loosened.


I think you need to be careful when deciding to rule out a problem on the basis of nothing being notable by thermography. The load may be light at the time, and the panel therefore not very exciting to look at, but there may still be a problem. For instance, a panel that contained a lot of baseboard electric heating breakers, when examined with an IR camera in the summer, may show nothing of note. This panel could still have a trashed bus behind all the heating breakers. Thermography is more useful to run in a problem rather than ruling it out, in the case of problems in a panel. If this transformer was examined when most of the people on that floor were away at work, I wouldn’t necessarily expect it to look very interesting.

Statement is based on what was presented and time of inspection.
Nothing more can be considered.
I see a lot of TI scans with no other information or baseline offered. This is always a problem and we can only work with what we are offered.

As for transformers, I see cell phone chargers, security panel system transformers etc. that run around 185F most of the time. They run hot without any load. This is what baseline info gathering is all about.

Probably so, but in the case of contactors, if you have one that is otherwise functioning ok, the source of the hum is usually rust on the faces of the armature pieces that will not allow them to come completely together. A quick cleaning with a fine abrasive and a very light coat of machine oil will likely reduce the contactor hum to a whisper.

I made money because the client wanted the noise to go.
There was nothing wrong, just some parts changing by request.


It is normal for many dry-type transformers to hum during operation. An in-depth explanation of why this occurs can be found on the IRINFO.ORG website at the following URL:

Common reasons for excessive transformer humming include, but are not limited to:

  • Improper installation
  • Component or enclosure looseness
  • Defects within the transformer

In order to help ensure transformer integrity, it would be advisable to have the transformer electrically tested in accordance with NFPA 70B and The National Electrical Testing Association (NETA) Maintenance Testing Specification.

Hope this helps.

More and more I see home inspectors doing troubleshooting when they are not trained to do so nor should they unless they are qualified in that discipline. We have many inspectors who were tradesmen or contractors in a previous life and are probably competent in those fields and if they feel comfortable making assessments should do so. Those who are not trained / qualified should simply tell the customer they are not instead of making comment. If someone ever ends up in court the easiest way I know of to defuse an issue is to tell the court, “I am not qualified to make that determination and I told the customer that at the time of the inspection. Further, I recommended they hire a professional in that field to do further investigation.” Many inspectors have long since jumped onto this slippery slope. Some whom I know ARE certified, trained and have vast experience in one or more trades and they wouldn’t dream of treading into those areas while doing an inspection. Additionally, in many States you can be fined and or lose your license if you perform troubleshooting or operate as a tradesman in a field you are NOT licensed in.

Often symptoms can be caused by numerous different reasons. Guessing at causes is just that…guessing. Proceed at your own peril.