mstrlucky74
Senior Member
- Location
- NJ
I see these asked for a lot for underground work? What's the longer sweep for(or help) and where is the sweep measurement take from...say 36" or 60"? Thanks.
Long sweep Elbows
- Thread starter mstrlucky74
- Start date
- Status
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
i believe that the 36" or 60" figure refers to the radius of the curve at the centerline of the pipe. That also gives you the approximate perpendicular offset of the end of one straight piece to the end of the other.
romex jockey
Senior Member
- Location
- Vermont
- Occupation
- electrician
it's to visually verify a 3' depth
~RJ~
~RJ~
Adamjamma
Senior Member
- Location
- Jamaica and london
I thought you used the 60 if you were worried about sidewall tension due to size of conductor... the 60 offers an easier sweep to pull the wire through.
romex jockey
Senior Member
- Location
- Vermont
- Occupation
- electrician
yes on long runs / bigger triplex Adam
~RJ~
~RJ~
Adamjamma
Senior Member
- Location
- Jamaica and london
Know the bigger I make my loops the easier the wire pulls through them... then I watch other electricians trying to pull wire through tight bends and I weep watching thm cuss...
Strathead
Senior Member
- Location
- Ocala, Florida, USA
- Occupation
- Electrician/Estimator/Project Manager/Superintendent
Yup, I see that and I use factory standard elbows when they make that statement. It is rare for the Engineer that writes such a lazy spec. to visit the job, but if one did and attempted to insist I provide a longer radius elbow, I would require a change order. "long sweep" does not define anything. The sweep is longer than the minimum sweep of a standard elbow as required in the NEC. If they want a specific sweep they need to specify a specific sweep.
romex jockey
Senior Member
- Location
- Vermont
- Occupation
- electrician
The poco's here have spec'd 36" rad sweeps for decades
~RJ~
~RJ~
mstrlucky74
Senior Member
- Location
- NJ
Strathead
Senior Member
- Location
- Ocala, Florida, USA
- Occupation
- Electrician/Estimator/Project Manager/Superintendent
- Location
- New Jersey
- Occupation
- Journeyman Electrician
There are actually two minimum sweeps, one for field bent elbows and one for manufactured elbows with the latter being smaller.
Carultch
Senior Member
- Location
- Massachusetts
According to the friction theory that is used for calculating pulling tension, the pulling tension is independent of the bend radius. What causes a larger sweep to help?
I thought extra large radius bends help with reducing sidewall pressure, but have no effect on pulling tension.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
To a first approximation the friction is the same regardless of bend radius, but the sidewall force to make the 90 degree change in direction of the pulling force will be lower with the larger radius. The lower sidewall pressure times the high sidewall length makes the total friction similar.
Where you get significant differences in pulling behavior are where:
1. The group of wires being pulled is subject to jamming. The code fill percentage for different numbers of wires is supposed to mitigate this.
2. When the bend is tight enough that the force required to bend and straighten the wire as it moves through the curve is significant. This is more likely to happen with solid wire and with the extreme case of using a plumbing elbow instead of an electrical elbow or sweep.
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
In theory, there is no difference between theory and practice.
GoldDigger has it right. The calculations used for pulling tension assume perfectly flexible wire, and also assume that coefficient of friction is a constant.
Under these assumptions bend radius does not alter pull tension. But both of these assumptions are clearly approximations.
-Jon
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
Just a note on the theoretical basis behind the phenomenon of wire jamming in a bend:
If a single conductor is laying against the conduit wall, the pressure against the wall depends only on the coefficient of friction and the wire tension divided by the radius of the bend.
But just as the tension in a span of messenger wire has to be much greater than the weight supported because of the angle of the forces, if two or more wires are pulled against a U-shaped wall the force against the sides of the U can be much greater than the force pulling the wire towards the bottom of the U. Thus for the same cable tension the normal force against the conduit walls and therefore the friction force for the same coefficient of friction can be much greater.
(This greater frictional force is also critical to the operation of v-belts.)
If a single conductor is laying against the conduit wall, the pressure against the wall depends only on the coefficient of friction and the wire tension divided by the radius of the bend.
But just as the tension in a span of messenger wire has to be much greater than the weight supported because of the angle of the forces, if two or more wires are pulled against a U-shaped wall the force against the sides of the U can be much greater than the force pulling the wire towards the bottom of the U. Thus for the same cable tension the normal force against the conduit walls and therefore the friction force for the same coefficient of friction can be much greater.
(This greater frictional force is also critical to the operation of v-belts.)
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