Douglas Krantz - Technical Writer - Describing How It Works
Get the book Make It Work - Conventional Fire Alarms

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

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.
Based on his electronics training, and his understanding of Life Safety, Douglas Krantz has compiled his knowledge of Conventional Fire Alarm Systems into the book Make It Work - Conventional Fire Alarms. The book covers the basics of the Conventional Fire Alarm System, and shows how Life Safety and internal supervision affects the fire alarm system.

Douglas Krantz -- Fire Alarm Engineering Technician, Electronic Designer, Electronic Technician, Writer

Share This With Friends:

See how Class A Wiring works

Go to the Fire Alarm Description Map Page of Douglas Krantzs Technicians Corner
Go to the Residential Life Safety Map Page of Douglas Krantzs Technicians Corner
Go to the Residential Life Safety Map Page of Douglas Krantzs Technicians Corner
Go to the Fire Alarm Description Map Page of Douglas Krantzs Technicians Corner
Go to the General Electrical Map Page of Douglas Krantzs Technicians Corner
Go to the Fire Alarm Maintaining Map Page of Douglas Krantzs Technicians Corner
Go to the Fire Suppression Map Page of Douglas Krantzs Technicians Corner
Go to the Guest Writer's Guidelines of Douglas Krantzs Technicians Corner
See Trivia on Douglas Krantzs Technicians Corner
This website uses cookies. See Privacy for details.
Get the book Make It Work - Conventional Fire AlarmsGet the book Make It Work - Conventional Fire AlarmsGet the book Make It Work - Conventional Fire AlarmsGet the book Make It Work - Conventional Fire AlarmsGet the book Make It Work - Conventional Fire AlarmsGet the book Make It Work - Hunting Ground FaultsGet the book Make It Work - Hunting Ground FaultsGet the book Make It Work - Hunting Ground FaultsGet the book Make It Work - Hunting Ground FaultsGet the book Make It Work - Hunting Ground Faults
Reader's Favorite Articles

What is a Stair Pressurization Fan (SPF)?

Which Way Does Electricity Flow?

What's the Difference Between Class A and Class B?

What Makes the End of Line Resistor So Important?

What is a Flyback Diode?

What is a Fire Alarm System?

What is an RTU (Roof Top Unit)?

What Causes an Open NAC?

Learn about fire alarms, one article at a time -

Keep up on the latest article!




No Charge - Unsubscribe Anytime