MC cable run though an emt raceway

[mm12866]

We are doing a lot of tenant improvements recently where they like to have offices with t-bar grid and hallways with no ceiling. I like to run a 10' stick of 2" EMT across the hallway and use it to run MC between offices. Here is the question:
If I run say eight 12-3 cables (24 current carrying conductors) through this EMT, then per Exception to (4) of 310.15(B)(3)(a)(4) I would have to derate the circuits to 18 amps (60% of 30 amps). Then, per 240.4(B) I can put these circuits on a 20 amp breaker (next higher standard breaker). The circuits will have been designed to maximum of 16 amps (per 210.19(A)(1).

Does this sound code compliant?

Thanks

 

[ActionDave]

Sounds fine in principal.

Is this a place with single phase service or three phase?

 

[infinity]

You can only us a 20 amp OCPD on an 18 amp conductor (next size up standard OCPD rule) if there are no multi-receptacle loads.

 

[goldstar]

Sounds fine in principal.

I have some questions regarding your answer :

1. What size EMT will be used for (8) MC cables ?
2. Will there be a "bundling" issue for this installation ?
3. If you use a length of 1/2" EMT with an MC cable enclosed you would have to use a "from-to" connector like this one https://www.bptfittings.com/store/emt-conduit-fittings.html. Is there a fitting that would allow you to clamp onto 8 MC cables ?
4. Doesn't the EMT need to be grounded ?

 

[infinity]

I have some questions regarding your answer :

1. What size EMT will be used for (8) MC cables ?
2. Will there be a "bundling" issue for this installation ?
3. If you use a length of 1/2" EMT with an MC cable enclosed you would have to use a "from-to" connector like this one https://www.bptfittings.com/store/emt-conduit-fittings.html. Is there a fitting that would allow you to clamp onto 8 MC cables ?
4. Doesn't the EMT need to be grounded ?

1-Since it's a sleeve it only needs to be big enough to pull in the 8 cables
2-This has been addressed by the 60% derating
3-The EMT is being used as a sleeve no fitting required
4-Sleeves are not required to be grounded

 

[goldstar]

4-Sleeves are not required to be grounded

Not according to EI’s in my area.

 

[Adamjamma]

You can only us a 20 amp OCPD on an 18 amp conductor (next size up standard OCPD rule) if there are no multi-receptacle loads.

So does that mean the receptacles and branch breaker would have to be 15 amp then? Just making sure in case I need to know for test or future?

 

[infinity]

Not according to EI’s in my area.

I'm not surprised, they need to read the codebook. :roll:

250.86 Other Conductor Enclosures and Raceways. Except as permitted by 250.112(I), metal enclosures and raceways for other than service conductors shall be connected to the equipment grounding conductor.
Exception No. 2: Short sections of metal enclosures or raceways used to provide support or protection of cable
assemblies from physical damage shall not be required to be connected to the equipment grounding conductor.

 

[infinity]

So does that mean the receptacles and branch breaker would have to be 15 amp then? Just making sure in case I need to know for test or future?

If the adjusted conductor ampacity is less than 20 amps then yes you would be limited to a 15 amp OCPD because you cannot use the next size up rule in 240.4(B).

 

[goldstar]

I'm not surprised, they need to read the codebook. :roll:

Thanks for pointing that out

 

[infinity]

Thanks for pointing that out

You can keep that one in your pocket for the next time an EI tells you to bond the sleeve.

 

[goldstar]

You can keep that one in your pocket for the next time an EI tells you to bond the sleeve.

I will, thanks. By the way we also have to define what A short length means

 

[infinity]

I will, thanks. By the way we also have to define what A short length means

Don't you just love the NEC ambiguity.

 

[kwired]

I will, thanks. By the way we also have to define what A short length means

Since EMT comes in 10 foot lengths - cut a quarter inch off one and you have a "short length"

 

[goldstar]

I remember somewhere, in one of my CEU classes that 24” was considered a short length. Was I daydreaming at the time ?:roll:

 

[infinity]

Since EMT comes in 10 foot lengths - cut a quarter inch off one and you have a "short length"

I like that.

 

[kwired]

I remember somewhere, in one of my CEU classes that 24” was considered a short length. Was I daydreaming at the time ?:roll:

It is, for different code section, ampacity adjustments for number of conductors in raceway aren't needed for less then 24". This discussion was about bonding of short lengths of metal sleeves/physical protection for cable assemblies.

 

[goldstar]

Since EMT comes in 10 foot lengths - cut a quarter inch off one and you have a "short length"

let’s run that one up the flagpole and see if it flies :thumbsup:

 

[kwired]

We are doing a lot of tenant improvements recently where they like to have offices with t-bar grid and hallways with no ceiling. I like to run a 10' stick of 2" EMT across the hallway and use it to run MC between offices. Here is the question:
If I run say eight 12-3 cables (24 current carrying conductors) through this EMT, then per Exception to (4) of 310.15(B)(3)(a)(4) I would have to derate the circuits to 18 amps (60% of 30 amps). Then, per 240.4(B) I can put these circuits on a 20 amp breaker (next higher standard breaker). The circuits will have been designed to maximum of 16 amps (per 210.19(A)(1).

Does this sound code compliant?

Thanks

Are all 24 conductors "current carrying"? If these are 120/240 MWBC's the neutrals don't count as current carrying for ampacity adjustments. If it is 120/208 MWBC's neutral does count as a current carrying conductor.

Just something to consider.

 

[tortuga]

I'm not surprised, they need to read the codebook. :roll:

I remember somewhere, in one of my CEU classes that 24” was considered a short length. Was I daydreaming at the time ?:roll:

24" is what I consider short. Its subjective for sure consider practical safety if its buried in a wall or accessible to the public.