12/37 SOW Cable feeding permanent Connector Strips?

[derekleffew]

An un-named local high school has a relatively new installation, and has about a 10' length of 12 AWG, 37 Conductor 90°C SOW cable running from the overhead junction boxes to the permanently installed connector strips (10 circuits) in the FOH catwalk.

I've only seen rigid or flexible conduit feeding such raceways that are not intended to be moved.

The name of the manufacturer is not relevant, as this issue (if it IS an issue) is on the electrical contractor. All stage lighting manufacturers' drawings I've ever seen include the phrase "Wiring by others."

Comments?

 

[mrb]

100% not ok. The use you described is not listed in 400.7(A) which lists the permitted uses for cord, and is specifically prohibited in 400.8(1) "unless specifically permitted in 400.7 flexible cords and cables shall not be used for the following: (1) as a substitute for the fixed wiring of a structure.

 

[Esoteric]

Not okay in the least. I always use EMT leading up to my circuit boxes.

Mike

 

[Dionysus]

Should be conduit or TECK... SOW is not meant for permanent installation like that. Would of been cheaper to use some sort of conduit as well.

 

[epimetheus]

Why would abybody do such a thing? Using EMT, flex, etc., in other words a RACEWAY, would be cheaper than splicing the single conductor to the #12/37 SOW in the junction box. How did they do the splice in the junction box?

 

[ship]

Hmm, I would have to go with the above and add the current capacity of a 12/37 SO cable even if only 10' would be just shy of the overcurrent protection.

 

[mrb]

Hmm, I would have to go with the above and add the current capacity of a 12/37 SO cable even if only 10' would be just shy of the overcurrent protection.

good point. I didnt bring it up since cord cant be used here even if it was 8/37, but the OCP and ampacity of the cord is worth mentioning.

 

[STEVETERRY]

Hmm, I would have to go with the above and add the current capacity of a 12/37 SO cable even if only 10' would be just shy of the overcurrent protection.

Err..while the application is totally non-compliant for reasons in the previous posts, the ampacity is actually OK per table 520.44. This table is almost always applied to multicable applications in the theatre. It assumes 50% diversity, which is very normal for a theatre application.

#12AWG 90C is rated at 35A from that table. We have 20 current-carrying conductors in the cable, so table 520.44 tells us we have to apply an ampacity adjustment factor of 70%. That gives us an ampacity of 24.5A, even though we cannot have an overcurrent device over 20A per table 520.44.

ST

 

[mrb]

Err..while the application is totally non-compliant for reasons in the previous posts, the ampacity is actually OK per table 520.44. This table is almost always applied to multicable applications in the theatre. It assumes 50% diversity, which is very normal for a theatre application.

#12AWG 90C is rated at 35A from that table. We have 20 current-carrying conductors in the cable, so table 520.44 tells us we have to apply an ampacity adjustment factor of 70%. That gives us an ampacity of 24.5A, even though we cannot have an overcurrent device over 20A per table 520.44.

ST

my previous statement was based off 400.5(A), which gives you 20 amps for SO cable with 3 ccc before derating. I wonder why the two different tables have such different figures for the same cord. Maybe 520.44 gives you the 90deg rating, and 400.5(A) does not.....

 

[STEVETERRY]

my previous statement was based off 400.5(A), which gives you 20 amps for SO cable with 3 ccc before derating. I wonder why the two different tables have such different figures for the same cord. Maybe 520.44 gives you the 90deg rating, and 400.5(A) does not.....

The two tables are different because:

1. 520.44 requires 50% diversity.
2. 520.44 trades off wire temperature rating for increased number of current carrying conductors, not increased allowable ampacity--hence the 20A OC limit on a #12AWG conductor that ends up with a 35A ampacity.

ST

 

[mrb]

The two tables are different because:

1. 520.44 requires 50% diversity.
2. 520.44 trades off wire temperature rating for increased number of current carrying conductors, not increased allowable ampacity--hence the 20A OC limit on a #12AWG conductor that ends up with a 35A ampacity.

ST

thanks for clarifying that for me.

 

[derekleffew]

NEC 520.46 - Connector Strips, Drop Boxes, Floor Pockets, and Other Outlet Enclosures. "...Supply cables for connector strips and drop boxes shall be as specified in 520.44(B)."

NEC 520.44(B)(1) allows the use of multi-conductor cable to feed borderlights, such as those rigged on a fly system batten.

So why is the same cable permitted backstage, but not on a catwalk? Just because the ones on that catwalk don't move? (Which I suppose they could, such as if they were in the way of a spotlight mounting position, for example.)

