Wire size calculations

Status
Not open for further replies.
E

eric7379

Member
Location
IL
I need advice so that a debate can be settled.

A new machine is being installed in the plant I work at. Part of the machine is fed by a 460 vac, 3 phase 800 amp panel and the main OCPD for the panel is set at 800 amps. I looked at the size of the wires that the EC pulled in and found that they pulled in 2 sets of THHN 400 kcmil's, copper. It is my understanding, that without taking any derating factors into account, that the paralleled ccc's would only be good for a maximum of 760 amps, going off of table 310.16. I questioned the person that is overseeing the installation and his response was "the EC said that according to his charts and calculations, they have the right wire size".

But, on another area of the machine, they pulled in 2 sets THHN 500 kcmil's, copper, for a 600 amp panel.

I keep asking to see what charts, tables or calcuations they used to come up with the wire sizes, but I keep getting the run-around.

I also think the conductors should be de-rated for temperature because the conduit that they are in run close to the ceiling for at least 50'(ceiling height of approx 30') and in the summer time, the temperature can easily exceed 110 degrees F. Even in the winter it is around 90 degrees.

Is my train of thought on the right track or am I misguided in my efforts?
 
400 kcm iis only good for 670 amps but you may still be allowed to use 800 amp breaker. What is the amperage of the motor?
 
The panel will feed several different blowers and stackers. Some of the load would be continuous, some non-continuous. I do not have the load calculations with me. I have only seen a few 25hp motors installed so far that will be fed from this panel, but a lot of the equipment has not been installed yet.
 
According to 240.4(B) 400 kcm can be protected by 700 amps (art. 240.6(A) not 800 amps being that it is feeding a panel and not one motor as I thought.
This is assuming the calculated load is not greater than 670 amps.
 
Can the OCPD that is set at 800a be adjusted down from 800a ? The size and style of the dist. panel and breaker may be an effect of the type of loads being served.
They may be serving a lesser over-all current, w/ the 400's, but at larger individual ampacities (spelling?) necessitating a larger framed dist. panel. ( Ex: the 25hp motors. ) Where as the panel fed w/ 500's could be serving more current but to smaller loads, allowing a much cheaper smaller framed panel. I have also seen feeds to machine panels over sized in some areas to address the possibility of an added "option" not currently being used.

Until you see a schedule of loads served you really are not certain.

All that being said, if I were your boss I'd encourage the investigation of your suspicions sooner rather than later. =)


Regards,
Doug S.
 
Status
Not open for further replies.
Top