Four MC Cables Run in 3" Conduit

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mkgrady

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Massachusetts
I need to run four three phase 48- volt circuits from the top (18th) floor of a high-rise building up through the elevator machine room and up through the roof of the machine room. With the help of some folks here I have learned that I can't run through the elevator machine room but it is virtually the only way to get to the roof where I need to feed four draft fan motors. The fans will be fed and controlled by four VFD's. I also learned here that running the four VFD outputs in one conduit is a bad idea.

Today I found what I hope is a good solution. There is a 3" conduit that runs from the 18th floor to the roof. The conduit is about 15 feet long. Would the code allow me to use the conduit as a sleeve? I believe the four cables would easily fit. I would be running four 10-3 jacketed MC cables. The motors are 15HP. The contract calls for the conductors to be #10.

Would derating be an issue with 12 CCC's?

I would run the four cables to a junction box on the roof. Would splicing the conductors in the box be an issue? Code wise I think not. I would run from the box on the roof to each fan disconnect using thhn/thwn in individual conduits. Would having the VFD outputs spliced in the same box be a problem or am I trying to keep all them all shielded from each other?

If the VFD outputs must remain shielded from each other I could continue the MC run all the way to the fan disconnect.
 
Dont think that table 310.15(B)(2)(a) nor exception #5 would allow you to not derate, however using innerduct if possible may effectively give you 4 separate 'conduits' where derating to 50% may not come into play.

Any way you could put two motors on a circuit and use #8 or #6 to reduce the # of CCC? Or use large conductors to a roof mounted subpanel/VFDs? No way to get another conduit next to the existing?

I'm sure others will have much easier/more workable ideas.
 
JFletcher said:
Dont think that table 310.15(B)(2)(a) nor exception #5 would allow you to not derate, however using innerduct if possible may effectively give you 4 separate 'conduits' where derating to 50% may not come into play.

Any way you could put two motors on a circuit and use #8 or #6 to reduce the # of CCC? Or use large conductors to a roof mounted subpanel/VFDs? No way to get another conduit next to the existing?

I'm sure others will have much easier/more workable ideas.
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If I have to derate I could use 8-3 MC. With derating 50% (of 50 amps) I would still exceed the 21 amp circuit requirement.

I cant use a subpanel. They want the VFD drives on the floor below the roof.

Interdict sounds interesting. Never used it. Not even sure what it is.
 
mkgrady said:
With the help of some folks here I have learned that I can't run through the elevator machine room
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Someone here please educate me on this one. I would assume it is an elevator code or a life safety NFPA101 code, not an NEC code. I know you can't run through a stairwell and you can mount other equipment in an elevator room, but never heard you couldn't run conduit through one.
 
Strathead said:
Someone here please educate me on this one. I would assume it is an elevator code or a life safety NFPA101 code, not an NEC code. I know you can't run through a stairwell and you can mount other equipment in an elevator room, but never heard you couldn't run conduit through one.
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Here you go.



620.37 Wiring in Hoistways, Machine Rooms, Control
Rooms, Machinery Spaces, and Control Spaces.
(A) Uses Permitted. Only such electrical wiring, raceways,
and cables used directly in connection with the elevator or
dumbwaiter, including wiring for signals, for communication
with the car, for lighting, heating, air conditioning, and ventilating
the elevator car, for fire detecting systems, for pit sump
pumps, and for heating, lighting, and ventilating the hoistway,
shall be permitted inside the hoistway, machine rooms, control
rooms, machinery spaces, and control spaces.
Click to expand...
 
Strathead said:
What I love about this site is that you learn something important on a regular basis. I used to read the code for leisure, I guess I need to get back in to that habit again.
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:thumbsup:

It goes both ways, I may have learned tag out is more acceptable in construction than it is in maintenance.
 
mkgrady said:
I... I would be running four 10-3 jacketed MC cables.

Would derating be an issue with 12 CCC's?
...
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Jacketed, yes.

310.15(B)(3)(a)...
(4) Adjustment factors shall not apply to Type AC cable
or to Type MC cable under the following conditions:
a. The cables do not have an overall outer jacket.
b. Each cable has not more than three current-carrying
conductors.
c. The conductors are 12 AWG copper.
d. Not more than 20 current-carrying conductors are
installed without maintaining spacing, are stacked, or are
supported on “bridle rings.”
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Time for RFI using #12 copper conductors, unless there is some other factor I'm not seeing about using #12.
 
Smart $ said:
Jacketed, yes.

310.15(B)(3)(a)...


Time for RFI using #12 copper conductors, unless there is some other factor I'm not seeing about using #12.
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I assume I need to use jacketed MC in order to make them suitable to be run to the JB on the roof. The conduit only rises 16" above the roof but I;m guessing that "wet location" means I need thwn conductors. I don't think unjacketed MC can be used in a wet location. I hope someone tells me I am wrong.

