Douglas Krantz - Technical Writer - Describing How It Works

Wiring - Is It Power Limited or Non-Power Limited?

By itself, wiring does not show if a circuit is power limited or non-power limited. If the power source is power limited, the circuit is power limited; if the power source isn't limited, the circuit is non-power Limited. The wiring has to be good enough for the type of circuit.

The question is whether it should be power limited wiring or non-power limited wiring.
Does the wiring practice determine the power supply, or does the power supply determe the wiring practice?

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: 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.
Life Safety
This website uses cookies. See Privacy for details.
Fire Alarm Q&A Articles

No Charge - Unsubscribe Anytime

Make It Work Series of Books by Douglas Krantz
Free - Click and Download
Make It Work Series of Books by Douglas Krantz
Make It Work Series of Books by Douglas Krantz