Electrical Diagram Symbols
These symbols are the ones used most often on electrical ladder diagrams.
They are standard JIC (Joint Industry Council) symbols as approved and adopt-
ed by the NMTBA (National Machine Tool Builders Association). They have
been extracted from NMTBA specification EGP1-1967.
Limit Switches Selector Switches Proximity Temperature Resistors
1 2 Switches Switches
Limit Switch, N.O.
Non-Actuated
Potentiometer
Limit Switch, N.O. Temperature Switch
Held Closed Proximity Switch Normally Open
Normally Open Resistor
Limit Switch, N.C.
Selector Switch
Non-Actuated
2-Position
Temperature Switch
Normally Closed Rheostat
1 2 3
Limit Switch, N.C.
Proximity Switch
Held Open
Normally Closed
Relay Coils and
Contacts Tapped Resistor
Pressure or
Vacuum Switches H
Limit Switch
Relay or Timer Coil Heating Element
Neutral Position
Non-Actuated
Selector Switch Pressure or Vacuum
Capacitors
Relay Contact
3-Position Switch, N.O.
Normally Open
Foot Switches Capacitor, Fixed
Limit Switch
Neutral Position
Relay Contact
Actuated Pressure or Vacuum
Normally Closed
Switch, N.C.
Foot Switch Capacitor, Adjustable
Normally Open
Pushbuttons
+ –
Relay Contact, Time
Limit Switch
Delay After Coil Capacitor, Electrolytic
Maintained Position Foot Switch
Pushbutton, N.O. Energized – N.O.
Normally Closed
Toggle Switches Load Devices
Flow Switches Pushbutton, N.C. Relay Contact, Time
Toggle Switch Delay After Coil
Energized – N.C.
Load Device
Rotary Selector Flow Switch Solenoid Valve, etc.
Switches Normally Open
Pushbutton
Double Circuit Relay Contact, Time
Delay After Coil
De-energized – N.O.
Rotary Selector Switch Flow Switch
Non-Bridging Contacts Normally Closed
Three-Phase
Electric Motor
Relay Contact, Time
Liquid Level Pushbutton Delay After Coil
Mushroom Head
Alternate Symbol
Switches De-energized – N.C.
Rotary Selector Switch
A
Non-Bridging Contacts Meters
DC Electric Motor
Liquid Level Switch Pushbutton
Normally Open Maintained Contact VM
Rotary Selector Switch
Bridging Contacts Voltmeter Thermal Overload
Disconnect Switch
AM
Alternate Symbol
Liquid Level Switch Alternate Symbol
Rotary Selector Switch Ammeter
Normally Closed Disconnect Switch Thermal Overload
Bridging Contacts
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Miscellaneous
X1 H1
Diode
(Crystal Rectifier) H2
Thermocouple
Horn, Siren, etc.
AC H3
Conductors
Not Connected X2 H4
+ – Buzzer
Control Transformer
Circuit Interrupter AC
Conductors
Bridge Rectifier 10
Connected
11
Earth Ground
+ – 12
13
Battery Bell
Chassis Ground
Circuit Breaker R
Fuse Terminal Board
Pilot Light Plug & Receptacle
Device Designations
These are intended for use on diagrams in connection with the correspond-
ing symbol to amplify the information on the function of a device. Suitable suffix
numbers (1, 2, 3, 4, etc.) may be added to distinguish between several similar
devices. Similar letters (A, B, C, D, etc) may be added to distinguish between
several sets of contacts on the same device. Examples: CR1A, CR1B, CR3A, etc.
AM - Ammeter FU - Fuse RH - Rheostat
CAP - Capacitor GRD - Ground RSS - Rotary selector switch
CB - Circuit breaker LS - Limit switch S - Switch
CI -Circuit interrupter LT -Pilot light SOC - Socket
CON - Contactor M - Motor starter SOL - Solenoid
CR - Control relay MTR - