Wiring Lower Volt Systems
Not long-ago systems having 26 volts and below were exempt from the National Electric Code. They do not generate enough electrical current to do harm to humans considering the body’s resistance to current flow. Higher voltages do. The NEC has significantly changed opinions of lower volt circuits since then.
Automobiles are not in the code unless having a 120v alternating current or larger generator on them. Vehicles differ from normal wiring situations. Rubber tires prevent current from going to ground. Best to stay in cars made of metal during lightning storms. Earth is where lightning wants to go.
Cars and trucks can be used as examples for low volt systems. Most auto batteries are 12 volts and do little harm to us when also touching the metal chassis but can deliver more than 300 hundred amps when starting fuel engines. These batteries make large sparks when metal is placed between terminals. When changing batteries, the cable should be taken off the negative terminal first since a conductive metal wrench touching the vehicle from there makes no difference, that terminal being connected to the frame anyway. The wrench can now safely be used on the positive terminal because the return circuit to the battery from the frame has been removed from the negative terminal.
Small 1.5v household batteries connected (+) to (-) add up to make 12 volts dc or more and can be handled safely. A metal object touching them end to end creates a direct short but makes little spark. Because small batteries only produce so much energy they are referred to as power limited.
Many Types of Low Voltage Circuits
The index in the NEC makes clear definition for low voltage circuits as being an electromotive force 24 volts nominal or less found in Article 551 for to Recreational Vehicles & Parks. Some direction for installing low voltage wiring is there, but for the most part Article 720 must be referenced. This article pertains to Circuits and Equipment Operating at Less Than 50 Volts and requires conductors not to be smaller than 12 AWG copper or equivalent. Makes little sense for most lower volt installations until 720.2 is considered referencing exceptions in Other Articles.
Moderately complex when and how exceptions apply are discussed in Other Articles for Low Voltage Lighting, Health Care Facilities, Park Trailers, Floating Buildings, Pipe Organs, Electroplating, Solar Photovoltaic (PV) Systems, Remote-Control, Signaling, and Power-Limited Circuits, and Power-Limited Fire Alarm (PLFA) Circuits. Where smaller wires are allowed for home furnaces and air conditioning thermostats, doorbells and so forth is in Article 725 for Class 1, Class 2, and Class 3 Remote-Control, Signaling, and Power-Limited Circuits. Although Class 1 reduced power limited circuits are somewhat restricted, those for Class 2 and Class 3 are not and require either the device producing or converting electricity, as small batteries or transformers, be power limited or fused accordingly. Tables 11 (A) & (B) found in Chapter 9 can be used to calculate equipment power source and fuse ratings, but it’s easier when marked on equipment by manufacturers approved by United Laboratories (UL listed). Some Canadian standards apply.
Hazardous (Classified) Areas I, II & III
Concerning electricity, hazardous areas are where high concentrations of gases & vapors, combustible dust, or ignitable fibers or flying’s are or can be present in the atmosphere or on light fixtures and other equipment with hot surfaces. These substances result in explosions due to electrical sparks and fall into Classes, different from classes used for power limited circuits by using Roman numerals.
Hazardous Classes are further subdivided by Divisions depending on how great concentrations are likely to be and extent electrical material and equipment must be installed in them to ensure safety and prohibit damage. Prevalent in European models, Zones may be used instead of Divisions. Groups further determine concentrations needed for ignition of different types of gases, dusts or fibers. Industrial facilities, such as natural gas wells and refinement sites, determine boundaries between Divisions or Zones. Corporations are responsible if damage results if their boundaries don’t encompass the entire threat. Divisions are determined in Commercial services, like gas stations, later in Chapter 500.
Power limitations on lower voltage equipment and circuits in hazardous locations are more critical because tiny sparks can ignite explosions and fires. For this lower voltage equipment and wiring to be exempt from costly manufacturing and installation, they must also be Intrinsically Safe Systems with lower power limitations addressed in Article 504.
Optical Fiber Cables
Optical cables transmit light comparable to electricity through wires but any similarities stop there. Used for their own sake to illuminate medical operations or light up artificial Christmas trees, optic cables are usually manufactured for communication and data.
