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Old 06-27-2013, 09:42 PM   #1
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Default Duct Bank Tying

Back when I was a first year I was taught never use tie wire to go completely around the duct bank and fasten the conduit to the chairs that are holding it. Instead go over the conduit (top) then through the chair and then tie back over the top again. This is so there's never a complete circle of tie wire around any duct bank.

I was told if you go all the way around it can induce voltage etc = it isn't good. I never thought about it since. I left that company and all the duct banks here they use string to tie everything down. I preferred the tie wire method but I'm just wondering how you guys do it?

Here's a pic off the net of what they said "not to do".
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Old 06-27-2013, 10:09 PM   #2
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With 3 phase and a neutral, is there any chance of that? Any more than passing through the knock out of a steel panelboard? You would not want to pass individual conductors through individual knock outs of course.

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Old 06-27-2013, 10:10 PM   #3
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Yeah, only a problem if these are isolated phases.
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Old 06-27-2013, 10:12 PM   #4
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Yeah, only a problem if these are isolated phases.
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Old 06-27-2013, 10:13 PM   #5
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Refailure, you don't know anything about electrical work.
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Old 06-28-2013, 12:13 AM   #6
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Default Duct Bank

What I do is tie bottom row of chairs. Lay conduit in chairs. Set top set of tied chairs on top and snap in. Tie around top chair , around conduit, thru bottom chair. Repeat. Depending on chair lay out, I stake and tie every other support. Bundle evey other one. Pour corncrete. In my opinion, concrete is grounded and you are tied in a complete loop. Make sure you double up your tie wire.

A little tip is to put a top chair every so often, so you can see the depth of your corncrete, while you are pumping it. Make sure you stake it, or it will float.
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Old 06-28-2013, 06:16 AM   #7
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Quote:
Originally Posted by CoopElec
What I do is tie bottom row of chairs. Lay conduit in chairs. Set top set of tied chairs on top and snap in. Tie around top chair , around conduit, thru bottom chair. Repeat. Depending on chair lay out, I stake and tie every other support. Bundle evey other one. Pour corncrete. In my opinion, concrete is grounded and you are tied in a complete loop. Make sure you double up your tie wire.

A little tip is to put a top chair every so often, so you can see the depth of your corncrete, while you are pumping it. Make sure you stake it, or it will float.
Nice I like the top chair Idea for giving you an idea of how high you are How far apart do you stake it?
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Old 06-28-2013, 06:40 AM   #8
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Quote:
Originally Posted by Aegis View Post
Back when I was a first year I was taught never use tie wire to go completely around the duct bank and fasten the conduit to the chairs that are holding it. Instead go over the conduit (top) then through the chair and then tie back over the top again. This is so there's never a complete circle of tie wire around any duct bank.

I was told if you go all the way around it can induce voltage etc = it isn't good. I never thought about it since. I left that company and all the duct banks here they use string to tie everything down. I preferred the tie wire method but I'm just wondering how you guys do it?

Here's a pic off the net of what they said "not to do".
I think the formula for impedance includes length so, I wouldn't worry so much.
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Old 06-28-2013, 07:05 AM   #9
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I think the formula for impedance includes length so, I wouldn't worry so much.

That's what she said.
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Old 06-28-2013, 08:57 AM   #10
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Default Stakes

How far apart do you stake it?


Depends on your spacing. Maybe every 8 to 12 feet.
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Old 06-28-2013, 11:41 AM   #11
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DWP in LA makes you pull any emt stakes during pour. They site inductive eddy current losses.
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Old 06-28-2013, 12:24 PM   #12
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I think the formula for impedance includes length so, I wouldn't worry so much.
I tend to agree, while possible, I'd bet in a real world application such as the above, minimal risk.

But avoid it and avoid being posted on an electrical forum.
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Old 06-28-2013, 12:39 PM   #13
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If the tie wire is a problem then the use of ferrous straps that completely encircle the raceway or the use of a metallic 90 in the run would also be a problem. Neither of those are a real world problem unless you have the some what rare isolated phase installation.
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Old 06-28-2013, 05:29 PM   #14
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I'm wondering if some were taught this a while ago and just passed it down to others through out the years without actually knowing. I'll be sure to ask a few inspectors and post what they say in here.
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Old 06-28-2013, 06:57 PM   #15
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Been on a job that the engineer would not allow encircling of any metal on the duct banks. Straps used had to have bolts replaced with nylon ones. I think they had value engineered the job so any little issue might be bigger than normal. Or maybe he was wound a little too tight, who's to say?
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Old 06-28-2013, 07:25 PM   #16
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Refailure, you don't know anything about electrical work.
maybe if I camped out on this forum all day you could teach me
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Old 06-28-2013, 07:41 PM   #17
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Originally Posted by gaffer1 View Post
maybe if I camped out on this forum all day you could teach me
You've been here for years but you still don't know anything about electrical work. I don't get it.
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Old 06-28-2013, 07:43 PM   #18
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You've been here for years but you still don't know anything about electrical work. I don't get it.
maybe your a bad teacher.
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Old 06-28-2013, 07:44 PM   #19
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maybe your a bad teacher.
Could be, I'm just a useless troll.
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Old 06-28-2013, 07:47 PM   #20
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Could be, I'm just a useless troll.
or a plumber

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