Duct the return-air in a zone. You can bring big changes. Make mine non-plenum, please.

Cabling choices are abundant; there are so many options these days that it is almost impossible to list them without leaving a few out. Throughout the years, the variations have continued to grow, and vendors even have their own “lines” of cable depending upon your preference.

There are some things, however, that remain constant when it comes to choosing the right cable for the job. You can usually start by making the decision whether you need to install CMP plenum or non-plenum cable. Almost universally in the United States and, due to National Electrical Code (NEC) requirements, this choice is based on the cable construction and materials used on the insulation and jacket of the cable. So after you choose fiber optic or copper UTP or STP, Category 5e or Category 6 cabling, you still have another issue to address: Code compliance. It is important because plenum and non-plenum are so different.

The term, “plenum cable,” is an abbreviation for the term, “return-air plenum space cabling.” The history of the return air plenum design evolved from the introduction of air conditioning into modern buildings. Prior to central air conditioning, most buildings did not have raised floors or drop ceilings, and communications was an afterthought in the design. As designers tackled the challenges of providing air conditioning in the new buildings, two design choices developed. First, the cooled or conditioned air needed to be delivered to the request area by duct, and the exhaust air could be returned and recycled by duct or by flowing through the return air plenum space (usually above a drop ceiling). Many designers opted for the design of using the return-air plenum spaces above a drop ceiling or below a raised floors. The new building design favored the drop ceiling design because of costs. There was no need to duct the return air back to the air handler.

The NEC required all communication cabling to be placed in steel conduit. The telecos that owned the cable required the customer to install the conduit in order to meet the Code and get the service. After the Carterfone Decision (1968), customers could buy their own telephone systems and cabling. Shortly after the customer-provided equipment and cabling, the National Fire Protection Association (NFPA), with the help of the chemical industry, introduced the concept of cabling without the need of conduit. The cost savings were huge (averaging over 75 percent less than cable and conduit) in return-air plenum spaces. Very quickly, building designers accepted the new NEC for the United States with the provision for return-air plenum cable (tested for low smoke generation and reduced flame spread). The use of return-air plenum without the ducted return-air flow became almost universal in the United States, and the demand for return-air plenum approved cable (tested by UL or ETL for compliance with NFPA 262 burn testing and CMP rated and listed) was significant.

Demand swelled beyond supply and prices soared while availability shrank. Competitors to DuPont’s Teflon FEP sprang up, and CMP cable prices fell as supply due to competition increased.

The CMP cable was challenged over the next three decades to deliver higher performance and meet expanding standards. The transmission performance and code compliance of FEP insulating material placed the Cat 5e and Cat 6 UTP and STP cable in a superior market position again. The only two manufacturers of FEP (Daikin and DuPont) are struggling to meet demand, and naturally, prices have soared again. The standards were enhanced, but the NEC CMP code remained unchanged. The CMP-approved cables are not, and never have been, tested for toxic gas generation. However, numerous independent tests of CMP cables have revealed a dirty secret. NEC-approved CMP cables may outgas highly toxic materials during heat decomposition. Some of the gases released have been rated the same as phosgene nerve gas (a chemical warfare weapon) by the U.S. government.

Late last year, Dyneon announced their exit from FEP production. Shortly after that, DuPont and Daikin began a series of significant price increases. I see no end in sight for the climbing prices of CMP cables in the United States.

Sealed air systems mean new concerns
Plenum spaces allow fire and smoke (and gases) to travel quickly. Today, we have the technology to test the airflow for smoke and gases, but the current codes require only smoke detectors. The modern building is like a fish bowl. Windows do not open, and the air is recycled without addressing toxicity and harmful materials. Currently, many cables are made and sold in the United States that are not compliant with the reduction of hazardous substances (RoHS) requirements because it is (like the toxicity test of CMP cables) not required.

What if the building design was altered to put the return airflow in ducts? It can be done, but you must look at these scenarios on a case-by-case basis. Cost and safety are issues to be considered. The cost of adding the ductwork needs to be compared to cost of non-plenum cable. The non-plenum cable is usually made of polyethylene or polyolefin insulating materials and PVC jackets. If the consumer specifies RoHS compliant cable, it is generally available. Some cable market veterans have proposed replacing the current NEC requirements for CMP cable with the European Union code LSZH RoHS compliant code. Totally eliminating CMP cable is not an option due to the almost 18 million miles of cable already installed in the United States. Changing the airflow may be much cheaper in the grand scheme, and it promotes a higher level of safety for the building’s occupants.

To summarize, plenum-rated cable gets its name from an HVAC term: plenum spaces. The plenum spaces include those that lie between a drop and standard ceiling (or a similar version in the floor space), and it is this section where the air in a building circulates, thus aiding in heating and cooling functions. When no conduit is used, CMP plenum cable is often required to comply with the building and electrical code or you may use conduit and install non-plenum cable.

The alternative
Non-plenum cable is the less expensive of the two choices. You are likely to see many projects stipulating non-plenum to save on costs, since, more often than not, the decision on the type of cable construction is made by the building owner or end user based upon their code circumstances.

In projects where CMP plenum is not required, non-plenum is nearly ubiquitous. It is not uncommon for non-plenum cable to be more than one-third less expensive than the cost of the CMP plenum variety. In the case of a fire, I have found that CMP plenum cable outgases highly corrosive materials. Any electronics exposed to gases like hydrogen fluoride (HF) may be catastrophically damaged. FEP (a halogen material) in CMP cables can decompose and release up to 49 percent of material as HF gases. HF, when exposed to moisture (even as little as humidity), converts to hydrofluoric acid. Hydrofluoric acid can even dissolve glass.

Check your insurance coverage for HF damages. You may not be covered. As more and more businesses are becoming burdened with higher insurance premiums, some may choose to go ahead and spend the extra money upfront for ducts and non-plenum cables in case of fire. By taking such fire safety precautions, companies can lower their liability, since non-plenum cable may causes less overall physical and secondary damage.

Because non-plenum cable is considered to be less toxic, it is used in locations such as schools and hospitals. Because of the number of people generally contained within these facilities, the potentially toxic gas emissions from the CMP cable in a fire could prove to be deadly. The return-air ducts would make it safer and easier to manage the smoke and gases in an emergency.

Know the difference
Perhaps the most important thing to remember, from a contractor’s standpoint, is that the biggest difference between the two is cost. Therefore, it is essential to always remember two things.

First, make sure the project in question truly does require the use of CMP plenum cable or conduit. If it is not necessary, then explain that to the customer, the non-plenum cable design may reduce costs now and in the future. This design may offer safety factors not previously considered.

Second, when initiating an estimate for a project, always check and recheck with the supplier to make sure whether the pricing received is for CMP plenum or non-plenum cable. This can save a lot of trouble and money.

In the end, it is the client’s choice. Some are bound by factors that they cannot control and their cabling choices may not be their own. As a contractor, you need to understand that, as with most things in business, cost is usually a primary deal maker or deal breaker, and the plenum/non-plenum decision is definitely one that falls into the categories of costs and safety. Code compliance is not debatable.

In the United States, most states and cities have adopted the NEC. The NEC is not perfect. It’s more like a work-in-progress, but it is still leaps and bounds ahead of many other building requirements, except possibly the European Union. However, you can utilize a design variation within the Code to the best advantage of your client. Changing the design from CMP plenum to non-plenum cable requirements may deliver some major benefits in safety and savings.

BISBEE is with Communication Planning Corp., a telecom and datacom design/build firm. He provides a free monthly summary of industry news on www.wireville.com.