Jamming
Jamming is the wedging of three or more cables as they are pulled into a conduit. This usually occurs as a result of crossovers when the cables are twisted or are pulled around bends in the conduit. Jam ratio is defined as the ratio of conduit inner diameter (D) to the cable outside diameter (d).
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Probability of Jamming
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| Jamming probability using the jam ratio - (Jam Ratio = D/d) |
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< 2.3
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Very small
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2.3 – 2.6
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Small
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2.6 – 2.8
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Moderate
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2.8 – 3.0
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Significant
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3.0 – 3.1
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Moderate
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3.1 – 3.2
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Small
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3.2 and up
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Very small
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The oval cross section of conduit bends was accounted for with a 5% factor.
Note: Measured cable diameters should be used to determine the jam ratio since actual cable diameters vary from published values.
Minimum Bending Radii
Single Conductor 600 Volt Power Cables (without metallic shielding) for conductor insulation 155 mils and less are as shown below. These should be used for training cable into final position (when there is no tension on the cable). When pulling cable through conduit (when cable is under tension), the minimum bending radius should be at least double the minimum bend radius shown below.
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OD of Cable
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Minimum Bending Radius as a Multiple of Cable Diameter
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1.000" and less
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4
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1.001" - 2.000"
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5
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Maximum Pulling Tensions
The following recommendations are based on a study sponsored by the ICEA. These recommendations may be modified if experience and more exact information so indicate.
| 1. Maximum Pulling Tension on a Cable |
| A. With pulling eye attached to copper conductors: |
| T=0.008 x n x CMA |
| Where: |
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T = Maximum tension in pounds |
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n = Number of conductors |
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CMA = Circular Mil Area of each conductor (see Table 1) |
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| When more than three conductors are pulled together, the maximum pulling tension should be reduced by 20%. |
| 2. Maximum Permissible Pulling Length: L=(T)/(f x W) |
| Where: |
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L = Pulling length, feet (straight section) |
| |
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T = Maximum tension, pounds |
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f = Coefficient of friction (per type of lubricant) |
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W = Weight of cable per foot, pounds |
| 3. Calculated Pulling Tension (straight section of conduit) |
| For straight duct sections, the pulling tension equals the length of the duct multiplied by the weight per foot of the cable and the coefficient of friction (per type of cable and lubricant). |
| Ts = L x W x f |
| Where: |
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Ts = Pulling tension at end of straight section in lbs. |
| |
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L = Length of straight section in feet |
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W = Weight of cable in lbs. /ft. |
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f = Coefficient of friction (see Table 2) |
| 4. Calculated Pulling Tension (curved or bent section of conduit) |
| For curved sections, the following formula applies: Tc = T1 x efa |
| Where: |
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Tc = Tension exiting curved section, pounds |
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T1 = Tension entering curved section, pounds |
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e = Napierian logarithm base (2.718) |
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f = Coefficient of friction (see Table 2) |
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a = Angle of bend in radians (1 radian = 57.3°) |
| 5. Cable Sidewall Pressure at Bends |
| Sidewall pressure is caused by the tension in the cable acting horizontally and the weight of the cable acting vertically. Generally, the pulling tension as the cable exits a bend should not exceed the factor shown in the table below multiplied by radius of curvature of the bend in feet. |
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Cable Type
600 Volt, 16 – 10 AWG
600 Volt, 8 AWG and larger
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Factor
300
500
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| Tbm = Factor x r |
| Where: |
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Tbm = Maximum allowable pulling tension at bend in lbs. |
| |
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r = bend radius in feet |
| P=Tb/r |
| Where: |
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P = actual sidewall pressure on cable in lbs./ft. |
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Tb = pulling tension at end of bend in lbs. |
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r = bend radius in feet |
| The maximum allowable pulling tension at a bend (Tbm) is the limit that the calculated pulling tension (Tb) should be compared to. If Tb is greater than Tbm, redesign or rerouting should be considered. |
| Cold Weather Installations |
| Low temperatures will cause the insulation on the conductor to become brittle. Wires should be kept in heated storage for at least 24 hours prior to installation in cold weather. Conductors should not be installed at temperatures below those shown in Table 3. |
| "Megger" (Insulation Resistance Testing) |
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Megger testing is a commonly used method of assuring that 600 volt cables have been installed in conduit without damage to the insulation. DC voltages of 500 or 1000 volts are acceptable to use. Megger readings may vary considerably due to ambient conditions. Humidity, moisture in the conduit, and residue from pulling lube will affect the megger reading. These should be taken into account. The length of the run and the ambient temperature will also affect the reading but adjustments can be made using the formula below to normalize these factors.
IR (insulation resistance per 1000 feet) = (L / 1000) X R X CF
Where: L = length of the conductor in feet
R = megger reading in megohms
CF = temperature correction factor (see table)
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Insulation Resistance Temperature Correction Factor (CF)
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Temperature in °F
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THHN
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45
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0.24
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50
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0.39
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55
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0.62
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60
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1.00
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65
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1.61
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70
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2.59
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75
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4.18
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80
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6.73
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85
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10.8
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After the selected voltage (500 or 1000 volts DC) is applied for 1 minute, the reading in megohms is taken from the megger. If the normalized reading is 50 megohms or greater, the reading is considered passing. If the normalized reading is from 2 to 50 megohms, the cable installation should be examined closely. Readings in this range are often accompanied by long runs or moisture or contamination that causes current leakage near the bared conductor ends. A reading in this range usually does not mean that the conductor insulation is damaged or defective. Readings in this ranged should be confirmed and evaluated by an experienced electrical testing specialist. A normalized reading of less than 2 megohms is considered failing if the testing conditions have been scrutinized to assure that current leakage due to moisture or contamination near the bare conductor ends and test leads is not present.
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Table 1
Circular Mil Areas
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AWG
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CMA
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12
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6,530
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10
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10,380
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8
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16,510
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6
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26,240
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4
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41,740
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3
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52,620
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2
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66,360
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1
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83,690
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1/0
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105,600
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2/0
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133,100
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3/0
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167,800
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4/0
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211,600
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Kcmil
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CMA
|
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250
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250,000
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300
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300,000
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350
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350,000
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400
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400,000
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500
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500,000
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600
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600,000
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750
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750,000
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1000
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1,000,000
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Table 2
(Use if exact numbers for cable type and lubricant are not known)
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Typical Coefficients of Friction with Adequate Cable Lubrication during the Pull
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Type of Conduit
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Cable Outer Surface
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Steel or Aluminum
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PVC
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PVC
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0.40
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0.35
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Nylon
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0.28
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0.24
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XLPE
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0.25
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0.14
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Table 3
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Minimum Installation Temperatures
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Jacket/Installation Type
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Minimum Installation Temperataure
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PVC
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-10°C
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Nylon
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XLPE
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