installation of cables

[dinkelja]

Any advice / opinions for a young engineer - would be helpfull. I am working with a designer on sizing feeders to a freezer / cooler room. The PDP requires a 3000A service (480v, 3ph, 4w), but has 75deg C terminations. The designer wants to run 10 sets of 500kcmil in seperate RMC. The run is 250'. (I wonder what that comes down to per foot, $30, anyways)? I don't know if that is relitevely short way for an electrician to pull that much cable; it is about 2 bends plus the rise and drop, I called out 4"C. I am looking at the NEC tables and this seems reasonable, but in practicality, how would any of you do it? The combined ampacity would then be 3800A. I am also having trouble finding the NEC article on grounding with multiple conductors 250-?. He says 1250 kcmil in a seperate conduit. This is all copper wire by the way. I haven't addressed the 1 hour fire rating yet, supposedly the coolers need to stay running through the fires of he** and back. I need to get to reading that code quickly. (we are having a disagreement about feeders for fire pumps as well) thanks, Jared - Cincinnati, Ohio, USA

 

[cammax]

Have you thought about MIC Cable?
we have used it to feed Mission critical locations and Fire pumps as well, speed of installation offset the the material cost for our project.

 

[dinkelja]

I am not familiar with an MIC cable, is is like type MI cable (Mineral Insulated)?

 

[BJC]

In his case it would be cheaper to run bus.
SquareD, GE and Seimens all make pre-fabbed busway.
You can get it up to 5,000 amps.
It has to be engineered, most vendors will do it for you if you send them the locations of the end points and things to miss along the way.
To fire proof the easiest thing will be to build a sheet rock enclosure around it ( with access doors at bus joints).
I would bet lunch that you don't have enough money in you budget to use MI cable.

 

[cammax]

yes MI cable, sorry is Bus duct cheaper then MI cable?
we went with MI cable due to retro-fitting and space limitations.

 

[davidbeach]

For conductors in conduit, you need to run a full size grounding conductor in each conduit, except that the grounding conductor is not required to be larger than the phase conductors in the conduit, so a 500kcmil grounding conductor along with the 500kcmil phase conductors. Big runs are expensive.

There is also one type (from one manufacturer) of RHH insulated conductor available that when installed in metal conduit becomes a fire rated assembly. It's been too long since I spec'd something like that that I don't recall whether it is one hour or two, but it is suitable for fire pumps.

 

[EEJaime]

Although MI an cable installation is expensive, I would recommend it for the fire pump feeders. Code requires that the feeders to the Fire pump, both normal and emergency feeds, (assuming the transfer switch is at the pump), must be fully enclosed in a fire rated enclosure or in concrete. This can be quite expensive as well, not to mention difficult to achieve in many locations. Look at a company named "Pyrotenax" for installation and specification guidelines.

As for the large feeder, remamber that each conduit must have a full sized ground conductor. Not just sized for the conductors in that conduit, but for the overall feeder kcmil equivalent.
Good luck

 

[itsmoked]

Another problem that comes up here commonly is runs with the phases all kept neat and separate. Because of this they end up with horrible circulating current problems.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[itsmoked]

See: thread238-170740 pertaining to my point.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[mc5w]

Placing type X drywall around busway will void the UL listing and current rating. Busways are rated for free air heat dissipation. Also, the rating of a totally enclosed nonventilated busway is based on a 100 degree Celsius temperature rise and about 2% or 3% voltage drop for a 100 foot run. Based on what you have, what you need is a combination high voltage fusible switch, dry transformer, main breaker, and switchboard lineup which is how Coca-Cola in Twinsburg, Ohio did their 3rd electrical service.

Why on earth is your electrical engineer specifying a 3,000 amp main breaker and feeder run? I know somebody who built a 4,000 amp 480 volt service for McNeil and NRM in Akron, Ohio using five 800 amp fusible switches. Each switch is 80% standard rating which means that the service can deliver 3,200 amps maximum. That is so much easier than a 3,000 amp single circuit breaker. Also, the largest motor that is practical on 480 volts is 500 horsepower if you are using ABB motor controllers. If you have a chiller that is over 500 HP or even 250 HP you need to think about breaking it up into multiple units or running it on 2400Y4160 volts.

National Electrical Code allows the secondary of a power transformer to have 6 fusible switches or 6 circuit breakers or some other combination totaling 6 devices for secondary protection.

If you look in Annex B of NEC you will find that there is no such thing as a 3,000 amp passively cooled underground duct bank.

 

[tommom]

Use MI cable for the fire pumps. I would think that for 3000 amps the busway would be good, but if you had to use wire & conduit, I think it would be (8) sets of 500 kcmil in 4" conduit with a minimum 400 kcmil (odd size) ground wire in EACH CONDUIT. Then you can put the conduits in a 2-layer drywall enclosure to get 1-hour rating (drywall is 1/2 hour rated).

The 75 degree termination rating is standard above 100 amps.