By Douglas Krantz
It's the power supply, not the wiring, which determines whether it is a Class I, II, or III wiring system.
Class I goes to wall plugs, lighting systems, HVAC motors, industrial power systems, etc. The electricity comes directly from the power station. Able to supply enough voltage or current to electrocute or start a fire, power from Class I power supplies is not limited.
Class II and Class III is "low voltage" wiring commonly used to furnish power to fire alarm systems, security systems, overhead paging systems, computer networking, etc. The electricity has a buffer power supply the stands between the Class I wiring system and the Class II or Class II wiring system. Not supplying enough voltage or current to electrocute or start a fire, power from Class II and Class III power supplies is limited.
Wiring shortcuts in Class I wiring is a bad thing - there are no limits to the electrical power. Electrical shorts at the end of the line can heat up the wire causing a fire, and electrical contact can easily electrocute a person.
On the other hand, when the power is limited, wiring isn't as critical. Smaller diameter wire, less insulation, and less protected routes through a building aren't dangerous when power is limited.
It's the cross-powering through wire with damaged insulation that's dangerous. Power limited wiring should never come in contact with non-power limited wiring because once in a while the insulation breaks down.
Even inside electrical boxes and conduit, insulation breakdown can happen. No matter how good the insulation looks on a new installation, after a period time, if the wires contact each other, there is a potential for the wires to cross power with each other.
The power limited wiring, if there is this cross-powering, will then receive power from a non-power limited source. Not good.
To prevent this cross-powering between the classes of wiring:
- The wires can be permanently separated in such a way that even momentarily they cannot come together
- There can be an insulated barrier, like the outer sheath of Romex, the outer sheath of an extension cord, or plastic or grounded metal conduit around the Class I wires
- There can be a grounded metal barrier preventing the power limited wiring from possibly coming into contact with the power un-limited wires
So no contact between power limited and non-power limited systems can happen, now or in the future, keep wiring systems separated when routing wires.