Douglas Krantz - Technical Writer - Describing How It Works

Why Perform Voltage Drop Calculations on Each Circuit?

By Douglas Krantz | Descriptions

The load only gets the voltage left over after the power supply's voltage is dropped or lost into the wires of the circuit

By Douglas Krantz

Stress Test Tool

Voltage Drop Calculations are a confirmation tool. Once the circuit has been designed, the calculations stress-test the design to see that the circuit is going to perform: that it will carry power all the way from the power supply to the last device on the circuit.
  • The resistance of the wire, based on its total length and its size or gauge, is different for each circuit

  • The current used to operate the device(s), based on the voltage reaching the device(s), is different for each circuit

  • The voltage lost (dropped) into the wires of the circuit, based on the resistance of the wire in the circuit, multiplied by the current used in the circuit, is different for each circuit

  • The voltage reaching the last device on the circuit, based on the voltage of the power supply minus the voltage lost in the circuit, is different for each circuit

In other words, the design of each circuit has to be confirmed with a Voltage Drop Calculation before the installation of a single wire.

Whether it's a Notification Appliance Circuit (NAC), door holder circuit, power carrying circuit, unlocking doors or opening roof hatches "on-fire-alarm" circuit, and any other power carrying circuit.

Electrical engineers do it, so should anyone else tasked with laying out a circuit. This includes those making submittals for fire alarm circuits, and includes even the installers when the design hasn't been stress-test Power Drop Calculated for a power carrying circuit.

The examples shown below are real instances, and the required repairs came after the new building was finished. Some of the poor installation practices weren't even discovered and dealt with until a year after occupancy.

Example One: A temporary six-classroom building was constructed. The electrician installing the horns and strobes asked his supervisor about what size wire should be used. The supervisor said to use 16 AWG wires. The electrician then used 16 AWG wires.

16 AWG wires were not adequate to carry the power all the way through all six classrooms, plus the hall.

Fortunately, the NAC power supply was set up for Class B circuits, which was all that was needed for the devices, but the electrician wired the NAC circuit as if it was a Class A circuit. Because the NAC circuit was basically wired with both ends of the circuit inside the box for the NAC power supply, the circuit could be split, with half the devices on each half-length Class B circuit.

Example Two: Roof-top smoke exhaust hatches were installed. The roof hatch people said that they needed an active 24 volts to open the latches. The fire alarm installer didn't ask for the Installation Sheet or Installation Manual that came with the hatches because 24 volts is 24 volts, right?

What the roof hatch Installation Manual said, and what the fire alarm installer failed to ask about, is the current needed for the activation of the roof hatch. At issue was that the roof hatches had thermal releases, which required 2 amps at 24 volts.

There was no room for the power supply to be mounted within 100 feet (30 meters) from the roof hatch. That's a long distance to run the 2 amps of current.

A couple of years after the installation, when investigating intermittent roof-hatch activations, the issue was found. The installed wires between the power supply and the roof hatch were the smallest size allowed, so the measured voltage at the roof hatch was only 19 volts.

Eventually, to solve the voltage drop problem in the wires, and to solve a couple of other problems with the roof hatches, the roof hatches themselves were replaced with a different brand of roof hatches.

Example Three: Door holders were installed in a three-wing, four-floor condominium building. There were more than 20 door holders on the single circuit that was connected to the power supply. The power supply could easily handle the current from just 20 door holders.

However, because of the long distance for the door holder circuit, and the number of door holders on the single circuit coming from the power supply, some of the door holders at the ends of the wings didn't receive enough voltage; they just barely held the doors open.

Well, the building was complete at the time the problem was discovered, so the building owners had to live with weak door holders.

These real-world examples show that problems arise when, before installing wires, the stress-test Voltage Drop Calculations are not used to confirm the design.

Anytime wires are going to be used to carry power, perform voltage drop calculations . . . before any wires are installed.
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