Pullboxes in Slab

[mstrlucky74]

Can you have pulboxes in the slab. Spec indicated no more than 180 degree in bends without a pullbox for conduits 2" and larger but most of these conduits are shown to be run in the slab. This is for low volatag.

 

[Smart $]

Can you have pulboxes in the slab. Spec indicated no more than 180 degree in bends without a pullbox for conduits 2" and larger but most of these conduits are shown to be run in the slab. This is for low volatag.

Yes, you can put pullboxes in the slab... i.e. as long as they remain accessible from above the slab.

Regarding the spec', most in-slab or under-slab conduit runs are point to point, but not necessarily in a straight line. Typically, no one counts long sweeps (i.e. large radii) as bends, so you only have bends, usually 90?, for entering and exiting the slab... thus 180?.

EDIT: "?" after numbers are supposed to be degree symbol (dang forum software )

 

[mstrlucky74]

Yes, you can put pullboxes in the slab... i.e. as long as they remain accessible from above the slab.

Regarding the spec', most in-slab or under-slab conduit runs are point to point, but not necessarily in a straight line. Typically, no one counts long sweeps (i.e. large radii) as bends, so you only have bends, usually 90?, for entering and exiting the slab... thus 180?.

EDIT: "?" after numbers are supposed to be degree symbol (dang forum software )

Thanks. Makes sense about the large sweeps but even then there would be some degree say 30,45 degree bends. Are those calculated? The routing on the drawings shows the pipe turning left/right(90 degree) about 4 times. How could you do that point to point without 90's? How would the engineers show that when submitting a shop drawing for approval? Thanks

 

[GoldDigger]

Thanks. Makes sense about the large sweeps but even then there would be some degree to the bend..say 30,45 degree. Are those calculated usually? Thanks

When the bend is gradual enough, then even though the difference between the starting angle and the ending angle is, for example, 45 degrees, it is not counted as a bend at all for pulling purposes.

 

[mstrlucky74]

When the bend is gradual enough, the even though the difference between the starting angle and the ending angle is, for example, 45 degrees, it is not counted as a bend at all for pulling purposes.

Ok, thanks

 

[Smart $]

Thanks. Makes sense about the large sweeps but even then there would be some degree say 30,45 degree bends. Are those calculated? The routing on the drawings shows the pipe turning left/right(90 degree) about 4 times. How could you do that point to point without 90's? How would the engineers show that when submitting a shop drawing for approval? Thanks

To add to GoldDigger's reply, I would question the intent of the drawing...

Conduit runs indicated on drawings do not always reflect required or actual routing. In many to most cases, the drawing's lines indicate only the functional aspect (i.e. start and end). You have to remember that drawings are made to look neat, structured, organized, and somewhat easy to discern which line is which. To have lines that could be likened to a bowl of spaghetti would be considered extremely unprofessional, no matter how close to actual... at least on pre-construction drawings.

The other side of the coin would be drawings that reflect an intent for locating runs after built and/or avoiding other systems or structural components; minimal disruption to operations if a run needs to be exposed for repairs; if runs are to be substantially under slab such that trenching is imminent and the routing is to minimize disturbed soil.

IMO, if routing depicted on the drawing is as required, so as to preclude runs would have more than 180 degrees of bends between termination points, then in-slab pullboxes should also be indicated... and I emphasize only my opinion. Ultimately, if in doubt, an RFI is in order.

 

[mstrlucky74]

To add to GoldDigger's reply, I would question the intent of the drawing...

Conduit runs indicated on drawings do not always reflect required or actual routing. In many to most cases, the drawing's lines indicate only the functional aspect (i.e. start and end). You have to remember that drawings are made to look neat, structured, organized, and somewhat easy to discern which line is which. To have lines that could be likened to a bowl of spaghetti would be considered extremely unprofessional, no matter how close to actual... at least on pre-construction drawings.

The other side of the coin would be drawings that reflect an intent for locating runs after built and/or avoiding other systems or structural components; minimal disruption to operations if a run needs to be exposed for repairs; if runs are to be substantially under slab such that trenching is imminent and the routing is to minimize disturbed soil.

IMO, if routing depicted on the drawing is as required, so as to preclude runs would have more than 180 degrees of bends between termination points, then in-slab pullboxes should also be indicated... and I emphasize only my opinion. Ultimately, if in doubt, an RFI is in order.

Got it, thanks.

 

[Knightryder12]

To add to GoldDigger's reply, I would question the intent of the drawing...

