Power limited means that the maximum possible voltage is low enough that it won't electrocute anyone, and the maximum possible current is low enough that even when the wires shorted, they won't heat up and burn down buildings.
The wires may be rated, but it's the power supply that prevents anything bad from happening.
Testing laboratories, like UL, ULC, CE, CCC, DE, FM, etc., test the wire and insulation for safety, and if the testing shows it is safe, the testing laboratory puts the wire on its list of wire that is OK to use; the wire is "Listed". The power supplies needs to be tested and listed, also.
The NEC (National Electrical Code or NFPA 70) defines "Class II" and "Class III" as power limited.
Is Power Limited Cable the same as Other Cables?
The American Wire Gauge of a cable is the AWG of a cable, whether it's power limited, bare copper, or 5,000 volt wire. The same with metric sized wire. 0.75 mm2 wire is the same whether it's power limited, bare copper, or 5,000 volt wire.
The AWG or mm2 number doesn't show the insulation, the AWG or mm2 number shows the cross section of the copper wire.
On the other hand, the type of insulation is defined by the letters after the wire size. Example:
- 18 AWG THHN means Thermoplastic High Heat-resistant Nylon coated insulation - commonly 600 volts
- 18 AWG FPL means Fire Rated Power Limited insulation - 300 volts
- 18 AWG FPLR means Fire Rated Power Limited Riser rated insulation - 300 volts
- 18 AWG FPLP means Fire Rated Power Limited Plenum rated insulation - 300 volts
What is a Power Limited Transformer
A power limited transformer cannot provide more current than what it's rated to provide. Sometimes, an internal fuse on its output is what prevents the transformer from exceeding the rated current.
Other times, the transformer is inherently limited - there isn't a fuse, but it cannot ever exceed its rated current. It is limited by the amount of iron in its iron core.
Remember, a transformer uses magnetism to transfer power from one coil of wire to the other. If the magnetism is limited, by limiting the iron in the core, the current transferred cannot be more than a certain amount of current. The transformer is inherently limited.
Is there a Power Limited Fire Alarm Battery?
The output from a listed power-limited power supply, like a fire alarm control panel, a fire alarm booster power supply, or an auxiliary power supply is limited in voltage and current. In other words, its power is limited.
On the other hand, the input to a power-limited power supply might not be power limited. For instance, the input might be utility power, or it might be a battery.
From the utility power source, the voltage could easily electrocute people, and the current could be enough to heat up wires and burn down buildings. From a battery, usually the voltage isn't enough to electrocute anyone, but the current, especially from a lead-acid, or a sealed lead-acid battery, is enough to heat up wires and burn down buildings.
By themselves, sealed lead-acid batteries used in fire alarm systems are not power limited because they are not limited in the amount of current. You won't find a "Listing" stamped on the side of the batteries saying they are "Power Limited", "Class II", or "Class III".