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Below you will find a general guide for the installation of shielded and unshielded cables, jacketed cables rated 600 to 35,000 volts in conduit, underground ducts, racks, trays or direct buried.

Safety Measures Prior to Pulling Cable

Temperature Considerations
It is important to consider that low temperatures create handling and pulling difficulties of varying degrees, depending on cable construction and installation location. Most cables can be safely handled if not subjected to temperatures lower than -10°C (14°F) in the twenty-four (24) hour period prior to pulling and bending the cable.

If temperatures do fall below these values during the following 24 hour period, measures should be taken to transport the cable to a warm storage area. The cable should be exposed to a temperature of 15°C (60°F) for 24 hours to ensure warm-up. Pulling eyes, if necessary, should be set up. If the cable is installed during cold weather conditions, cable should be pulled more slowly and trained in place the same day it is removed from the warm storage area. The cable should also not be dropped or bent if cold outside.

Duct Sizing
Select duct size in such a way that the difference between the hoop diameter of the cable(s) and the inside diameter of the duct will not be less than ½”. Also check that the cross-sectional area of the cable is not more than the percentage of the interior cross-sectional area of the conduit, as recommended by the National Electric Code (NEC). In addition, consider using larger ducts or additional pull boxes if long pulls are required.

Jam Ratio
Jamming might occur in bends if three cables are pulled in parallel in duct. This happens when the cables adjust from a triangular pattern to a cradled pattern as they are pulled in through the bend.

This pattern change will force the two outer cables to move farther apart. The cables will also jam if the conduit diameter is too small to contain the wider pattern.  

To prevent this, the jam ratio should be checked. The jam ratio corresponds to the inside diameter of the duct to the cable diameter, such that:

J = D ÷ d

J = Jam ratio
D = Inside diameter of duct (in)
d = Outside diameter of cable (in)

The proper cable configuration can be determined if the above jam ratio is calculated. The likely configurations are as follows:

Jam Ratio
J < 2.4

2.4 < J < 2.6
2.6 < J < 2.8
2.8 < J < 3.0
J > 3.0

Cable Configuration

More likely triangular
Either triangular or cradled
More likely cradled

Cable jamming tends to occur between J = 2.8 and J = 3.1. This is true if the sidewall bearing pressure (SWBP) in a bend surpasses the 1,000 lbs/foot.

Cable Clearance

In order to make sure that the cables can be pulled through the conduit, specifically in applications where the National Electric Code (NEC) limits on conduit fill do not apply, one needs to calculate the clearance between the cable(s) and conduit. The recommended calculated clearance should not be less than 0.5 inches.

However, a lesser clearance, such as 0.25 inches, may be suitable for primarily straight pulls.

In addition, the clearance should contain the pulling eye or cable grip, which is used for the cable pull. The formulas below can be used to calculate the cable clearance for a single cable pull and for a three-cable pull. (Please Note: To allow for differences in cable and duct dimensions and ovality of the duct at bends, the nominal cable diameter “d” has been increased by 5%).

a. Single Cable Pull

C = D – 1.05 x d

b. Three Cable Pull (triangular pattern)

C =
 − 1.366 (1.05 × d) + 
(D - 1.05 x d)
 1.05 x d
D - 1.05 x d

C = Cable clearance (in)
D = Inside diameter of duct (in)
d = Outside diameter of cable (in) 

Please reference the following table in dealing with applications where the National Electric Code (NEC) is compulsory. The table shows the most ordinary scenarios concerning the fill ratio of many cable configurations in various duct sizes.

Conductor Fill Per NEC Code
1 Conductor
(53% Fill Ratio) 
2 Conductors
(31% Fill Ratio) 
3 Conductors
(40% Fill Ratio) 
4 conductors
(40% Fill Ratio) 
½ 0.16 0.453 0.09 0.245 0.12 0.227 0.12 0.197
¾ 0.28 0.6 0.16 0.324 0.21 0.301 0.21 0.261
1 0.46 0.764 0.27 0.413 0.34 0.383 0.34 0.332
1 ¼ 0.8 1.005 0.47 0.543 0.6 0.504 0.6 0.436
1 ½ 1.08 1.172 0.63 0.634 0.82 0.588 0.82 0.509
2 1.78 1.505 1.04 0.814 1.34 0.755 1.34 0.654
2 ½ 2.54 1.797 1.48 0.972 1.92 0.902 1.92 0.781
3 3.91 2.234 2.26 1.208 2.95 1.12 2.95 0.97
3 ½ 5.25 2.583 3.07 1.397 3.96 1.296 3.96 1.122
4 6.74 2.931 3.94 1.585 5.09 1.47 5.09 1.273
5 10.6 3.674 6.2 1.987 8 1.843 8 1.596
6 15.31 4.415 8.96 2.388 11.56 2.215 11.56 1.918

Please Note: “dmax(in)” is the maximum single conductor diameter that will comply with the above requirements. “Area (in2)” is the area of the conductor(s). Ground wires have not been considered in the above table. The NEC requires that “Equipment grounding or bonding conductors, where installed, shall be included when calculating conduit or tubing fill. The actual dimensions of the equipment grounding or bonding conductor (insulated or bare) shall be used in the calculation.”

The below formula can be used when a calculation must be made to comply with the NEC fill ratio requirements:

FR = [NPC x PCD ÷ 2)2 + NGC x (GCD ÷ 2)2] ÷ (CD ÷ 2)2


FR = Fill Ratio (%)
NPC = Number of Phase Conductors with the same diameter
PCD = Diameter of Phase Conductor (in)
NGC = Number of Ground Conductors with the same diameter
GCD = Diameter of Ground Conductor (in)
CD = Diameter of Conduit or Duct (in)


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