Conduit runs indicated on drawings do not always reflect required or actual routing. In many to most cases, the drawing's lines indicate only the functional aspect (i.e. start and end). You have to remember that drawings are made to look neat, structured, organized, and somewhat easy to discern which line is which. To have lines that could be likened to a bowl of spaghetti would be considered extremely unprofessional, no matter how close to actual... at least on pre-construction drawings.

The other side of the coin would be drawings that reflect an intent for locating runs after built and/or avoiding other systems or structural components; minimal disruption to operations if a run needs to be exposed for repairs; if runs are to be substantially under slab such that trenching is imminent and the routing is to minimize disturbed soil.

IMO, if routing depicted on the drawing is as required, so as to preclude runs would have more than 180 degrees of bends between termination points, then in-slab pullboxes should also be indicated... and I emphasize only my opinion. Ultimately, if in doubt, an RFI is in order.

Thanks for that answer Smart. If in doubt, send an RFI to the engineer.

 

[don_resqcapt19]

...
Regarding the spec', most in-slab or under-slab conduit runs are point to point, but not necessarily in a straight line. Typically, no one counts long sweeps (i.e. large radii) as bends, so you only have bends, usually 90?, for entering and exiting the slab... thus 180?.
...

The use of long sweeps does not reduce the pulling force so why would you not count them. The long sweeps reduce the sidewall pressure that can crush the insulation, but not the pulling force. Many low voltage cables have unrealistically low pulling forces specified by the manufacturer or other standards.

 

[don_resqcapt19]

When the bend is gradual enough, then even though the difference between the starting angle and the ending angle is, for example, 45 degrees, it is not counted as a bend at all for pulling purposes.

The bends are all counted in every pulling force calculation I have ever seen. Two 45? bends results in the same pulling force as a single 90? bend.

 

[growler]

When the bend is gradual enough, then even though the difference between the starting angle and the ending angle is, for example, 45 degrees, it is not counted as a bend at all for pulling purposes.

The bends are all counted in every pulling force calculation I have ever seen. Two 45? bends results in the same pulling force as a single 90? bend.

Don I think you are a smart guy but I don't think you and GoldDigger are talking about the same thing.

A 45 degree bend is a 45 degree bend but I can turn a conduit run 45 degrees over a distance of say 15-20 feet and never even use hot box for PVC. There is no real bend in the conduit but it does change direction, it is very gradual. Would I have to count this change in direction?

I am a big fan of the very long sweep.

 

[GoldDigger]

Don I think you are a smart guy but I don't think you and GoldDigger are talking about the same thing.

A 45 degree bend is a 45 degree bend but I can turn a conduit run 45 degrees over a distance of say 15-20 feet and never even use hot box for PVC. There is no real bend in the conduit but it does change direction, it is very gradual. Would I have to count this change in direction?

I am a big fan of the very long sweep.

There are at least two components to the pulling force.
1. Friction against the sidewall. This depends on the wall surface, insulation surface, and the force pushing the wire against the wall. (and it can be reduced by lubricant.)That side force can come from gravity and from tension in the wire pulling it against the curved wall. It can also come from being wedged between other wires.
2. When a wire goes through a tight bend it requires force to bend it, and that can cause both direct side force and increase the tension making forces closer to the pulling end greater.

A sweep will not have a significant contribution from 2 , but will increase 1 by a non-zero amount which will still be less than the effect of an elbow and the same length of straight pipe.

The pulling force added for a elbow at the far end of a straight section will not be as great as the force added at the near end of that same pipe. Pulling calculators need to be quite complex to model all of that , but the code has to make do with worst case approximations.

 

[don_resqcapt19]

Don I think you are a smart guy but I don't think you and GoldDigger are talking about the same thing.

A 45 degree bend is a 45 degree bend but I can turn a conduit run 45 degrees over a distance of say 15-20 feet and never even use hot box for PVC. There is no real bend in the conduit but it does change direction, it is very gradual. Would I have to count this change in direction?

I am a big fan of the very long sweep.

Yes, that is still counted. The pulling force is the same as when you would use a standard radius 45? sweep.

 

[Smart $]

The use of long sweeps does not reduce the pulling force so why would you not count them. The long sweeps reduce the sidewall pressure that can crush the insulation, but not the pulling force. Many low voltage cables have unrealistically low pulling forces specified by the manufacturer or other standards.

The bends are all counted in every pulling force calculation I have ever seen. Two 45? bends results in the same pulling force as a single 90? bend.

Yes, that is still counted. The pulling force is the same as when you would use a standard radius 45? sweep.

