PVC duct bank question.

[ITO]

PVC duct bank question.

(I have already been looking but can?t find a reference to this.)

Question:
Is there a mandate or a code requirement in NEC 2005 that requires that I put chairs on PVC in a duct bank?

Specifics

1. (5) Five feet deep
2. No concrete
3. 6? sand envelope (bottom, top and sides).
4. (8) eight 2? PVC with 3 phase 480 feeders
5. Located under building pad.

 

[rcarroll]

I can only come up with 310.15(B)(2)(b) & annex B. Annex B has separations from 7.5" to 10" center to center of pipe. Hope this helps.

 

[chris kennedy]

I never use them. Here are two 3" and two 2" over four 3". At 480V your 300.5 not 300.50


View attachment 868

Sorry about the crappy pic.

 

[charlie b]

310.15(B)(2)(b) does require that spacing be maintained. But that is similar to the requirement that for parallel conductors the distances be equal. How equally spaced do they have to be? For my money, if you can find a way to keep the conduits spaced in a reasonably even way through the use of sand or other material, I would be happy.

 

[chris kennedy]

310.15(B)(2)(b) does require that spacing be maintained. How equally spaced do they have to be?

I'm thinking more along the lines of what is space. A mm, 4", mabey 10' ?

 

[don_resqcapt19]

I submitted a proposal to specify the minimum space between conduits 3 or 4 code cycles and the panel rejected it with a comment that if you have enough space to install couplings and locknuts you have spacing.
Don

 

[chris kennedy]

I submitted a proposal to specify the minimum space between conduits 3 or 4 code cycles and the panel rejected it with a comment that if you have enough space to install couplings and locknuts you have spacing.
Don

Well then Don that means the conduits are touching each other. So you should propose that the term raceway is taken out of 310.15(B)2(B).

BTW what was the min. spacing you had in mind and why? Have you seen an underground failure due to heating?

 

[don_resqcapt19]

Chris,

Well then Don that means the conduits are touching each other.

No, if the couplings and locknuts are touching, the conduits will have small spaces between them.

BTW what was the min. spacing you had in mind and why?

Not sure what spacing I wanted, and the only reason was that the current rule is vague and unenforceable.

Have you seen an underground failure due to heating?

I have not, but there are a number of documented cases where they have, but only when the loads were not calculated using Article 220. These cases were the supporting information for what is now Annex B. At one time those tables were going to be in the actual code rule. They were published in Article 310 with an effective date 3 years later, but moved to the annex in the next code cycle.
Don

 

[chris kennedy]

Chris,No, if the couplings and locknuts are touching, the conduits will have small spaces between them.

I beg to differ. Given the inherent flex of RNC conduit I have long runs that are pushed together during backfill. So as far as ITO's OP, we do not worry about or enforce this.

 

[iwire]

I beg to differ. Given the inherent flex of RNC conduit I have long runs that are pushed together during backfill. So as far as ITO's OP, we do not worry about or enforce this.

That may be one way to look at or I might be inclined to say that the spacing of at least the couplings and connectors must be maintained and that because of PVC flexibility we have to provide something to maintain that spacing.

 

[chris kennedy]

That may be one way to look at or I might be inclined to say that the spacing of at least the couplings and connectors must be maintained and that because of PVC flexibility we have to provide something to maintain that spacing.

I see your point. The difference between 1/4" and 1/16" makes a huge impact on heat dissipation in wet and compacted soil. I'm no engineer but common sense....

I would be very intrested to see the failures Don has seen. My guess is that they would apply to 300.50 not 300.5.

 

[iwire]

I see your point. The difference between 1/4" and 1/16" makes a huge impact on heat dissipation in wet and compacted soil.

It does not have to make sense it only has to meet code. :wink:

I would be very interested to see the failures Don has seen. My guess is that they would apply to 300.50 not 300.5.

I expect it to be 300.5

Higher voltage, less current, less heat.

