Decoding NEC Compliance: O-Z/Gedney™ Cable Supports as Essential Elements in Vertical Cable Installation
Emerson O-Z/Gedney™
Vertical cable supports are one of the electrical industry’s unsung heroes. Engineered to safely support non-armored cables in vertical raceways or risers, they can make the critical difference between power being connected or disconnected. When installed according to NEC code, cable supports provide enhanced safety, greater reliability, and lower maintenance wherever heavy power cables traverse between floors in multi-story buildings. Yet cable supports can be deceptively simple, leading to no shortage of confusion for the electrician when it comes to their requirements. In this blog, we are going to explore what codes apply to them and how they are installed.
There is a mistaken belief that the NEC does not come into play on low voltage installations. While NEC 300.19 doesn’t explicitly cover data cables, the same rules apply. Additionally, there is a misconception that the use of wire mesh grips or even sheet rubber wrapped around conductors and tied off in the box is enough to satisfy the code. Neither approach meets NEC Article 300.19 for supporting conductors. While wire mesh grips are very useful in shorter runs, they have no place in applications outlined in the NEC chart.
Installing Cable Supports
There are two basic types of cable supports: the one-piece plug and the multiple segment plug. The one-piece has a single wedging plug with a groove for each wire. It is ideal for all types of non-armored cables 600 Volts or less, plus it is considered the easiest to install. In contrast, the multiple segment version has a wedging plug constructed so that each cable is supported between grooves in adjacent segments. This uniform pressure distribution is required by the softer types of insulations frequently used at higher voltages.
The electrician begins by threading the cable support body to conduit or resting it on existing bushing atop a conduit raceway in which non-armored cables pass through. A wedging plug is then inserted into the cable support body that exerts pressure around the cable, providing holding force without damaging the insulation or excessively reducing the amount of insulation over time. Without it, an unsupported power cable will pull significantly on the upper terminal until it fails.
Installing a cable support is much easier on a new cable. But what if conductors have already been pulled and terminated. For instance, an inspector or AHJ conducting a final inspection of a high-rise finds a code violation involving the vertical raceways, specifically the lack of cable supports? Or the lead electrician finds sub-par work done by an apprentice who installed wire mesh grips instead of supports? Thankfully, O-Z/Gedney supplies retrofit-style cable supports. Because its split body rests on an existing bushing atop the conduit raceway it is possible to use it where cables are already installed. Problem solved.
Ordering Cable Supports
The first step in ordering a cable support is to specify the number of conductors being supported and to provide each conductor’s outside diameter (OD). You’ll also need to know the conduit trade size, voltage, and whether the cable run is in dry or wet location. Slight variations in cable OD can render a support plug either too large or too small. Jacket sizes can fluctuate. So can stranded wire depending on things like the number of strands, amount of twist and manufacturing processes. O-Z/Gedney recommends that you use a digital micrometer or calipers to accurately measure the cable’s OD.
NEC Requirements
The National Electric Code (NEC) calls out specific installation methods for long vertical runs in NEC Article 300.19 (Supporting conductors in vertical raceways). It states that cable support is needed at the top of a long raceway, with intermediate supports installed at specified lengths according to a formula based on the size of the wire as outlined in the chart below. Essentially, the heavier the wire, the shorter the unsupported cable length.
|
|
Conductors
|
||||
|
|
|
Aluminum
or Aluminum
|
Copper
|
||
|
Size of Wire |
Support
of |
m |
ft |
m |
ft |
|
|
|||||
|
18 AWG through 8 AWG |
Not greater than |
30 |
100 |
30 |
100 |
|
6 AWG through 1/0 AWG |
Not greater than |
60 |
200 |
30 |
100 |
|
2/0 AWG through 4/0 AWG |
Not greater than |
55 |
180 |
25 |
80 |
|
Over 4/0 AWG through 350 kcmil |
Not greater than |
41 |
135 |
18 |
60 |
|
Over 350 kcmil through 500 kcmil |
Not greater than |
36 |
120 |
15 |
50 |
|
Over 500 kcmil through 750 kcmil |
Not greater than |
28 |
95 |
12 |
40 |
|
Over 750 kcmil |
Not greater than |
26 |
85 |
11 |
35
|
Installing Cable Supports
There are two basic types of cable supports: the one-piece plug and the multiple segment plug. The one-piece has a single wedging plug with a groove for each wire. It is ideal for all types of non-armored cables 600 Volts or less, plus it is considered the easiest to install. In contrast, the multiple segment version has a wedging plug constructed so that each cable is supported between grooves in adjacent segments. This uniform pressure distribution is required by the softer types of insulations frequently used at higher voltages.
The electrician begins by threading the cable support body to conduit or resting it on existing bushing atop a conduit raceway in which non-armored cables pass through. A wedging plug is then inserted into the cable support body that exerts pressure around the cable, providing holding force without damaging the insulation or excessively reducing the amount of insulation over time. Without it, an unsupported power cable will pull significantly on the upper terminal until it fails.
Installing a cable support is much easier on a new cable. But what if conductors have already been pulled and terminated. For instance, an inspector or AHJ conducting a final inspection of a high-rise finds a code violation involving the vertical raceways, specifically the lack of cable supports? Or the lead electrician finds sub-par work done by an apprentice who installed wire mesh grips instead of supports? Thankfully, O-Z/Gedney supplies retrofit-style cable supports. Because its split body rests on an existing bushing atop the conduit raceway it is possible to use it where cables are already installed. Problem solved.
Ordering Cable Supports
The first step in ordering a cable support is to specify the number of conductors being supported and to provide each conductor’s outside diameter (OD). You’ll also need to know the conduit trade size, voltage, and whether the cable run is in dry or wet location. Slight variations in cable OD can render a support plug either too large or too small. Jacket sizes can fluctuate. So can stranded wire depending on things like the number of strands, amount of twist and manufacturing processes. O-Z/Gedney recommends that you use a digital micrometer or calipers to accurately measure the cable’s OD.
Photo courtesy of Emerson O-Z/Gedney™
