Main Distribution Panel with a Loud Hum

[ksmith846]

I am stumped at this point and am relying on the wealth of experience in this forum to shed some light on my dilema.

We just completed a small grocery store and they opened this week. Now that they are running at full capacity we are having an issue with one of the 400 amp MDP's.

The problem is that under full load, which is peaking at about 385 amps, there is a loud humming noise resonating from the panel. I cannot pin point if it is the Main or a branch breaker or if it is the bus. What I do know it increases greatly at peak load. Take the load down and the frequency lowers.

It is an existing Panel and Main (as requested by the owner and architect to be reused).

The service is 800 amps (which I feel should have been 1000 amps, but I do not draw or sign the plans). The service has an 800 Main Disconnect/Manual Transfer Switch outside. From which we installed one set of #500 KCMIL to two 400 amp MDP's (one new one and the reused one in question). Each set is less than 10 feet from the ATS to each MDP. Both MDPS are drawing between 375 and 385 amps. This place has steamers,ovens,RTUs....it's a grocery/pizzeria/resturaunt......
We have checked all of the wiring/breaker/bus connections......the Main Breaker under full load becomes extremely hot and smells of hot plastic.....when we alleviate 60 amps or so it cools down and the hum is more tolerable.
I am going to move (4) 3 phase 40 amp loads into a new 200 amp panel which I am going to mount outside next to the MTS (less than 10 feet). This will alleviate about 120 amps on all three phases from this panel and allow for the load to be reduced, as well as the extra heat. However I am still confused on what is causing the hum and it will still be present.

My question is what can cause this loud hum? Is it result of old equipment....a failing Main.....I am stumped at this point:-?

 

[charlietuna]

Your operating the 400 amp panel at near "FULL" load for long periods of time!! It's designed to operate at 80 percent load for periods over three hours. Other than infrared inspecting the panel at full load which would probably tell you where it's going to fail (breaker-bus-terminals) you need to explain to your customer they need to replace this panel with a larger capacity or shift about 70 amps somewhere else? This way they can plan the power interuption around operating store hours or face a future emergency shutdown.. Eighty percent of 400 is 320 amps...

 

[cadpoint]

... I cannot pin point if it is the Main or a branch breaker or if it is the bus.

I stated this one before:
One use to be able to tone cars the one trick to isolating a bad lifting cylinder was to use a paper towel to your ear.

In your case you can or form an open question mark of PVC about 2.5' in length.
or longer and CAREFULLY put it to your ear. as a matter of fact you can just put the PVC NEAR each breaker and you will hear the variables, with out trying to get your ear close to the end of the PVC.

A fellow student in Physic class became crazy about tubular bells (some might remember that) and asked Physics Teacher about them and starting building them for weekend sales, Etc...

My wild guess your breaker isn't up to the new requires of modern day breakers, IE AIC rating and you have HVAC breakers even though assumed new and in new panel its running all the way to the Main Breaker!

 

[hardworkingstiff]

Did you check the voltage drop across the main breaker?

 

[ksmith846]

to replace this panel with a larger capacity or shift about 70 amps somewhere else? This way they can plan the power interuption around operating store hours or face a future emergency shutdown.. Eighty percent of 400 is 320 amps...

I agree completely.....this is why I am installing a new 200 amp Panel.....

 

[ksmith846]

Did you check the voltage drop across the main breaker?

Yes same voltage both sides

 

[Ragin Cajun]

Panelboards are rated at 100%, not 80%.

The limitation is the molded case CB's which are only 80%.

You can feed a panel board with a 100% rated breaker and be OK.

My guess in this case it's limited by the source 400A breaker, likely 80%.

But . . . the noise needs to be fixed at the source.

RC

 

[buzzbar]

So, does anyone actually have an idea as to what CAUSES the hum? I'd be interested in this as well, as I've come across this MANY times over the years. I mean, what causes the hum? Is it a worn out circuit breaker? Bad connection to buss bar?
I don't think that a panel with a load close to 100% capacity would cause a hum, but maybe I'm wrong.

On a related matter, why do some transformers hum more than others? I think that we could all guess, but does anyone have a definitive answer?

Thanks.

Andrew

 

[cadpoint]

...
On a related matter, why do some transformers hum more than others? I think that we could all guess, but does anyone have a definitive answer?