Pulling force and sidewall pressure are dynamically related, but not always directly proportional. The problem is the calculations are generalizations, and do not take into account every variable. For example, say you have a 90? pull. However, the conduit is two sections used only two protect the cable. Instead of using a standard bend or elbow at the 90? turn, a sheave of the same radius is placed there. In this case, both the pull force and sidewall pressure is reduced compared to an installation with a bend or elbow. How many calculations are you aware of that account for that difference???

Another example which disproves your claim... Most pulling force calculations are based on the length of conduit run (or the length of cable). Using numbers which are easy to calculate, let's say you have a pull that measures 141 ft entry point to exit point. Now for comparison we have two means of routing the conduit. One is 100 ft east, a standard 90? bend, then 100 ft north. The second is to arc it starting in an easterly direction and ending in a northerly. In case one, we have 200 ft of conduit. In case two we have 157 ft of conduit. Now in both cases we have a 90? bend, but in the latter we have substantially less distance... which will calculate to a lower pulling force. This is how long sweeps differ.

FWIW, my original statement regarding the counting of the extremely long sweep bends had nothing to do with pulling force or sidewall pressure, but rather compliance with the NEC max bend requirement in a run.

 

[don_resqcapt19]

Pulling force and sidewall pressure are dynamically related, but not always directly proportional. The problem is the calculations are generalizations, and do not take into account every variable. For example, say you have a 90? pull. However, the conduit is two sections used only two protect the cable. Instead of using a standard bend or elbow at the 90? turn, a sheave of the same radius is placed there. In this case, both the pull force and sidewall pressure is reduced compared to an installation with a bend or elbow. How many calculations are you aware of that account for that difference???

I have never worked with cable tray pulling calculators, but I assume they account for the use of a sheave. That has a substantial reduction in friction at the sheave as compared to pulling around a conduit bend and the pulling tension will go down as will the sidewall pressure.

Another example which disproves your claim... Most pulling force calculations are based on the length of conduit run (or the length of cable). Using numbers which are easy to calculate, let's say you have a pull that measures 141 ft entry point to exit point. Now for comparison we have two means of routing the conduit. One is 100 ft east, a standard 90? bend, then 100 ft north. The second is to arc it starting in an easterly direction and ending in a northerly. In case one, we have 200 ft of conduit. In case two we have 157 ft of conduit. Now in both cases we have a 90? bend, but in the latter we have substantially less distance... which will calculate to a lower pulling force. This is how long sweeps differ.

The the total length is a big factor in the pulling tension as the weight per foot of the conductors is big part of the pulling tension along with the friction on the length of the raceway.
Are you saying that if the conduit length is the same for both standard bends and very long radius bends, that the pulling force will be substantially less for the very long radius bend?

 

[Fulthrotl]

I have never worked with cable tray pulling calculators, but I assume they account for the use of a sheave. That has a substantial reduction in friction at the sheave as compared to pulling around a conduit bend and the pulling tension will go down as will the sidewall pressure.


The the total length is a big factor in the pulling tension as the weight per foot of the conductors is big part of the pulling tension along with the friction on the length of the raceway.
Are you saying that if the conduit length is the same for both standard bends and very long radius bends, that the pulling force will be substantially less for the very long radius bend?

well, if the radius of the bend is greater than 48", i believe it's not considered a bend.

a practical consideration is that with SimPull and the copycats, you can pull stuff pretty much without drag....
i had a 700+ foot run of #2 in inch and a half pipe, with 7 factory 90's in it and a kick, and C condulets
every so often, and that pulled without ever hitting 1,000 lbs on the strain gauge. most of the time it was
under 500#, except when the head went around a 90.

if you really want to see how slippery that stuff is, leave a 2' long of #2 on the concrete floor, and step on
it accidentally.

 

[GoldDigger]

Are you saying that if the conduit length is the same for both standard bends and very long radius bends, that the pulling force will be substantially less for the very long radius bend?

Yes, especially if lubricant is used. That reduces what I list as component 1 without doing as much for component 2. And component 2 is much higher with a tight radius bend.
For completeness in the specification, let's also stipulate that the standard bend is exactly in the middle of the overall length of the pull.

We are also assuming substantially horizontal pulls in the analysis.

 

[Smart $]

...
Are you saying that if the conduit length is the same for both standard bends and very long radius bends, that the pulling force will be substantially less for the very long radius bend?

No. I'm saying given identical the entry and exit points a larger radii route will require less pulling force because the developed length of conduit is shorter. Consider the four routes depicted...

 

[Smart $]

...

We are also assuming substantially horizontal pulls in the analysis.

That, too.

Where any portion of the pull is vertically down, the pulling force required through that portion is...???

 

[curt swartz]

well, if the radius of the bend is greater than 48", i believe it's not considered a bend.

Can you post the exception in the NEC that states that? I have never seen it.