Supporting Cable in Inclined Raceway

Status
Not open for further replies.
D

DMeigs

Member
Location
Albuquerque, New Mexico, USA
Hey Everyone,

I have an application that calls for 3 #2/0, 1 #6 ground (individual conductors or cable) in an existing 2" RGS conduit system installed along a pipeline at a steep incline, and I am concerned about how to support the conductors. The installation is approximately 1600 feet long with a vertical drop of about 1100 feet. The conduit has a few boxes along the way, but they have up to 320 feet of elevation change between them. If this was a vertical raceway, Article 300.19 would require me to support the (copper) conductor every 80 feet. Does 300.19 apply to conduit on a slope as well?

If 300.19 does not apply, at what interval should the conductors be supported?

Thanks!
 
Welcome to the forum. Are you saying there is an 1100' sheer vertical drop in the run, or that the total drop along the 1600' is 1100'? If the latter, then 1600' is the hypotenuse, 1100' is drop (A), giving a ~43* angle, assuming the pipeline is straight from A to B. Not vertical or close to it, dont think 300.19 would apply. Also, the raceway should be plenty of support for the wire; I think the surface area and friction of the conductor insulation would keep it in place. Supporting it with wedges, grips, or other 300.19 support methods wouldnt be a bad idea tho. Only thing I can think of that might change the equation is being in a seismic zone, where ground shake could cause the conductors to try to shimmy down the conduit (think of like a wire avalanche). Never done such an install, and I'm not an engineer, so dont take my words as gospel.
 
Thanks JFletcher!

To clarify, the raceway averages 43 degrees, and it is composed of six segments. Four segments are pretty mild, but two are at 61 degrees and 68 degrees.

If 68 degrees is not considered "vertical" and Article 300.19 does not apply, then it's a simple statics problem to see what forces will be exerted on the cable at each support and make sure the support can handle that.

So, I guess the question comes down to this: Would code consider a 160 foot conduit that has a 145 foot rise "vertical"?

Thanks!
 
DMeigs said:
Thanks JFletcher!

To clarify, the raceway averages 43 degrees, and it is composed of six segments. Four segments are pretty mild, but two are at 61 degrees and 68 degrees.

If 68 degrees is not considered "vertical" and Article 300.19 does not apply, then it's a simple statics problem to see what forces will be exerted on the cable at each support and make sure the support can handle that.

So, I guess the question comes down to this: Would code consider a 160 foot conduit that has a 145 foot rise "vertical"?

Thanks!
Click to expand...

You're welcome.

The NEC doesnt define vertical, but vertical is straight up and down. So I would not think 300.19 could be cited.

That said, a 70*+ angle of that length would probably need some support, especially if it's 6 separate segments of cable and not a continuous pull.

Just curious, how would one access this conduit/j-boxes, repelling down a cliff?

You could also maybe use Riser MC cable:

http://www.southwire.com/commercial/aluminum-riser-mc-cable.htm

Perhaps contacting Southwire would be a good idea on using that to the lengths you need, or supporting the conductors in the 2" conduit.
 
The definition of vertical does not include a slope so I agree with JFletcher that technically that table is not applicable.
 
I would support every run of that. You have gravity forces pulling it down. You can't rely on friction between the insulation and the wireway or raceway.
If the code prescribes an 80' vertical run, do the math for the 80' rise and that's the spacing interval of the support in each run.
That's one job you don't want to get wrong.
 
DMeigs said:
If 68 degrees is not considered "vertical" and Article 300.19 does not apply, then it's a simple statics problem to see what forces will be exerted on the cable at each support and make sure the support can handle that.
Click to expand...


I concur with that thought process. If the traction in the raceway can support the weight of the cable without transferring the load to the equipment terminals at the top, then you shouldn't need to add additional support fittings. Technically, the cable is supported as required by the NEC. The traction of the conduit on the wire jacket applies a force up/along the raceway, preventing it from sliding down. Bear in mind, that if you could have one cable resting on another wire, instead of the inner conduit wall, then THAT should be the worst case coefficient of friction to use. The nylon on nylon, or whatever your wire insulation material(s) may be.

If however, the wire will slide right down under its own weight, in the event that you let it go, then you should have a vertical cable support fitting at the top. It is only the pure vertical component of distance that would matter for the table. Not the inclined length along the conduit.
 
Whether or not the NEC requires it, I would still do it.

The NEC is a design minimum, there is nothing wrong with adding a little common sense to it!
 
mgookin said:
...If the code prescribes an 80' vertical run, do the math for the 80' rise and that's the spacing interval of the support in each run. ...
Click to expand...

