20 Numbering System

Control circuits can range from the very simple to the extremely complex. Ladder diagrams show how a system works and are used for troubleshooting purposes. Wiring diagrams show where equipment is and how it is connected.

Being able to convert from one style of diagram to the other is an essential skill for electricians and people working with control circuits. To accomplish this, we use a simple numbering system.

The numbering system is a way of identifying and “naming” each electrically common point in a circuit. A wire is considered “electrically common” with another wire if they share a direct electrical connection with no switches or loads between them.

Consider the circuit below:

A three-wire numbered circuit

It is a simple three-wire circuit with single stop and start pushbuttons. Single-phase power is supplied from Lines 1 and 2 and a motor coil is the load.

Starting at Line 1, we label the first wire “1.” When we jump through the first device, (the normally closed stop button) we jump up to the next number. Wires 1 and 2 are not electrically common. Coming out of the stop button with wire number 2, we are going to two different places. The first is the normally open start pushbutton, the other is the normally open instantaneous contact. Since there are no loads or devices between the load side of the stop button, the line side of the start button, and the holding contact, these points are considered electrically common and can all share the same number. Jumping through these devices, wire number 3 is connected to the load side of both normally open devices and the motor starter itself, and so these three points are also electrically common.

Once the control circuit has been numbered, we can use that information to help complete wiring diagrams or determine the number of conductors needed in conduits for installation purposes.

In summary:

  • L1 and L2 (or L1 and N) are used to designate the control circuit power.
  • Assign number 1 to the first wire entering the control circuit.
  • Trace through the schematic diagram increasing the numbers given to wires each time you pass through a device.
  • Give the same number to all wires that are electrically common, spliced, or connected to the same terminal.



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Basic Motor Control Copyright © 2020 by Aaron Lee and Chad Flinn is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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