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I have seen a number of answers here from our esteemed electricians about running wiring through conduit, and many of them include a warning similar to:

You're not allowed to piece the conduit together over the wires - they must be pulled only after the conduit is complete

While I understand that it must be done this way because That's What The Code Says™, my question is why is this specified in code?

I understand that code is generally written based on objective laboratory testing and/or real-world experience. What real-world experiences could have led to this being codified?

What is it about "pull wires through a piece of conduit, attach the conduit to the source box, pull wires through an elbow, secure elbow to existing conduit, pull wires through another piece of conduit, secure conduit to elbow, lather, rinse, repeat" that is inherently and/or potentially dangerous?

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    Doing it by the book also has the advantage of testing that the wires actually can be pulled through as intended, while there is still time to fix possible issues. I can imagine many ways in which the piecewise assembly could lead to wires that won't come out (tangled, pinched, too thick to bend in elbows) or make it impossible to pull new wires through (too many bends, …). You would only discover that years or decades later. – TooTea Jan 27 at 15:02
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    From my experience mostly with repairs is trying to install conduit over wires often results in pinch points and harsh radius on wire bends that often reduces integrity of the wire insulation. Also sliding pipe over wire causes the wire to zigzag in the pipe creating a choke that can prevent you from being able to tell if the conduit is firmly seated in the fitting. Insulation of small conductors could get caught in the pinch of fittings too. If using plastic pipe there is no way to properly apply solvent. – NoSparksPlease Jan 27 at 16:12
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    There is a way to still benefit from the access of the conduit before assembly, pass a pull rope through as you assemble. – ratchet freak Jan 28 at 10:08
  • I don't know if it's relevant, but I've seen contractors repairing buried conduits under streets and sidewalks that have been damaged during excavation, and they'll often use "half pipe" conduits that can be assembled around the dangling wires, without re-pulling the wires. But I think they're playing by a different set of rules, so to speak. – Steve Summit Jan 29 at 23:25
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    You're quoting random people on the internet. What code? After every comma in your rundown it needs to say detach fish tape leader, insert [device], reattach fish tape, DO NOT scrape the wires. - I've had to do it that way... it sucks. - Re. the title: because you're not a magician that can make a piece of EMT teleport around some wires, and if you do it piecemeal chances of damaging the wire are high. - Seriously, what code; if you follow all the rules, w/e you're talking about wouldn't need to be stated. – Mazura Jan 30 at 2:39
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Guaranteed Reusability

If you run the wire as you are putting together the conduit, there is a possibility, unless you are truly careful about all the details, that you could end up in a situation where your initial set of wires are perfectly fine, but that pulling them out to replace them - or more likely pulling in new wires (out is "easy") will run into unexpected problems. If you assemble everything first then the first time you pull wires through you will find and fix any problems. Plus you will be more careful (especially if you don't do this kind of thing every day) to play by the rules so that you will be able to pull that first set of wires without a problem.

Remember, conduit serves three different functions:

  • Physical protection
  • Grounding (for metal conduit)
  • Ease of use - i.e., add or replace wires as needed

The first two, which obviously are the "safety" issues, will be the same whether you do conduit-then-wire or conduit-with-wire. But for the 3rd, it can make a big difference.

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Far more opportunity to damage the wire insulation - either by mechanical damage from the exposed ends of the conduit/fittings being slid along the wires (metal or PVC), or from cement/primer if PVC.

An assembled conduit (or duct) will have all the various ends joined (and de-burred to remove any internal sharp edges, if properly assembled.) wires or cables slide along and don't get torn up.

In addition, there's increased opportunity for damage from the wires being draped around the workspace unprotected while you have them laid out but the conduit is not assembled.