Maybe using #12 would be OK. The FLA of the 15HP motor is 19 amps. Wouldn't it be weird if using #12 would be OK, but if I am required by contract to run #10 I have to derate and use #8?
 
mkgrady said:
I assume I need to use jacketed MC in order to make them suitable to be run to the JB on the roof. The conduit only rises 16" above the roof but I;m guessing that "wet location" means I need thwn conductors. I don't think unjacketed MC can be used in a wet location. I hope someone tells me I am wrong.

Maybe using #12 would be OK. The FLA of the 15HP motor is 19 amps. Wouldn't it be weird if using #12 would be OK, but if I am required by contract to run #10 I have to derate and use #8?
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You said the conduit is about 15'. Code does not say anything about stripping the jacket off for that 15', less the 16", as not qualifying for 310.15(B)(3)(a)(4)(a)'s "The cables do not have an overall outer jacket."

The 16" part can qualify for the "under 24-inch bundling" allowance?
:D
 
Smart $ said:
You said the conduit is about 15'. Code does not say anything about stripping the jacket off for that 15', less the 16", as not qualifying for 310.15(B)(3)(a)(4)(a)'s "The cables do not have an overall outer jacket."

The 16" part can qualify for the "under 24-inch bundling" allowance?
:D
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I'm confused but you do make me think. Only the top 16" of the conduit is above the roof and of course so is the JB that will sit on top of the conduit. Seems to me I could get rid of the jacket for the entire 15 feet. The only reason I thought of jacketed is because that type would have wires suitable for wet location, whereas regular MC would not.

I guess it has been settled that using jacketed 10-3 MC would need to be derated which would then require #8 and if I use # 12 jacketed with the jacket removed I don't have to derate. Do I have that right?
 
mkgrady said:
I'm confused but you do make me think. Only the top 16" of the conduit is above the roof and of course so is the JB that will sit on top of the conduit. Seems to me I could get rid of the jacket for the entire 15 feet. The only reason I thought of jacketed is because that type would have wires suitable for wet location, whereas regular MC would not.

I guess it has been settled that using jacketed 10-3 MC would need to be derated which would then require #8 and if I use # 12 jacketed with the jacket removed I don't have to derate. Do I have that right?
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[310.15(B)(3)(a)(4)(a)] You would need unjacketed #12 to not have to derate....
[330.10(A)(11)] but you need jacketed in the 16" above the roof to qualify for the wet location usage...
...so you use jacketed and strip off the jacket for all but the 16" above the roof, and
[310.15(A)(2) Exception] use the adjacent rule to not have to derate that 16".
 
mkgrady said:
I assume I need to use jacketed MC in order to make them suitable to be run to the JB on the roof. The conduit only rises 16" above the roof but I;m guessing that "wet location" means I need thwn conductors. I don't think unjacketed MC can be used in a wet location. I hope someone tells me I am wrong. Maybe using #12 would be OK. The FLA of the 15HP motor is 19 amps. Wouldn't it be weird if using #12 would be OK, but if I am required by contract to run #10 I have to derate and use #8?
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What's the input current on the drives? The motor FLA doesn't really matter.
 
So what is up with 310.15(B)(3)(a)(4)? I read it as the NEC saying "We understand that the present derating rules are excessively conservative, but we can't/won't fix them. However, we agree than in the particular case of #12 copper AC or MC cable, the discrepancy is particularly onerous, so here's an ad hoc exception." Or is there something else going on?

Cheers, Wayne
 
wwhitney said:
So what is up with 310.15(B)(3)(a)(4)? I read it as the NEC saying "We understand that the present derating rules are excessively conservative, but we can't/won't fix them. However, we agree than in the particular case of #12 copper AC or MC cable, the discrepancy is particularly onerous, so here's an ad hoc exception."
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My impression is that you summed it up perfectly. :cool:

Much like they know the service calculations for dwelling units are too conservative and instead of fixing them they add exceptions to ignore the normal ampacity tables.
 
Smart $ said:
[310.15(B)(3)(a)(4)(a)] You would need unjacketed #12 to not have to derate....
[330.10(A)(11)] but you need jacketed in the 16" above the roof to qualify for the wet location usage...
...so you use jacketed and strip off the jacket for all but the 16" above the roof, and
[310.15(A)(2) Exception] use the adjacent rule to not have to derate that 16".
Click to expand...

I like the sound of that. Thanks
 
Smart $ said:
If I'm not mistaken, these cables are between the drives and the motors. The input current only matters on the supply side of the drives.
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That is correct. The drives are downstairs and the fans are on the roof. The contract has me running #10 wire to the drives on a 40 amp 3 pole breaker but I don't know the input current of the drive
 
mkgrady said:
That is correct. The drives are downstairs and the fans are on the roof. The contract has me running #10 wire to the drives on a 40 amp 3 pole breaker but I don't know the input current of the drive
Click to expand...
Can't be more than 28A using #10 copper.
 
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