Well that answer has been given and exists in other threads! :grin:

 

[charlietuna]

This panel is operating at 95 % of full load--now how many times does this load exceed 100% for short periods of time? Consider the motor loads and starting currents and when you exceed 100% of equipment you begin to create voltage drops, overheating connections which are like snowballs rolling down a hill ! The panel may be designed to carry 100%--but it's on it's way to failure. Being in the infrared business since 1989, i have personnelly watched equipment fail due to constant full load or even close to it!

 

[growler]

the Main Breaker under full load becomes extremely hot and smells of hot plastic.....

There you have it. The main should not be hot or smell of hot plastic.
Either the lugs are a bit loose or the breaker is bad. Once breakers get hot it's best to replace and re-torque the lugs.

( sometimes the threads on the lug's set screw will be buggered up a bit and even with the correct torque value will still be loose).

 

[Ragin Cajun]

Somehow I missed the "Main Breaker under full load becomes extremely hot and smells of hot plastic....." part.

Get someone in the infrared business to check out ALL the panels. Have you checked the demand reading on the utility meter?

In another thread I grouse about excessive oversizing required by the code. I wonder if here the engineer did a poor job of service equipment sizing? I have never had a job where I didn't have all kinds of capacity following code, generally way too much capacity.

RC

 

[charlietuna]

Any breaker--new or old that is subjected to 80 percent of it's rated load is going to be hot and look hot on an infrared scan -- Does this mean the breaker is bad? NO! Thats normal! The same breasker operating at 50 percent of it's rating will feel and look fine on a scan. I question the design of this system? The main breaker might fail, and you'll replace it, and a year later, you'll replace it AGAIN ! Is this providing your customer proper service?

 

[nakulak]

Well that answer has been given and exists in other threads! :grin:

as stated, that answer has been given quite a few times (they don't know the words)

in my limitied experience I would think that any breaker that is extremely hot and smells like burning plastic is in an overload condition. I would suggest that if the owners don't want to spend the money to have the equipment re certified/inspected/replaced, that you let them know that they should at least institute a fire watch.

 

[electricalperson]

we had a problem like this at a plastic factory in cape cod. the service was at full capacity and the main breaker was very hot. we ended up installing a 1000 amp 480v 3 phase service and put the original 400 amp main panel as a subpanel and removed some of the bigger loads off of it and put it on the 1000 amp panel

 

[danickstr]

we had a problem like this at a plastic factory in cape cod. the service was at full capacity and the main breaker was very hot. we ended up installing a 1000 amp 480v 3 phase service and put the original 400 amp main panel as a subpanel and removed some of the bigger loads off of it and put it on the 1000 amp panel

This to me is the way to go. Give yourself lots of headroom with the power. Why stress stuff out to the max? To save a few grand? Never worth it in the end, when failures plague the operations of the plant.

 

[dbuckley]

So, does anyone actually have an idea as to what CAUSES the hum?

Is it a deep hum (60Hz) or a buzz (180Hz)?

 

[quogueelectric]

So, does anyone actually have an idea as to what CAUSES the hum? I'd be interested in this as well, as I've come across this MANY times over the years. I mean, what causes the hum? Is it a worn out circuit breaker? Bad connection to buss bar?
I don't think that a panel with a load close to 100% capacity would cause a hum, but maybe I'm wrong.

On a related matter, why do some transformers hum more than others? I think that we could all guess, but does anyone have a definitive answer?

Thanks.

Andrew

The difinitive answer is that it hums because it doesnt know the words.................Or the jerk installing it never released the shipping pads mounted under the xformer transfering the vibration to the rubber pads.

 

[growler]

It is an existing Panel and Main (as requested by the owner and architect to be reused).

Here is another key phrase I missed the first time I read the post. "as requested by the owner and architect to be reused".

Makes you wonder which one of them did the load calculation and inspected the panel for serviceabilty. They really should pay either the contractor or an engineer to do a load calculation.

You see a lot of this sort of thing where the owner or architect try to make important decisions about the electrical system and you couldn't find two people less qualified. Normally neither the owner or architect has had any electrical training.

 

[iwire]

I am willing to be that if an NEC load calculation is done it will indicate that the loads served by this panel need a supply ampacity of greater then 600 amps.

If the actual load is hitting 385 amps I would be very surprised if NEC load calcs would even allow a 600 amp supply. (The existing 400 amp and the proposed 200 amp panels)