Douglas Krantz - Technical Writer - Describing How It Works

What are the Power Supply Voltages used for Voltage Drop Calculations?

By Douglas Krantz | Descriptions

When performing voltage drop calculations, there is the voltage produced by the power supply, the voltage that is dropped into the resistance of the circuit, and the leftover voltage that finally arrives at the load.

By Douglas Krantz

Power Supply Types

The first concern when calculating the voltage drop for a Notification Appliance Circuit (NAC), a door holder circuit, or any other power carrying circuit in a building's fire alarm system is the power producing ability (both current and voltage) of a power supply. However, a power supply isn't just a power supply; all power supplies have their own quirks. In other words, they're all very different.

The Installation Manuals that come in the same box as the power supplies are extremely important. However, the specifications fall short of giving a complete description of how the power supply works, and how the specifications fully affect the design of the installation.

Real-Life Current Ability for a Power Supply

Look carefully at the specifications for the power supply. The maximum current consumption for each NAC circuit output or auxiliary power output will be shown.

Also, look in the specifications for the maximum shared current value for the entire power supply. The current consumption for all the NAC outputs of the power supply is often shared with the current consumption of other outputs, like auxiliary power, resettable auxiliary power (sometimes called Smoke Power), etc.

The accumulated current on all of the outputs will affect how many circuits can be loaded onto a single power supply. Never exceed any of the power supply's specifications, the power supply won't work when any of the specifications are exceeded.

To find out the current capability of a power supply, read the specifications in its Installation Manual and also in the specification's footnotes. If it's hard to figure out, call technical support; that's what technical support is there for.

Real-Life NAC Power Supply Voltages

Real-Life Voltages vary greatly between a Regulated Power Supply, a Nominal Power Supply, and a Rectified Power Supply. All three types are commonly made right now. With all the different power supplies, I only know of a single company that produces a 29-volt regulated power supply, I don't know of any fire alarm manufacturing company that makes a regulated 24-volt regulated power supply.

A 29-volt Regulated Power Supply provides a steady 29 volts DC, with no ripple, and at all times, The voltage does not vary between daytime and nighttime.

The output voltage of a regulated power supply stays at the same 29 volts, even during a utility power blackout.

A 29 volt regulated power supply will produce a constant 29 volts.

When determining the current consumption of the horns, strobes, or other devices on the circuit, the STEADY DC voltages are what should be looked at.

To find out the voltage that should be used for calculations, read the specifications in its Installation Manual and also in the specification's footnotes. If it's hard to figure out, again, call technical support; that's what technical support is there for.

A 24-volt Nominal Power Supply provides ~27.5 volts DC, without much ripple, and during normal times (when there isn't a utility power blackout) the voltage does not vary between daytime and nighttime.

The utility power blackout output voltage starts out at ~27.5 volts and at the end of a 24-hour utility power blackout, drifts down to ~20 volts. The Nominal DC Voltage is the backup battery voltage.

A 24 volt nominal power supply will provide a voltage somewhere in a range of 20 volts to 27.5 volts.

To find out the voltage that should be used for calculations, read the specifications in its Installation Manual and also in the specification's footnotes. If it's hard to figure out, as I said, call technical support; that's what technical support is there for.

A 24-volt Rectified Power Supply commonly provides 24 volts DC RMS, with a 100 Hz or 120 Hz ripple voltage (twice the utility's line frequency) that constantly goes from 0 volts, to ~34 volts, and back to 0 volts. Because the utility's voltage varies as the community uses more electricity during the day, the voltage level on the output of the power supply varies as much as +/- 5% from daytime to nighttime.

A 24 volt RMS rectified power supply will produce a rippled voltage that is jumping between zero volts and 34 volts.

Commonly, in order to be battery backed-up, when there is a utility power blackout, the output voltage switches internally, from the 24 volts DC RMS Rectified, to a 24 volts DC Nominal.

A 24 volt nominal power supply will provide a voltage somewhere in a range of 20 volts to 27.5 volts.

Once the power supply switches to Nominal DC Voltage, the Nominal DC Voltage starts out at ~27.5 volts and drifts down to ~20 volts at the end of a 24-hour utility power blackout. The Nominal DC Voltage is the backup battery voltage.

When determining the current consumption for the horns, strobes, or other devices on the circuit, the RECTIFIED DC voltages of the device Installation Sheets are what should be looked at. The current consumption values for the devices of rectified DC voltages are almost always greater than the steady DC voltages.

Read the Installation Manual

To find out the voltage that should be used for calculations, read the specifications in its Installation Manual, and also in the specification's footnotes. If it's hard to figure out, really, call technical support; that's what technical support is there for.

Regulated - Nominal - Rectified

The type of power supply is shown in the Installation Manual, and these voltages will affect the results of the Voltage Drop Calculations. Knowing how the different power supplies types work, and how they affect the devices is important when performing the calculations.

Further reading about performing Voltage Drop Calculations can be found at:

Why Perform Voltage Drop Calculations on Each Circuit?


What are the Power Supply Voltages used for Voltage Drop Calculations?



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