Cable Pull Box

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That Man

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Electrical Designer
Mike Holt friends,

I have a design problem I want your opinion on, which may have an established solution, since I imagine this problem is often faced for city utility installations. I'm designing a relatively large underground conduit system (industrial, not for a utility). To make the system easy to use, I am placing 4'W x 4'L x 4'D concrete pull boxes at strategic locations, to allow for easy cable pulling, while also making the system easy to expand at a later date, if necessary. It occurs to me that there may be a problem, however. I'm running both power and instrument cables in my conduit system. I separate the analog from the power by using different conduits, but within the pull box itself, there is no separation. I don't think insulation will be an issue, since it's easy to simply spec 600V insulation for power as well as control cables. The two elements of my problem are:

1. Is there any code violation using a pull box as I describe for multiple service types (DC analog, 120VAC, 480VAC all together)? I couldn't find any, but I also couldn't find any specific allowances for this scenario either.

2. I'm worried about signal interference between the power and instrument cables when they run through these boxes without a barrier between them. Does anybody have any experience with this?

Thank you all in advance for your help.
 
Thanks for the reply. You know what? I've been engineering electrical and control systems for 12 years, and have never noticed article 700 before. I guess it doesn't come up much in an industrial setting. Or rather, the construction methods used in an industrial setting have satisfied this article without me even knowing it. Since, I usually use conduit without pull boxes, cable tray with barriers, or MC cable, the code sections for those wiring methods, along with industry standards, have managed to satisfy the article 725 requirements you mentioned. I'm going to have to study this whole section of the NEC--this portion of the code seems so important, I'm amazed it has never come up before.

For the problem I am currently facing, however, it appears that there is a code violation unless the cables are separated by a barrier. I can think of a few ready solutions for this, but I don't like them:

* Conduit through the box for class I circuits

* MC cable for the class I circuits

* Cable tray with barrier in the pull box (absurd, but it looks like it will satisfy the requirement)

* Separate pull boxes for class I circuits

What I'm hoping, is that there is some pre-existing solution to this problem that would allow me to utilize the exception listed in 725.136 (F)(3). I'll be researching this. If anyone knows of a solution though, please speak up.
 
That Man said:
Mike Holt friends,

I have a design problem I want your opinion on, which may have an established solution, since I imagine this problem is often faced for city utility installations. I'm designing a relatively large underground conduit system (industrial, not for a utility). To make the system easy to use, I am placing 4'W x 4'L x 4'D concrete pull boxes at strategic locations, to allow for easy cable pulling, while also making the system easy to expand at a later date, if necessary. It occurs to me that there may be a problem, however. I'm running both power and instrument cables in my conduit system. I separate the analog from the power by using different conduits, but within the pull box itself, there is no separation. I don't think insulation will be an issue, since it's easy to simply spec 600V insulation for power as well as control cables. The two elements of my problem are:

1. Is there any code violation using a pull box as I describe for multiple service types (DC analog, 120VAC, 480VAC all together)? I couldn't find any, but I also couldn't find any specific allowances for this scenario either.

2. I'm worried about signal interference between the power and instrument cables when they run through these boxes without a barrier between them. Does anybody have any experience with this?

Thank you all in advance for your help.
Click to expand...

What size conduits are you running? Could you reduce the size of the boxes and just put two next to each other?
 
I might have uncovered an exception which renders the whole 725.136 issue moot. 725.130 exception 2 states:

"Class 2 and Class 3 circuits shall be permitted to be reclassified and installed as Class 1 circuits if the Class 2 and Class 3 markings required in 725.124 are eliminated and the entire circuit is installed using the wiring methods and materials in accordance with Part II, Class 1 circuits."

It sounds like I can simply declare that all remote control and signalling are Class 1, and then they are Class 1. This doesn't really solve my problem, but at least it gets rid of the confounding factors of having to separate Class 2 and 3 from Class 1, in addition to Class 1 from power cables. I'm still held to the Class 1 circuit installation requirements as listed in 725.48(3), which means I will have to run MC or UF cable for ONE of the systems, add a fixed nonconducting barrier, attach control cables to racks (not defined here), or install these cables in a separate vault system. I'm going to put my cable schedule together first, and calculate my fill requirements before I evaluate this further. It may be a few days.
 
That Man said:
...
It sounds like I can simply declare that all remote control and signalling are Class 1, and then they are Class 1. ...
Click to expand...
And you have to use all Class 1 wiring methods and all of the connected equipment must be suitable for use on a Class 1 circuit. Not very easy to accomplish in the real world.
 
What Don said means if you declare something like a typical class 2 HVAC thermostat control circuit to be class 1 you now can not use the typical class 2 HVAC thermostat that often has open terminations on back side, you need something rated for class 1 like a line voltage HVAC thermostat, you can't pick and choose to run CL2 cables wherever it is convenient, you need to use wiring methods suitable for class 1 for the entire circuit. The 24 volt run out to a condenser unit - can't be 18-2 CL 2 cable - would need to be UF cable, MC cable, raceway method, etc.
 
don_resqcapt19 said:
And you have to use all Class 1 wiring methods and all of the connected equipment must be suitable for use on a Class 1 circuit. Not very easy to accomplish in the real world.
Click to expand...

725.151 States that a Class 2 or Class 3 circuit must be sourced from a Class 2 or Class 3 power supply. 725.41 States that circuits that do not meet requirements of Class 2 or Class 3 are Class 1. The power supplies I am using do not meet the requirements of Class 2 or Class 3 (they are not listed as such), therefore, I'm interpreting this to mean that all of my control and signalling circuits are Class 1. Am I missing something there? I may very well be, as I've never looked into this section of the code before.

