If you’ve ever installed tray cables in an industrial or commercial setting, you likely relied on conduit to do the job safely. 

Conduit is a low-cost solution to protect tray cable and other electrical wires from damage during and after installation. The tubing is typically either plastic, like polyvinyl chloride (PVC), or metal, like steel or aluminum, and provides several benefits for your wire installations. 

The conduit protects tray cable and other wires from environmental and physical damage, especially in places where contact with chemicals or moisture could occur. It also reduces the threat of electromagnetic interference (EMI) from affecting signals and machines. Meanwhile, metal conduit offers a safe grounding path for electricity, protecting people and objects from accidental shocks. 

Luckily, no matter how and what you choose to install, the National Electrical Code (NEC) has rules to ensure the best level of protection possible. 

The Risk of Incorrect Conduit Use 

It might seem like a time and cost-saving idea to jam as many wires as possible into your conduit, but it’s reckless and potentially dangerous. 

NEC guidelines, including fill capacities, are meant to keep everyone safe. It might not seem like a huge deal, but the rules differ based on the type of wire and conduit used. Not following regulations, especially fill rates, opens the door to problems during and after installation. 

What are the NEC’s Conduit Fill Capacity Rules? 

The NEC outlines conduit fill rates based on several factors, including the number of conductors used and their size. 

Maximum fill rates can change based on conduit type, though there is a standard general fill rate for reference. Rigid metal conduit (RMC) and intermediate metal conduit (IMC) have similar capacities. Lighter and thinner walled options like electrical metallic tubing (EMT) and flexible metallic conduit (FMC) have lower fill capacities. This is because the conduit is designed to flex and move but still provides protection for the wire and cables inside.

Number of Conductors RMC and IMC EMT FMC 
1 53% 40% 40% 
2 31% 25% 25% 
3 or more 40% 30% 30% 

Plastic conduits like PVC have two common options, Schedule 40 and Schedule 80. Schedule 80 is stronger and thicker, helping it withstand higher pressures and take more damage. The trade-off is that although Schedule 80 has better protective qualities, fewer wires fit in it whether it’s standard 10’ lengths or longer. 

External vs. Internal Measurements 

Conduit is sold based on nominal external diameter (1″ conduit, 2″ conduit, etc.). However, this is not the measurement used for capacity. 

When installing tray cable and other wiring to power machine tools or other equipment, measurements are taken using the internal diameter. This subtle distinction dictates the space available. 

Once you have the internal diameter measurement, you can use it to figure out the internal area of the conduit. The area gives you the conduit’s fill capacity. Fill capacity rules ensure installed wires have enough room for heat dissipation while preventing damage caused during pulling. 

Overfilling Conduit Creates Concerns 

Everyone loves a good shortcut, but they can sometimes turn into pitfalls. The same goes for jamming as many wires as possible into a single conduit to save time and money. 

It seems like a great idea, but stretching beyond capacity risks several dangerous situations. Heat is generated whenever current passes through a wire. If enough heat is produced, it can damage conductors by melting the insulation and fusing together. When this happens, conductors become exposed, allowing air, water, and chemicals to enter through cracks in the insulation. 

Exposed conductors also increase the risk of sparks, arcs, or faults, putting people, appliances, and machines in danger. 

Ten Pounds of Flour in a Five-Pound Bag… 

Time is money, but doing a job right is better than doing it fast, especially where safety is a concern. 

If you’ve ever driven on a multi-lane highway, the easiest way to travel is by staying in one lane. When drivers start weaving through the lanes, they become jampacked with too many people fighting for too few spots. That leads to traffic jams forming, and everything moves MUCH slower. 

The same can be said for electric power installations in conduit. For every wire you try pulling through the system, there’s more friction, abrasion, dragging, and potential damage. Cable lubricant works in some situations, but when too many wires are moving through the conduit, you can end up with snags, harder pulls, and tension damage. 

Penalties and Fines 

Running more than the accepted number of wires through a conduit system is one thing; it’s another story to try sneaking the installation past code enforcement. 

At the very least, a code enforcement officer can require you to remove the wires you installed and reinstall them correctly. This could mean using fewer wires to fit within the NEC guidelines or adding another conduit for the other cables. In either case, the time, material, and labor costs far outweigh the perceived results. 

In other cases, a codes official may issue a fine or penalty, depending on the severity. By adhering to NEC guidelines, you can avoid time delays, fines, and other costs associated with redoing the installation correctly. 

What Counts Toward Fill Capacity? 

It might seem obvious, but when measuring for fill capacity, every wire must be included. 

Sometimes NEC codes are misapplied to situations, resulting in wrong data and potentially incorrect installations. One common problem is workers will apply Article 310 to figure out fill capacity when it applies to ampacity. 

Ampacity excludes bonding and ground wires from its calculations, resulting in the wrong fill rate. When tray cable and conduit are involved, every insulated and uninsulated conductor should be factored into the equation to prevent overfilling. 

Gauges and Stranding 

It takes far fewer large marbles to fill a jar than small ones. The same thing happens when trying to run tray cable through conduit. 

Because capacities are based on the area filled by the wire and not the number of wires, it takes fewer large gauge wires to reach the same conduit capacity as smaller ones. Fill capacities also vary based on the type of conduit used. The more flexible the conduit is, the less it takes to fill it. The same can be said for thicker- versus thinner-walled tubes with the same nominal outer diameter. 

Strand types, whether it be Class B, C, etc. are decided by the engineer and codes. It seems like a small detail, but the air void left behind increases the wire’s overall outer diameter (OD). 

Standard round concentric stranding leaves space between the individual strands, making it larger than compressed or compact options. For reference, compressed stranding is about 97% as large as round. Compact stranding, which removes the air pockets from the insulated conductor, is only 90% the size of a standard round configuration. 

Improving Your Conduit Experience 

The NEC codebook is an electrician’s best friend, but don’t be afraid to consult other sources, including local code enforcement, to ensure your next install is compliant. 

Understanding fill capacities simplifies and streamlines installations by preventing damage during and after the process. When done correctly, tray cable installs move faster, result in little (if any) damage during pulling, and can safely dissipate heat long after the project is done.

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