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    Plus, piecing the conduit together as you go along might come up with a configuration that doesn't allow you to replace the conductors in the future due to too many bends, etc. – JACK Jan 27 at 14:42
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    Counter point: If I'm assembling EMT properly, I'm going to de-burr the ends of the tubes. If I'm doing that prior to slipping it over existing wires, then I'll have smooth edges either way. Can't fix the cement issue, though. I suppose those assembling around the wires are less likely to do other parts properly, too, though. – FreeMan Jan 27 at 15:09
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    Much better answer than the accepted one; this one describes actual problems/dangers with the proposed practice aside from "well you might make a mistake!" – TylerH Apr 6 at 17:07
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Well, it's never a good idea to "divining rod" the intent of the NFPA. Because jackasses mainly are looking for one reason that they can rationalize around, so they can fabricate a reason why the Code doesn't apply to them. Really, they just don't want to follow Code, but they are not honest enough to call themselves outlaws.

So, it is a stupid game. In every rule I have researched, there are usually a variety of "whys" which overlap to provide safety.

But I'll give you one.

It stops you from building "unpullable" conduit!

Building the conduit first, as Code requires, keeps you honest. If you build it unpullable, you won't be able to pull it! :)

We had one person on here, we finally convinced them that a route between boxes would be best done as THHN and conduit. But they had to follow a zigzag on the building wall, and when they showed us a picture, they did not use a sweep in the inside corner -- they just put the exact same common "pulling elbow" they had used on the outside corner! The cover plate was facing the wall. (they make a special corner pulling elbow with a side door, by the way).

It was completely obvious that a) the person had assembled the conduit around the THHN wires, and b) they were completely oblivious that you would do it any other way. What a pain in the butt to do that, only to be told it's wrong.

Oh yeah,

It's much, much harder to do it that way

It really gives you the worst of both worlds. You have the "coils of cable splayed all over the place all day" like you do with a direct cable installation. And then you also have the "fidgeting with conduit" part too, except the conduit is much, much harder to do, because every piece must be slid down all the wires.

I do a ton of conduit work, and SMH... that's nuts. What a royal mess. Making it 5 times harder than it has to be.

Building empty conduit makes the job easier.

  • The pipe goes up easily, obviously.
  • Your work area is a space about 10 feet long - no bigger than the next stick. That means for instance you don't need to clear belongings out of the entire route, just the immediate <10' work area.
  • Take a break anytime you want. Tidy up, and come back to it after lunch or next month. I have a conduit run somebody installed ten years ago but didn't finish the last 20 feet, it's not in anyone's way.
  • It's all an erector set, which is particularly important to novices, who aren't going to get it right first time. You can disassemble and reassemble any way you like, with no loss of material except maybe some tubing. You can seek feedback off forums, and if something needs changing (like that pulling elbow pushed against the wall), you just change that little bit. It all un-bolts. (or if it's PVC just don't glue it yet).

There's no throne. Threading pipe over wires doesn't buy you anything.

Honestly, after you build the conduit, actually pulling the wires is a "victory lap". It's the easiest part of the job. I can't grasp why someone would want to make the job so much harder by trying to assemble conduit around wire. It's like doing direct cable, but with 10 times more mess.

Do people really think conduit work must surely be harder merely because it is different than what they're already familiar with?

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    This looks more like 3 "good reasons" than just the one you claimed to be providing... :) – FreeMan Jan 28 at 14:36
  • It provides you one thing: Maybe you hate the person likely to be working with the conduit in the future. – DKNguyen Jan 29 at 0:20
  • @FreeMan Reasons grow in number as the weapons of the Spanish Inquisition ... – Hagen von Eitzen Jan 30 at 18:22
  • Just spend a few extra $ and get the handbook, sorry it’s not free online but it explains most code articles so there is no guessing why. – Ed Beal Apr 6 at 16:51
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Although this has an accepted answer there is a lot of guessing going on here.

NEC 300.18 requires raceways “shall be installed complete between outlet, junction, or splicing points prior to installation of the conductors”.

The hand book then identifies the reason: the handbook is an expanded version of the code (and the book that most inspectors use with the reasoning for the code in most articles. These explanations are not enforceable like a fine print note but explain many things like this code requirement).

One of the primary reasons of a raceway is to provide physical protection for the conductors. If raceways are incomplete at the time of conductor installation, a greater possibility of damage to the conductor exists.

It then goes on to show the exception of motors and exhibit 300.15. It has nothing to do with reusability. if conductors are not damaged they would be reusable though.

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