As rlundsrud pointed out, I still need to separate the Class 1 circuits from the power wiring. That's do-able. What was becoming unmanageable was also separating the Class 2 & 3 as well.
 
So, I've been doing some research. The power supply line we tend to use for our control panels is the Allen-Bradley XLS Line. For the control panel in this instance, we are using the XLS480E (480W available, we're using about 399W). It turns out, Allen-Bradley does make Class 2 power supplies, but I've never seen them used before, probably because their power output is so meager--the largest in the line of power supplies we would use is the XLP-100E (100W). So I have a question--is there some advantage to using a class 2 power supply? I can see using one in the examples they mention in the code, such as thermostat or doorbell wiring, but those are occupancy scenarios, and we don't deal with occupancies; when we encounter them, we leave those to architects. Routing wires without RMC, MC cable, or TC cable in tray doesn't fly here. Even our Cat.5 is MC cable.

All of the requirements for class 1 circuits listed in 725.41 through 725.46 appear to be met by simply complying with articles 200 through 400, which we do anyway. Is there some restriction for class 1 wiring that I might not be complying with I wonder?

If so, NO ONE in my industry is complying with it; I talked to several of my colleagues about this issue, and a few of them had vaguely remembered something about wiring classes, but none of them applied this in their engineered designs. It just doesn't come up. Or does it? That's my chief concern. I strive to comply with the code, and I know about this now, so I want to make sure I am in compliance. My initial misgiving about routing all the cables through vaults proved to be valid--for the first time (I hope), this section of the code comes into play, and it provides for methods in which I can comply with the code, while alleviating my concerns about signal interference.

The solution I have settled on is to route all power wiring as MC cable ( 725.48 (B)(3)(1) ), which contains generated noise within its metal sheath, and route all my controls (class 1, per 725.41) as TC cable. I'm mostly finished with the power portion of my cable schedule, and so far, I've been able to get away with my current duct plan of 2-4", 3-3", 4-2", and 5-1" conduits between vaults. There is provision within these vaults to double that if I have to, but I want to avoid it to prevent installers from using the wrong set of ducts and violating pull box rules. I'll add more later if necessary.
 
Advantage of class 2 is lower incident energy levels allows for somewhat relaxed physical protection as it won't start the building on fire as easily as higher energy level sources during abnormal conditions.

Also is less electric shock potential.

Some routine maintenance and other procedures to control equipment may possibly be done on live circuits if at the class 2 level where if at the class 1 level may not be permitted or will at least require higher PPE levels.
 
That Man said:
Mike Holt friends,

I have a design problem I want your opinion on, which may have an established solution, since I imagine this problem is often faced for city utility installations. I'm designing a relatively large underground conduit system (industrial, not for a utility). To make the system easy to use, I am placing 4'W x 4'L x 4'D concrete pull boxes at strategic locations, to allow for easy cable pulling, while also making the system easy to expand at a later date, if necessary. It occurs to me that there may be a problem, however. I'm running both power and instrument cables in my conduit system. I separate the analog from the power by using different conduits, but within the pull box itself, there is no separation. I don't think insulation will be an issue, since it's easy to simply spec 600V insulation for power as well as control cables. The two elements of my problem are:

1. Is there any code violation using a pull box as I describe for multiple service types (DC analog, 120VAC, 480VAC all together)? I couldn't find any, but I also couldn't find any specific allowances for this scenario either.

2. I'm worried about signal interference between the power and instrument cables when they run through these boxes without a barrier between them. Does anybody have any experience with this?

Thank you all in advance for your help.
Click to expand...

the circuits you need to separate......

get a 6" x 6" x 4' gutter.

each pipe that needs to be separated from the herd,
have it use a separate gutter. if you've got several,
put them on the bottom of the concrete box, pull them
first, then pull the other conduits.

you'll need to put the gutters in when you set the box and
pipe to it. use an FA and a chase nipple into the end of the
gutter. if the concrete box is marginally smaller than 4', use
a 3' gutter, and extend the conduits into the concrete box
to reach it. you'll still have 8X the conduit diameter.

i'm assuming you aren't using any conduit smaller than 2".

LADWP puts everything in one vault all the time....
480vac, 250vdc, data, etc... but as a public utility, they
aren't bound by the NEC...
 
Fulthrotl said:
the circuits you need to separate......

get a 6" x 6" x 4' gutter...
Click to expand...

That's an AWESOME idea. That's just what I was looking for at the start of this, and I will incorporate this in the next pass if I get a chance (there will be a next pass, but the client has to decide to part with more money first; if they take too long, I might be on something else).

These guys are really averse to field fabrication without drawings, so I was struggling to come up with a solution that wasn't some Frankenstein, while at the same time, segregating cables as appropriate. I have a workable solution (pulling MC cable for power circuits), but I didn't like it. This one is better.

I'm still going to end up pulling some MC cable, since some of these are going into a Class I Div 1 area, but I can get rid of most of the MC cable with this method. This also allows me to force certain ducts to certain directions, as going other directions would violate NEC rules, but would be entirely possible while building it. And the best part is, it's entirely done with hardware that is familiar to these guys. Thanks a bunch Fulthrotl.

P.S. Why the concern about running no smaller than 2"? I'm intentionally adding several 1" conduits, since I don't know how many times I've needed to pull just 1 STP through an existing duct bank, and all that's available is a 3" or something similar. Is there a problem with this reasoning? Do you think it would be too hard to pull? I don't actually install this stuff, so I'm always worried I might do something to make these guys' lives difficult that I could have avoided.
 
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