|
 CONDUIT
AND DUCT SIZES
In
general it is recommended that conduit sizes be governed by the
National Electrical
Code. Unless strict adherence to NEC is specified, larger conduit
sizes may be
governed by the determination of 1/2
" clearance between the conduit inside diameter
and the circumscribing diameter of the installed cable(s).
Formulas
for computing the minimum conduit inside diameter based on the
above rules are shown below.
POWER
CABLE CONDUIT AND DUCT SELECTION
Number
of Cables
Per Conduit |
National
Electrical Code |
Standard
Utility
Calculation Method
2" and Larger Conduits |
Max
Percent
Occupancy |
Calculation
Formulas |
| One
|
Non-leaded
53% |
D
= 1.374 x d |
D
= 0.500 + d |
Lead
Covered
55% |
D
= 1.348 x d |
| Two |
Non-leaded
31% |
D
= 2.540 x d |
D
= 0.500 + (2 x d) |
Lead
Covered
30% |
D
= 2.582 x d |
| Three |
All
Types
40% |
D
= 2.739 x d |
D
= 0.500 + (2.155 x d) |
| Four
or More |
Non-Leaded
40% |
D
= 1.581 x ÖN x d |
D
= 0.500 + (F x d) |
D = Minimum conduit
I.D. (inches). Select nearest trade size conduit that is larger than
the computed figure
for D.
d = Individual cable O.D.
N = Number of individual cables in same conduit.
F = Multiplying factor for computing circumscribing diameter over
multiple conductor cables.
|
N
|
F
|
| 2 |
2.000 |
| 3 |
2.155 |
| 4 |
2.414 |
| 5 |
2.700 |
| 6 or 7 |
3.000 |
CAUTION:
When installing three or more parallel cables in the
same conduit, avoid ratios of 2.8 to 3.1 (Ratio
of conduit I.D. to one cable O.D.) to minimize potential jamming.
This caution does not apply
to twisted assemblies (triplexed, quadruplexed, etc.) in normal installations,
but should be considered when
multiple bends occur in the same conduit or long, difficult pulls
are anticipated.
APPROXIMATE MAXIMUM ALLOWABLE CABLE DIAMETERS
(IN INCHES)
| |
Per
NEC
|
|
Per
1/2" Clearance
|
|
|
Conduit
|
Internal
|
Conduit
|
|
3 or
more
|
|
Trade
|
Diameter
|
Area
|
1
|
|
3
|
1
|
3
|
parallel
|
|
Size
|
(Inches)
|
(Sq.
Inches)
|
Conductor
|
Conductor
|
Conductor
|
Conductor
|
(AVOID)
|
| |
Steel
or Schedule 40 PVC
|
|
|
1/2
|
0.602
|
0.29
|
0.44
|
|
0.22
|
|
|
0.19-0.22
|
|
3/4
|
0.804
|
0.51
|
0.59
|
0.29
|
|
|
0.26-0.29
|
|
1
|
1.029
|
0.83
|
0.75
|
0.38
|
|
|
0.33-0.37
|
|
11/4
|
1.360
|
1.45
|
0.99
|
0.50
|
|
|
0.44-0.49
|
|
11/2
|
1.590
|
1.99
|
1.16
|
0.58
|
|
|
0.51-0.57
|
|
2
|
2.047
|
3.29
|
1.49
|
0.75
|
1.55
|
0.72
|
0.61-0.73
|
|
21/2
|
2.445
|
4.70
|
1.75
|
0.89
|
1.95
|
0.90
|
0.79-0.87
|
|
3
|
3.042
|
7.27
|
2.21
|
1.11
|
2.54
|
1.18
|
0.98-1.09
|
|
31/2
|
3.521
|
9.74
|
2.56
|
1.29
|
3.02
|
1.40
|
1.14-1.26
|
|
4
|
3.998
|
12.55
|
2.91
|
1.46
|
3.50
|
1.62
|
1.29-1.43
|
|
5
|
5.016
|
19.76
|
3.65
|
1.83
|
4.52
|
2.10
|
1.62-1.79
|
|
6
|
6.031
|
25.57
|
4.39
|
2.20
|
5.53
|
2.57
|
1.95-2.15
|
Cement Conduit or Duct
| |
| 2 |
1.9 |
2.84 |
1.38 |
0.69 |
1.40 |
0.65 |
0.61-0.68 |
| 3 |
2.9 |
6.61 |
2.11 |
1.06 |
2.40 |
1.11 |
0.94-1.04 |
| 31/2 |
3.4 |
9.08 |
2.48 |
1.24 |
2.90 |
1.35 |
1.10-1.21 |
| 4 |
3.9 |
11.95 |
2.84 |
1.42 |
3.40 |
1.58 |
1.26-1.39 |
| 41/2 |
4.4 |
15.21 |
3.20 |
1.61 |
3.90 |
1.81 |
1.42-1.57 |
| 5 |
4.9 |
18.86 |
3.57 |
1.79 |
4.40 |
2.04 |
1.58-1.75 |
| 6 |
5.9 |
27.34 |
4.30 |
2.15 |
5.40 |
2.51 |
1.90-2.11 |
|
Fiber Conduit or Duct
| |
| 2 |
2.0 |
3.14 |
1.46 |
0.73 |
1.50 |
0.70 |
0.65-0.71 |
| 3 |
3.0 |
7.07 |
2.18 |
1.10 |
2.50 |
1.16 |
0.97-1.07 |
| 31/2 |
3.5 |
9.62 |
2.55 |
1.28 |
3.00 |
1.39 |
1.13-1.25 |
| 4 |
4.0 |
12.57 |
2.91 |
1.46 |
3.50 |
1.62 |
1.29-1.43 |
| 41/2 |
4.5 |
15.90 |
3.28 |
1.64 |
4.00 |
1.86 |
1.45-1.61 |
| 5 |
5.0 |
19.64 |
3.64 |
1.83 |
4.50 |
2.09 |
1.61-1.79 |
| 6 |
6.0 |
28.27 |
4.37 |
2.19 |
5.50 |
2.55 |
1.94-2.14 |
Example:
Determine
the minimum plastic conduit size for three 1/C 500 kcmil, 15kV,
copper stranded, 175 mil, 100% insulation wall, copper tape shield
with a PVC jacket. The three conductors are paralleled and the individual
conductor O.D. is 1.51".
Using the first
formula for three cables per conduit based on NEC requirements:
D = 2.739 x
d
D = 2.739 x 1.51"
D = 4.136"
The NEC requires
a 5" conduit for 1.51" diameter cable. Using the second formula
based on standard utility practice of 1/2 " clearance.
D = 0.5000"
+ 2.155 x d
D = 0.5000" + 2.155 x 1.51"
D = 0.5000" + 3.254"
D = 3.754"
A 4" conduit
may be used for a 1.51" diameter cable.
Note:
The 1.51" diameter cable does not fall into the jamming range 1.30"
to 1.44" for a 4" conduit or 1.63" to 1.80" for a 5" conduit. If
it did, simply twisting the assembly or a larger conduit size would
remove this objection.
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