Why would anybody do such a thing? Using EMT, flex, etc., in other words a RACEWAY, would be cheaper than splicing the single conductor to the #12/37 SOW in the junction box. How did they do the splice in the junction box?

I did not open the junction box, had no need or desire to do so, but I imagine they used the industry-standard terminal barrier strips, possibly provided or specified by the factory. It is arguable whether 10' of multi-cable is less expensive than EMT or BX when one adds the labor involved to pull single conductors through the conduit.

 

[mrb]

the cable is permitted in the fly system because it moves. Not permitted for the original scenario because that connector strip is bolted to the catwalk and is part of the building.

 

[STEVETERRY]

So why is the same cable permitted backstage, but not on a catwalk? Just because the ones on that catwalk don't move? (Which I suppose they could, such as if they were in the way of a spotlight mounting position, for example.)

There's moving, and then there's moving.

If a connector strip or drop box needs to travel once or twice a day over a 50' lift, that's portable SO cord moving.

If an FOH connector strip needs to be raised a foot once a year, that's Greenfield moving.

ST

 

[SHARYNF]

Steve is the expert here, but I would also believe it would depend on how the strip was mounted. If it were mounted via say c clamps to the catwalk then it probably would be considered movable. If on the other hand it was permantley mounted to the catwalk then it would not. I think a reasonable intrepretation fo the code is related more to how easily it can be moved vs how ofter it actually is moved. Otherwise we could get in to a silly situation where if the flys were only used a few times a year, that they would then be considered not suitable for S0

It would be interesting to check and see if the same so 37 conductor cable was used in a similar setup on the fly and if the designer simply spec'ed a series of these assemblies with various cable lengths, some to be installed on the fly some on the catwalk.

Sharyn

 

[epimetheus]

I did not open the junction box, had no need or desire to do so, but I imagine they used the industry-standard terminal barrier strips, possibly provided or specified by the factory. It is arguable whether 10' of multi-cable is less expensive than EMT or BX when one adds the labor involved to pull single conductors through the conduit.

I wouldn't have opened the box either...

My point was that if the connector strip is in fact permanently mounted and does not need to ever move, how could the cost of the junction box and SOW be justified? I agree the cost of SOW vs. EMT or BX is arguable for the section between the junction and the connector strip. It is not arguable that it would have been cheaper not to install the j-box and pull straight into the connector strip. The parts I'm leaving out here are consturctability concerns. It happens often in my business where we need to get the cable pulled before the end device has arrived on site. In this case a junction box and SOW makes sense. Maybe that's what happened here?

 

[Esoteric]

Facinating discussion. I defer to my ME on situations like this, he informed me that single conductors pulled through conduit is the best code passable solution (at least here in my local area). I asked about portable vs non-portable in his situation and his guide was intent. Is the cable intended to be moved (like on a batten or in a portable system like the one's we install in smaller theaters where they use a dimmer rack and raceways, but the raceways need to be able to attach to any point on the grid, in which case we use a multi connector attached directly to the raceway) or is it intended to be a permenant part of the building (a raceway installed on a catwalk, the back wall of the theater, etc). He said "the inspector will use his common sense, so just use yours" he then added "unless you are friends with him of course". *lol*

Mike

 

[derekleffew]

Esoteric, I think you, together with STEVETERRY and mrb, have nailed it.
NEC 520.44(B)(1) - General, says "...Such cords and cables shall be employed only where flexible conductors are necessary." The operative words being "only...necessary".

SHARYNF, I'm sure the electrical contractor used leftover scraps from the drops to the onstage electrics for the two runs FOH. Otherwise, there's no reason to use 12/37 for 10 circuits.


I agree these types of discussions can be both interesting and educational. One of my favorite passtimes is reading NEC-interpretation issues in the "Stumped by the Code" section of EC&M and on Mike Holt's Forum, even though they rarely, if ever, discuss places of assembly. If anyone has future questions about code-compliance/interpretation, please submit them to the Senior Team or the CBMods.

 

[TimMiller]

Code gets even better when you start to grandfather things in.

 

[STEVETERRY]

Code gets even better when you start to grandfather things in.

Note that the NEC adopted by the municipality at the time of installation is the one that applies to that installation. Therefore, a technique that was legal for an installation in 1984 may not be legal for a new installation in 2009. However, it does not become a violation in the original installation just because there is a new Code in effect that does not allow it on new work.

However, a portable installation has the potential to be inspected each time it is installed, so it is very important to keep the portable elements of the system compliant with current Code.

ST