Anyone see a logical reason

[roger]

A duct bank consisting of 66 concrete encased 4" PVC conduits and 18 manholes. Each manhole has 11 conduits entering and leaving, and the concrete encasement is all the way to each manhole. The last 5' straight section of each concrete encased conduit is specked to be RMC and they are installed per the specs

My question is, does anyone see a reason for this spec besides added expense for the RMC, labor for threading the RMC, bond bushings, copper EGC, and introducing half of a lethal circuit into the equation? Remember this is a complete concrete encasement for physical protection


Roger

 

[mcclary's electrical]

So you don't burn through the PVC on the pull in, especially on the 90's

 

[roger]

So you don't burn through the PVC on the pull in

Sorry but that doesn't cut it, these are straight sections, if we were dealing with 90's we would have a different reason to consider.

Roger

 

[iwire]

Just a guess ..... to make sure if the man hole and duct bank try to settle at different rates the RMC will keep things together instead of PVC being crushed or sheered off.

We get specs like this for conduits leaving the foundation of a building, the first 10' will be RMC then change over to PVC.

 

[Flex]

What do you mean by last 5' straight section? Could you be more specific otherwise I cant think of anything.

 

[SEO]

That would be a good question to pose for the designer. One reason may be they might think that the manhole could float and that the rmc would be more durable than the pvc?

 

[roger]

Just a guess ..... to make sure if the man hole and duct bank try to settle at different rates the RMC will keep things together instead of PVC being crushed or sheered off.

We get specs like this for conduits leaving the foundation of a building, the first 10' will be RMC then change over to PVC.

That is a possible reason but, it still seems as though there would be seperation and conduit damage where the RMC and PVC meet.

Roger

 

[Profish00]

RMC is to be used at each manhole for the 1st 5 foot. And now will need to be bonded.:grin:

 

[Flex]

That is a possible reason but, it still seems as though there would be seperation and conduit damage where the RMC and PVC meet.

Roger

Not really cause there encased in the same concrete and the manhole is separate from it.

 

[erickench]

Well if it's encased in concrete it would make a great grounding electrode though the NEC doesn't allow it.

 

[al hildenbrand]

. . . and introducing half of a lethal circuit into the equation? . . . .

But isn't a person, inside the manhole, already connected to earth? Or are you thinking of the arc flash from an energized conductor to EGC while a person is present?

 

[roger]

But isn't a person, inside the manhole, already connected to earth? Or are you thinking of the arc flash from an energized conductor to EGC while a person is present?

Actually both reasons but, IMO contacting the metal and associated bonding is a bigger danger than just being in the manhole with shoes on.

The truth is, with forty 500 kcmil's looped in each manhole there won't be enough room for a person to get into them anyways. :grin:


I still don't see the rational, I have installed probably 20 - 25 manholes in my career and have always used PVC with bell ends and have never had any problems.


Roger

 

[weressl]

A duct bank consisting of 66 concrete encased 4" PVC conduits and 18 manholes. Each manhole has 11 conduits entering and leaving, and the concrete encasement is all the way to each manhole. The last 5' straight section of each concrete encased conduit is specked to be RMC and they are installed per the specs

My question is, does anyone see a reason for this spec besides added expense for the RMC, labor for threading the RMC, bond bushings, copper EGC, and introducing half of a lethal circuit into the equation? Remember this is a complete concrete encasement for physical protection


Roger

If you don't provide end-bells, that are flush with the concrete wall then your conduit will protrude into the manhole and would be subject to breakage. An RMC could withstand more physical force than a PVC. It is also easier to install/replace threaded accessories on thee end. Damage by the pulling rope is yet another issue. Usually you have and anchor on the opposing wall for a pulley and one pulley will not line up with all 11 conduit, so some you will be pulling at an angle. (Hence an end bell - usually CI or PVC - would be a better solution.)

 

[roger]

If you don't provide end-bells, that are flush with the concrete wall then your conduit will protrude into the manhole and would be subject to breakage.

Non issue here, they are flush.

An RMC could withstand more physical force than a PVC. It is also easier to install/replace threaded accessories on thee end.

Not IMO, and I am an old installer.

Damage by the pulling rope is yet another issue. Usually you have and anchor on the opposing wall for a pulley and one pulley will not line up with all 11 conduit, so some you will be pulling at an angle. (Hence an end bell - usually CI or PVC - would be a better solution.)

This is mostly true of every big parallel wire pull and is also not a real issue. I would (and have used) use unglued male connectors with a metal bushings and replaced them with the bell end after the pull.

 

[Cow]

If you don't provide end-bells, that are flush with the concrete wall then your conduit will protrude into the manhole and would be subject to breakage.

:-?

Breakage? With 4"?? That's some tough stuff, there's no way I can see breaking the ends off a bunch of 4" conduits stacked on top of each other unless they're sticking into the manhole 12 inches. 3 or 4 inches, no way. Seems like if you're suffering from breakage, you've got bigger issues(like wire insulation damage), and should figure out how to pull wire without being so destructive.

 

[benaround]

In the past, the designer of your system had a problem within 5' of a manhole and he/she

said " This will never happen to me agian " , and using this method it hasn't. ????

 

[weressl]

Non issue here, they are flush. Not IMO, and I am an old installer.
This is mostly true of every big parallel wire pull and is also not a real issue. I would (and have used) use unglued male connectors with a metal bushings and replaced them with the bell end after the pull.

The question was asked WHY and I answered. Tell THEM it is not a real issue......

 

[Doug S.]

I could venture a few guesses, most already mentioned, but I'm really curious why.

The guess I would lend the most believability to would be what I-wire mentioned about settling, I could see the PVC shearing much easier than the RMC at the dry concrete junction. HOWEVER IF the manholes are NOT subject to vehicle traffic, that bet's off.

Ask 'm would ya?

 

[roger]

The question was asked WHY and I answered.

No, the question was, "does anyone see a logical reason" and you pointed out "if's". The "what if" argument is always present but is not the norm, it is the exception.

I got paid for the installation so it really doesn't matter to me, I am just curious for opinions as to if there is any real logical reason to do this.

Tell THEM it is not a real issue......

My opinionated point exactly, it was not an issue and probably just extra expense to the job

Roger

 

[weressl]

No, the question was, "does anyone see a logical reason" and you pointed out "if's". The "what if" argument is always present but is not the norm, it is the exception.

I got paid for the installation so it really doesn't matter to me, I am just curious for opinions as to if there is any real logical reason to do this.

So you're saying that my answers presented no logical reasons? If I recall your post correctly your dismissal of the reasons had to do with YOUR personal qualifications, not with the general measures that may be taken by a designer to avoid problems.

My opinionated point exactly, it was not an issue and probably just extra expense to the job

Roger

It is not an issue for YOU. But even you would be helped should you make a mistake.

BTW the protection of the ends is not from the linemen, but against all the other yokels that bang around in the manholes before the lineman gets there. (In you OP you did not make clear that the conduits are flush.)