Derating MC cable and SER Cable

[henbra]

Please can you let me know opinion regarding the value of the conductor derating factor, conductor size and allowable temperature derating for this project .
A 4 story building, 80 units, multifamily apartment complex will be served by 80 feeders rated 100A, 120/208 volts, single phase, 3 wires per each unit. The electrical room and metering room is located at the garage level, for this reason the multi-conductor cables will be stacked or bundled longer than 24 inches. We are planning using: Option 1. MC Aluminum multiconductor cable. Option 2. SER copper cable. Can you let me know your opinion regarding the most economic option.

 

[benaround]

Re: Derating MC cable and SER Cable

henbra,

First, The comment "--3 wires per each unit",that was a typo right? Oops,now I see it MC feeder cable using the sheath for EGC.

I would have to guess that the Alum. MC would be less $ than the copper SER,around here copper is pretty high right now. On the other hand,if the per/ft. price was close,I would go with the SER. personal preferance.

frank

 

[physis]

Re: Derating MC cable and SER Cable

Would I be right if I assumed you're going to have to derate to 35% because of table 310.15(B)(2)? And that if you did, you'd need 250 kcmil Cu. or 400 kcmil Al. for 100 amps services?

 

[henbra]

Re: Derating MC cable and SER Cable

Is a 4 wire, cable. My question is whether the adjustment factor Table 310.15(b)(2)(a) of 35%(41 conductors and above) applies to MC and SER.

 

[physis]

Re: Derating MC cable and SER Cable

I don't know of any exceptions for SER and the exception for MC is only for 12 AWG.

But the Exception to 310.15(A)(2) might help if you can keep the bundling to ten feet or less.

There's a lot of smarter guys than me here, someone should know a way to avoid the 35%.

 

[peter d]

Re: Derating MC cable and SER Cable

This feels like deja vu!

Without knowing all the details, I'm still not comprehending why you plan on running 80 feeders into one electrical room. From my minds eye, you may have to do some serious derating to the point where the feeders will become excessively large.

 

[benaround]

Re: Derating MC cable and SER Cable

I don't know what it is,but when a Consultant asks about certian things in this Forum,I feel like I'm getting conned into doing for free,What he/she gets paid to do.

Does anyone else feel this way?

frank

 

[henbra]

Re: Derating MC cable and SER Cable

"Exception N0. 5:
................
A 60 percent adjustment factor shall be applied where the current-carrying conductors in these cables that are stacked or bundled longer than 600 mm (24 in.) without maintaining spacing exceeds 20."

Can I use this exception for MC cables bigger than #12?

 

[physis]

Re: Derating MC cable and SER Cable

Does anyone else feel this way?

Maybe a liitle but if I was gonna be stuck with a 35% adjustment factor I'd be hoping I could get some ideas or advice too. I don't feel like he's asking us to design it. It's just the one issue.

 

[physis]

Re: Derating MC cable and SER Cable

Can I use this exception for MC cables bigger than #12?

No.

Edit: Can't you find a way to spread the cables out some?

And the neutral conductors don't have to be counted as current carrying, 310.15(B)(4).

What a mess. to get to a 70% factor you'd have to have 18 bundles of 9 cables.

I don't build apartment buildings so I don't know how this would usually be done but I'm thinking I'd be looking at how other buildings are done if it's not like this.

[ July 07, 2005, 06:27 PM: Message edited by: physis ]

 

[benaround]

Re: Derating MC cable and SER Cable

physis,

Ok, just this one time.

frank

 

[henbra]

Re: Derating MC cable and SER Cable

Sorry guys, but the facility has 5 electrical rooms already at the garage level.Each electrical room serves 80 units. The specific feeder routing through chases is requested by the owner and contractor. Thank you very much for your opinion of Consultants.

 

[infinity]

Re: Derating MC cable and SER Cable

Can the cables be installed without being bundled or stacked? I would build racks out of strut and secure the cables to them with spaces in between similar to running conduit.

 

[henbra]

Re: Derating MC cable and SER Cable

Infinity,
Thank you very much for your opinion. I agree. Is there another similar or better solution.