Douglas Krantz - Technical Writer - Describing How It Works

Why Double the Cable Length when Performing Voltage Drop Calculations?

By Douglas Krantz | Descriptions

A circuit is a circle. After passing through the load, electrons have to be returned to the power supply so the power supply can use the electrons again.



Greetings Douglas,

I would really appreciate if you could help me with some explanation regarding the following:

I have been searching the internet for a while with no success for an explanation of why the cable run is doubled (after doubling the first run) when calculating the voltage drop in a Class A NAC circuit, considering that this will increase the voltage drop.

Rather, it would seem that the current draw should be divided by coming from both terminals of the NAC, which should cause the current to drop and therefore the voltage drop to decrease.

Thank You, HP

A Circuit is a Circle

All electrical and electronic circuits are circles. In a circuit, electrons are what actually move. Electrons, also, are very durable; electrons are extremely difficult to create or destroy. In other words, in a circuit, the electrons have to be used over and over again.

The electrons:
  • Come out of the negative terminal of the power source (like a battery or a NAC circuit from a panel)

  • Travel toward the load in one of the wires of the circuit (feeding the electrons to the load)

  • Pass through the load of a circuit (like a resistor, a motor, or a fire horn)

  • Return from the load in the other wire of the circuit

  • Go into the positive terminal of the power source

  • Pass through the power source to be reused and returned to the circuit from the negative terminal

It's the reusing of the electrons that's important in the circuit. If the flow of the electrons are interrupted, like what a switch does, the electron flow in the entire circuit is stopped.

The Notification Appliance Circuit (NAC) is a Circle

The Notification Appliance Circuit (NAC) is a Circle

The cable on a fire alarm NAC circuit has two individual wires: the feed wire and the return wire. The electrons themselves pass through one of the wires on the way to the fire horn, and then through the other wire when they return to the panel. Alternatively, think of the two wires as being in series - double the resistance of a single wire in the cable, because the electrons have to go through both wires, one wire at a time.

That's the true length of the wires inside the jacket of the cable. It's the there-and-back distance that the electrons travel through in the two-wire cable for the NAC circuit.

NEC Resistance Charts

The National Electrical Code (NEC) shows the resistance of a single wire. They leave it up to you to figure out the total length of the two wires in the circuit.

Douglas Krantz
Life Safety
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