Carultch said:
...If however, the wire will slide right down under its own weight, in the event that you let it go, then you should have a vertical cable support fitting at the top. It is only the pure vertical component of distance that would matter for the table. Not the inclined length along the conduit.
Click to expand...

Cow said:
Whether or not the NEC requires it, I would still do it. ...
Click to expand...
Ding, ding, ding. We have a winner! :D

You may have a tough time getting wedge type supports for the #2/0 and #6 combination, though. BTW, a #6 ground does not appear to be the right size. What are the electrical details of this circuit?

As far as supports, I think OZ Gedney Type W Cable Supports or equivalent are ideal for this type installation. See page 255 (12/16) here: http://www.emersonindustrial.com/en...s/commercial_products/cable-supp-ozgedney.pdf
 
BTW, a #6 ground does not appear to be the right size. What are the electrical details of this circuit?
Click to expand...

Thanks Smart $, that was a good observation. I failed to upsize the ground when I upsized the phase conductors. It's a 480 V 3-phase 3-wire circuit protected by a 60 A ECB. Code requires (3) #6, (1) #8 G. Voltage drop requires upsizing the phase conductors to #1/0. Therefore, the ground needs to be increased to #2.
 
I think I made a mistake in my reply to Smart $. When I said code requires (3) #6, (1) #8 G, I was taking the #8 from Table 250.66, because it is a building service entrance. However, the requirement to upsize the ground conductor is associated with Table 250.122, which calls for a #10 ground. I upsized the phase conductors by a factor of 4.02 (from #6 to #1/0), so if I understand correctly, I need to upsize the ground by the same factor but starting with the size in Table 250.122, not Table 250.66. Starting with #10 gives me a #3 ground, not the #2 that I stated before.

Is that logic correct?
 
DMeigs said:
I think I made a mistake in my reply to Smart $. When I said code requires (3) #6, (1) #8 G, I was taking the #8 from Table 250.66, because it is a building service entrance. However, the requirement to upsize the ground conductor is associated with Table 250.122, which calls for a #10 ground. I upsized the phase conductors by a factor of 4.02 (from #6 to #1/0), so if I understand correctly, I need to upsize the ground by the same factor but starting with the size in Table 250.122, not Table 250.66. Starting with #10 gives me a #3 ground, not the #2 that I stated before.

Is that logic correct?
Click to expand...
Yes...and this instance exhibits an interesting set of parameters which I have tried to get changed in Code. First, factoring the upsize by AWG sizes is Code not the prescribed method of determination. 250.122 says the EGC increase must be based proportionally on the change of cmil areas. Chapter 9 Table 8 is the source for cmil areas recognized by the NEC, and it uses the commercial cmil conductor sizes... effectively, the original AWG size in cmils rounded to four significant figures.

Technically you should be permitted to round your "proportioning" calculation result to 4 significant figures, but Code does not tell you this, and it is not a widespread understanding in AHJ/IAEI circles. That is, change in AWG sizes are proportional by the very nature of the gauge standard.
 
Smart $ said:
Chapter 9 Table 8 is the source for cmil areas recognized by the NEC
Click to expand...
So, 90.3 says "Chapter 9 consists of tables that are applicable as referenced." I don't see Table 8 referenced anywhere else in the NEC. Which would mean that you don't have to use it. Which would be a good thing since it apparently has rounding errors.

Anyway, AWG sizes are in geometric series, so if an ungrounded conductor is upsized from 6 to 1/0, then upsizing an EGC from 10 to 4 would be proportionate.

Cheers, Wayne
 
wwhitney said:
So, 90.3 says "Chapter 9 consists of tables that are applicable as referenced." I don't see Table 8 referenced anywhere else in the NEC. Which would mean that you don't have to use it. Which would be a good thing since it apparently has rounding errors.
Click to expand...
The Table values do not have rounding errors. Using its values without knowing how its values are rounded and not carrying over that rounding into a calculation is where the error is introduced. However nowhere in the NEC does it say you can do that... and even if you do do it, there's nothing to say your AHJ has to do it the same way, especially since upsizing must be according to proportional cmil area. As such in some cases, your EGC may end up having to be one or two sizes larger than necessary.
 
Smart $ said:
As such in some cases, your EGC may end up having to be one or two sizes larger than necessary.
Click to expand...
I'm with you until this point--I'd say it may end up being one size larger than necessary if you use Chapter 9 Table 8 as described. But since you don't have to use that table (90.3), you can work off AWG size directly (until you hit 250 kcmil)--increase the ungrounded N sizes, then increase the EGC N sizes.

Cheers, Wayne
 
Status
Not open for further replies.
Top