the current project im at is a 4 story wood structure. there are 800 amp tap box on each floor feeding meter stacks on each floor. the tap boxes are coming off of a 800 discconect. the voltage coming in the building is 120/208. Right now in every suite on the first floor and second floor, the dryers have 4800w construction heaters connected to them. there are 21 suites a floor and 8 of the suites have two heaters. I calculated the max load on the service feeders feeding these tap boxes with all the heaters is around 375 amps per phase. Bassically the problem we have is just with this load, the main distribution panel where the 800 disconnect is located in, hums and the cabinet vibrates a little. the hum and vibration changes which the load so if i turn off all the meter stacks one by one the hum and vibration dissipates and once all load is off, nothing hums or vibrates. i know that transformers naturally hum but is it normal for a disconnect to hum, there are no tranformers as the proper voltage gets brought in from the supply authority. i have checked the tightness of the lugs on all the connections from the 800 amp disconnect all the way to the fourth floor tap box. because its all just resistive loads be causing the hum?
MDP Vibration/hum
- Thread starter joeypb89
- Start date
- Status
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
Magnetic forces between the current carrying wires and bus bars will cause some 120Hz forces on them, proportional to the square of the current flowing (the product of the current in each of the two wires involved).
It seems odd that at that low a current you have really noticeable hum from that cause, but I will not rule it out and leave it for those with actual experience at those current levels to comment further.
The bus bars are mechanically braced to handle fault current, not just full load current.
It seems odd that at that low a current you have really noticeable hum from that cause, but I will not rule it out and leave it for those with actual experience at those current levels to comment further.
The bus bars are mechanically braced to handle fault current, not just full load current.
- Location
- Massachusetts
I agree with goldmember.
That said it might be worth checking if your parallel sets are run correctly. You don't want to run a couple of one phase on one side of metal brace and the others on the other side etc.
That said it might be worth checking if your parallel sets are run correctly. You don't want to run a couple of one phase on one side of metal brace and the others on the other side etc.
It's 4 parralell runs of 250mcm, how do you mean they could be run incorrectly? The cables enter the MDP on the left but the connection point is on the right so the runs run directly perpendicular to the bus bars because there wasn't enough length to go to the bottom of the cabinet then over then up. Do you think it is that?
Phil Corso
Senior Member
- Location
- Boca Raton, Fl, USA
Joepb,
Presuming the problem is not new, or it started after some recent work on the system, then check for any penetration thru sheet-metal panels where 3-phase conductors are not passed thru the same hole!
Regards, Phil Corso
Presuming the problem is not new, or it started after some recent work on the system, then check for any penetration thru sheet-metal panels where 3-phase conductors are not passed thru the same hole!
Regards, Phil Corso
Assume:
MDP w/4 x 800 A disconnects
each disconnect feeds a flr
Heaters = 2 x 21 + 2 x 8 = 58
I = (58 x 4800) / (1.732 x 208) = 773 A or 387/flr
What are the MDP lugs/bus rated? 2000, 3000 A ?
have you checked that all bars are bolted down tightly?
some noise is normal
MDP w/4 x 800 A disconnects
each disconnect feeds a flr
Heaters = 2 x 21 + 2 x 8 = 58
I = (58 x 4800) / (1.732 x 208) = 773 A or 387/flr
What are the MDP lugs/bus rated? 2000, 3000 A ?
have you checked that all bars are bolted down tightly?
some noise is normal
- Status