 

[don_resqcapt19]

Chris,

The difference between 1/4" and 1/16" makes a huge impact on heat dissipation in wet and compacted soil. I'm no engineer but common sense

The issue of spacing is more important with burried raceways than with those in the air because the heat is trapped. The earth does not let the heat escape near as well as the air does. The only reason that the burning up of underground duct banks is not very common is the fact that the load calculations in Article 220 result in the use of oversided conductors for the actual or real load.
As far as having seen this, I haven't. My comments are based on the proposals and comments on this issue that covered two code cycles...the 1984 and 1987 cycles.
Don

 

[ITO]

I could not find anything in the NEC either and I have a client who has decided that about 6,000 feet of 2", that I have already, had inspected, buried, compacted and the plumber has come along and put his work in over it, buried, inspected and compacted his work, should have had chairs for proper spacing.

This is also the same engineer who argued that my duct bank, under the concrete building pad should have been concrete encased. While I appeared to have won the debate over the concrete, he is determined to make me dig it all back up and put chairs in now.

 

[LawnGuyLandSparky]

Tell him to pound sand. :grin:

 

[iwire]

he is determined to make me dig it all back up and put chairs in now.

Considering all the work that has happened after yours it appears to me that the EE did not move in a timely manner and has missed the opportunity to have the chairs installed.

That would be the way I would approach this.

I had an EE bring up the use of tie wraps to support MC cables in the walls of a job that went on for months with regular visits from the EE.

He never mentioned the tie wraps until after the entire building was finished. He brought it up at a close out meeting with the building owners. I pointed out that he had months to bring this up at a time when something could be done about it. The owners basically told him that it was water under the bridge and they where not impressed with his waiting to the end to sandbag me with it.

 

[ITO]

This is a big job and we just started about a month ago, it?s not a good way to start a job. I have a meeting this Monday with the owner and his engineer to determine how to proceed. The goal here is to somehow let the engineer win, without having to dig up my pipe, while making the owner feel good about it.

Thank you for all the code references, it helps a lot; I am going to do some serious reading this weekend and have all my ducks in a row for the meeting.

 

[nakulak]

I personally like chairs because if used properly they always meet ACI specs for concrete surrounding your pipe, and allow for the conc in between pipes to be adequate, and able to freely fill in the area also. However, the chairs, IMO, only consititute a constuction method, and are only an optional portion of the raceway system. Any other suitable method of providing for the correct fill under, around, or over the pipes is equally valid, and other methods are often used in conjunction with the chairs when they are used (for instance, I like to support the pipes at bends and the chairs only really lock together for the straight sections). IMO as long as you provided the correct surround, material, compaction (sand, I'm told, does not compact), etc., then you have complied with the section for the duct bank which the engineer designed.

 

[charlie b]

Here is my take on the situation. Tell your engineer that it came from an engineer.

There are two things that are impacted by the manner in which the conduits are laid out in the ground. One is ampacity. The closer the conduits are to each other, the more difficult it becomes for any single conductor to dissipate its heat to the surrounding environment. This issue, in my opinion, is the engineer?s responsibility.

The second consideration is local overheating. It is my opinion that this is the basis for the requirement to ?maintain spacing.? If you wish to discuss the question of how much spacing to maintain, then go to the paragraph above. But let?s consider conduits that are 1 inch apart for 20 feet, then are 5 inches apart for the next 50 feet, then are in physical contact for 10 feet, then are 2 inches apart for the next 30 feet, etc, etc, etc. The ?impedance per foot? of any single conductor passing through one of these conduits will vary from section to section. That is because of the local influence (i.e., inductive and capacitive coupling) of its neighboring conductors. The current flowing in a given conductor will be constant throughout its length. But since impedance varies, the amount of heat (i.e., I2R heating) will vary. It will therefore be possible that the conductor will overheat in one section, even though the total current passing through the conductor is within the ampacity limits of the conductor.

My conclusion is that if you have kept the spacing between conduits ?reasonably? constant throughout the length of the conduits, then it does not matter what the distance is between the conduits. So if I were the engineer of record, I would ask you to describe the method you used to maintain spacing, and would probably have wanted to see the method in place, before the trench was backfilled.