Electricity is transfer of electron energy in molecules present in tangible matter. Light is made of photons traveling faster in absence of molecules. It goes 186,000 miles per second in outer space. Light is immune to electromagnetic interference generated by electricity making optical cables efficient when placed beside conductive wires or themselves.
Optics includes laws of refraction and total internal reflection. Light is trapped in denser material surrounded by less provided there is a small angle of contact. For light to completely refract, this angle must be less than the critical angle determined by differences in mass. Diffusion causes irregularities from other than a completely smooth boundary. Optical cables can be made of many fibers the size of threads, thin enough to prevent light waves making contact from the inner core made of highly transparent flexible glass to surrounding cladding more than critical. Much data can be imposed on a single wave of light generated by lasers. Information is increased by many fibers and frequencies.
So why are optical fiber cables addressed in Chapter 7, Special Conditions by the National Electric Code since light has no known voltage? Transmitters and receivers interpreting this light do and cables are likely installed by electricians. Fundamentally there are two types of optic cables, nonconductive and conductive, the latter using metal jackets and wires for protection and strength to pull them in and hang in vertical risers. Hybrid cables have both electric wires and fibers. Optic cables can occupy the same conduits and cable trays as electric cables. Metal wires in optic cables need to be bonded and grounded. Types of cables are listed for various installations in Table 770.154(a) and (b). Another consideration is when they are installed in ducts used for environmental air, especially when used for heating.
Although no specific reference is made to family dwellings, optical cables are further discussed by Premise-Powered Broadband Communication Systems in Chapter 8.
Communication Systems
Residential and most commercial communication cables are low volt but not taken lightly by the NEC. In fact, Chapter 8 pertaining to them is the last chapter preceding Tables and Informative Annexes and is completely devoted to communication. This chapter is divided into five Articles depending how communication, TV, internet, etc. are provided. The NEC does not cover wireless devices regulated by the FCC.
Article 805 addresses Communication circuits as telephone land lines and is not all that complicated. Article 810 is for Radio and Television Equipment. It helps to understand how antennas work.
Antennas receive electromagnetic waves transmitted through air and are not to be mistaken for electromotive force (EMF) commonly known as voltage. Electromagnetic fields are generated by alternating current transferring energy from wires when coiled around an iron core in electromagnets and transformers. Energy also transfers between wires run next to each other. Electromagnetic waves are transmitted from ground towers and satellites outside earth’s atmosphere. Outer space is considered a vacuum.
This technology was used during WWI to receive Morse code in battlefields. Additional power was not needed for amplification by crystal radios. Electric flow results in voltage through resistive devises and small headphones were needed because voltage derived was small.
Antennas have become more complex collecting stronger electromagnetic waves of certain frequencies transmitted from a particular direction. Energy from batteries or a larger source is still required for amplification needed by radio and television power consumption. Further advancements allow large, circular antennas mounted on poles in the yard to receive satellite transmission by TV providers. Antennas are now smaller since electromagnetic fields are significantly stronger and more focused from satellites. Coaxial cable from antennas to receivers provide better results than wires not being wrapped in a jacket made of meshed wires or foil. Conductive jackets drain off outside wave interference when grounded from overhead transmission lines and higher volt wires providing power to devises using more current. Larger signal wires provide better reception, but due to ease of installation RG-6 Coax is commonly used.
Community Antenna Television and Radio Distribution (CATV) Systems are different in that electromagnetic waves are received by taller antennas and processed by receiving stations before distributed with overhead or buried cables to consumers. At first analog signals received from antennas were sent through coaxial cables but modern television technologies rely more on digital inputs converted by receiving stations. Hybrid cables incorporating optical fibers and electric signal wires are used as well by companies.
Powered Broadband Communication Systems aren’t so much different from CATV Systems except they’re more digital and deliver additional information and data with many frequencies. Network-Powered Broadband mostly rely on conventional electric cables. HDTV cables in buildings improve operation of devises. Premise-Powered Broadband Systems depend more on optical fibers.
Articles in Chapter 8 are divided into similar Parts. Cables are listed from Tables depending on installation, must be properly grounded, and determine insulation ratings for voltage and issues such as restricting fires from spreading rapidly in buildings.
Eberling@www.thndrsns.com
