Pullboxes in Slab

[don_resqcapt19]

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...

But that pulling force reduction is a factor of the reduced length of the run and not a factor of the larger bend radius.

 

[don_resqcapt19]

That, too.

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

The force required to move the cable though the previous sections of the run. Yes, gravity will be helping supply that pulling force.

 

[Smart $]

But that pulling force reduction is a factor of the reduced length of the run and not a factor of the larger bend radius.

Correct in a way... but the point in fact is the large radii bends do reduce the overall pulling force compared to short radii bends. It is the overall pull force that matters... not the factoring because of a bend.

 

[don_resqcapt19]

Correct in a way... but the point in fact is the large radii bends do reduce the overall pulling force compared to short radii bends. It is the overall pull force that matters... not the factoring because of a bend.

But my point is that it is not the radius of the bend that make the major difference ...it is the reduction in length when you are looking at very long radius bends.
Most of the time when this comes up we are talking about the comparison of the pulling force needed to pull around a "standard" bend and a 3 or 4 or even 5' sweep bend. In those cases, there is very little length change, and there will be very little difference in the required pulling force. The long sweep is used to prevent the sidewall forces from damaging the cable.

My personal experience with very long sweep bends in smaller conduit and pulling by hand is that the very long sweeps seem to pull harder. The long sweeps I am talking about are concentric bends on a horizontal rack. In some cases it took more than 10' of conduit to make the 90. I never used any type of tension meter, but those types of pulls seemed to require more force than runs with standard bends.

 

[Smart $]

But my point is that it is not the radius of the bend that make the major difference ...it is the reduction in length when you are looking at very long radius bends.
Most of the time when this comes up we are talking about the comparison of the pulling force needed to pull around a "standard" bend and a 3 or 4 or even 5' sweep bend. In those cases, there is very little length change, and there will be very little difference in the required pulling force. The long sweep is used to prevent the sidewall forces from damaging the cable.

My personal experience with very long sweep bends in smaller conduit and pulling by hand is that the very long sweeps seem to pull harder. The long sweeps I am talking about are concentric bends on a horizontal rack. In some cases it took more than 10' of conduit to make the 90. I never used any type of tension meter, but those types of pulls seemed to require more force than runs with standard bends.

I should have stated with regards to a basic calculation. Otherwise, I'll agree with that...

Out at the local refinery, we run very long radius bends (actually segmented bends) around the process and storage tanks. I don't recall any radii specifically, but thinking up to 50' radii, give or take.

I've never had a calculated pulling force provided to me for comparison with the actual... and have never been tasked to document the actual either. :happysad:

 

[teqniqal]

It's not just 'pulling tension' that matters

It's not just 'pulling tension' that matters

It is often overlooked that 'pulling tension' can be SIGNIFICANTLY REDUCED if the workers at the feed end of the work PUSH the cables into the conduit in synchronization with the efforts of the pulling crew. The resulting cable slack reduces the amount of wire jacket that is pressed against the conduit wall and makes wire installation A LOT EASIER due to reduced friction. If you need to demonstrate this to your crew, just take a 4' conduit with two 90 degree bends an run a piece of wire through it. You can easily feel the pulling tension difference between the 'Pull only' and the 'Push while pulling' approaches.

Other factors that can help, too: Pull a cleaner rag through the pipe BOTH WAYS to clear-out dry debris. A Scotchlite cleaning pad is good for this as it frees-up bits of dried concrete. Follow-up with a damp swab rag to clean-out the dust, then pre-swab the pipe with pulling lubricant BEFORE you try to pull in the wire. Lubing the cable as a part of the pushing process helps, too. Messy, and time consuming, but it can make a long run with many bends more do-able.

Also to note: If you PULL plastic jacketed cables through a PVC pipe too fast, the the friction between the cable jacket and the conduit wall will heat-up the conduit and MELT it at the turns (elbows). This can create a breach in the conduit wall that can allow ground water to flood-in. If you pause for a moment to take a breather, the melted PVC conduit wall can cool-down and the molten PVC will adhere to the wire jacket and the whole thing becomes one big mess stuck in-place. That can be very expensive. Pushing the wire into the conduit can reduce the sidewall friction that creates heat that melts the conduit.