Reading Motor Control Diagrams: IEC 60617 Symbols vs NEMA, IEC 81346 Reference Designations, Terminal Numbers, Wire Numbers and Cross-References

Power circuit and control circuit Basic

The power circuit (main circuit) is drawn as three lines L1 L2 L3 running down from the top through the breaker, contactor and overload relay to the motor; you read it top to bottom, following the flow of energy. The control circuit is drawn between an upper rail (L or +24 V) and a lower rail (N or 0 V). Each vertical branch is called a “path” or “current path” and has a column number at the top. Symbols belonging to the same device (e.g. coil -K1 and contact -K1 13-14) may sit in different columns or on different pages; they are tied together by the device designation.

What an IEC 60204-1 control schematic contains

• Column numbers 1 2 3 … above the L1 rail locate every device, with a function text (e.g. “hold-in”) under the N rail
• The box under a coil is the contact cross-reference (contact mirror): its NO / NC rows give the column of each contact of that coil; “PWR” = the power poles sit in the power circuit
• Coils on the N rail with all contacts on the L side: an earth fault between a contact and the coil cannot pick the coil up (IEC 60204-1 clause 9.4.3)
• Fuse -F3 protects the control circuit · overload contact -F2 95-96 (NC) is in series with the coil · 97-98 (NO) signals the trip
• IEC 60204-1 lamp codes GN green = normal/running · YE yellow = abnormal (trip) · RD red = danger/emergency · BU blue = action required · WH white = general status
• Power circuit: green-yellow dash-dot = PE conductor bonded to the motor frame · short dashes between poles = mechanical coupling · the overload is a bimetal element per phase linked to contact 95-96

The single-line (one-line) diagram Intermediate

A single-line diagram is the “map” of an electrical system: one line stands for all three phases (three oblique strokes show three conductors) and the sheet shows sources, transformers, busbars, feeders, protective devices, starters and motors with their ratings — but no control circuits, terminal numbers or internal wiring. It is used to check protection coordination, calculate fault levels and plan extensions.

Read it top to bottom

1. Source and transformer — voltage, frequency, rating (kVA), uk %, vector group (Dyn11) and neutral earthing (TN-S)
2. Main breaker (ACB) — rated current and breaking capacity Icu, which must exceed the prospective fault current at the bus
3. CT and meters — the circle around the line is the current transformer feeding an ammeter or power meter
4. Busbar — the thick line: 3N~ 400/230 V · every feeder branches from it
5. Motor feeders — breaker/MPCB → starter (contactor + overload, soft starter or VFD) → cable (size, cores, shield) → motor (kW, current)

What a good single-line diagram must show

• Voltage, frequency, system earthing (TN-S, TN-C-S, TT, IT) and the fault level at the bus (kA)
• Type, rating and Icu of every breaker · fuses · protection-relay settings
• Cable size, cores, material and length (for voltage drop and fault checks)
• Motors: kW or hp, rated current, starting method and device codes that match the control schematic (-Q1 -K1 -F2 -M1 or DISC M OL MTR)
• The symbol standard used (IEC 60617/81346 or ANSI/IEEE 315/NEMA), stated in the drawing legend

IEC 60617 symbols Basic

Drawing rules you must know

• Contacts are drawn in the “de-energised, not actuated” state — a limit switch that is normally held operated carries a note
• An NO contact is drawn with its blade tilted to the left; an NC contact has a short bar across at the contact point
• The half-circle on a timer contact shows the direction of the delay (like a parachute)
• A solid dot (●) is a junction; lines that cross without a dot are not connected
• A fuse is a rectangle with the conductor running through it · a contactor pole has a small bow at the contact · a circuit-breaker has a cross at the blade tip · an isolator has a bar across the blade tip
• Dashed lines are mechanical links between poles or devices — the three poles of a contactor, a breaker’s trip unit, or the mechanical interlock between coils -K1 and -K2

Device designations and terminal numbers Intermediate

A terminal number is always read together with the designation: “-K1:A1” is the coil terminal of contactor K1, “-X1:5” is terminal 5 of terminal strip X1. Wire numbers on drawings are often based on page and column number (e.g. wire 2.4 is a wire on page 2, column 4), which makes it practical to trace wires inside the panel.

Cross-reference tables

Under each coil on an IEC drawing there is usually a small table showing where its contacts are, for example

-K1
 1-2   │ 3.2      ← main contact, page 3 column 2
 13-14 │ 4.3      ← NO used for self-holding, page 4 column 3
 21-22 │ 4.6      ← NC used for interlocking, page 4 column 6
 43-44 │ 4.8      ← NO for the run lamp

Reading a control circuit as logic Intermediate

Current reaches a coil when there is at least one path on which every contact is closed, so the circuit can be written as Boolean algebra. This is the foundation of ladder programming in a PLC.

American (NEMA/JIC) and Japanese drawings Intermediate

In North America motor drawings follow NEMA ICS 19 (designations and symbols), NEMA ICS 2 (contactors, starters and NEMA sizes), NFPA 79 / JIC (electrical equipment of industrial machinery) and the NEC (installation); symbols come from ANSI Y32.2 / IEEE 315. The sheet below is the symbol set used in this series’ ladder diagrams.

Reading a NEMA ladder

• Read left to right: buttons and contacts on the left, loads on the right — coil M sits at the far right with OL, next to the grounded rail X2 (the same idea as IEC’s “coil on N”)
• Rung numbers outside rail X1 · wire numbers on the lines: one number per electrical node
• Numbers under a coil list the rungs that carry its contacts — underlined = NC, e.g. “1 2 3” = NO on rungs 1 and 2, NC on rung 3
• Contacts carry no terminal numbers such as 13-14: they are identified by device name (M, CR1, TR) · timer contacts state their type NOTC / NCTO (on-delay) or NOTO / NCTC (off-delay)
• Full examples with the power circuit: direct on line · reversing · wye–delta

IEC ↔ NEMA device codes

NEMA starter sizes (ICS 2)

The hp figures are maxima for full-voltage, non-plugging duty — check the manufacturer’s catalogue and NEMA ICS 2 before selecting. A NEMA size is chosen from the hp, not the measured current, whereas an IEC contactor is chosen from current and utilisation category AC-3.

Current Japanese drawings use JIS C 0617, which matches IEC 60617, but older machines still carry the former symbols. Ladder diagrams in every brand of PLC are drawn horizontally, much like NEMA.

Reading a drawing to find a fault

1. Start at the motor/load with the problem and find its device designation in the power circuit (e.g. -K3)
2. Find coil -K3 in the control circuit, then work down from the L rail one device at a time
3. Measure the voltage to the N rail point by point; the first point with “no voltage” lies after the device that is open
4. Use the cross-reference table to see which device the open contact (e.g. -K2 21-22) belongs to, and follow it from there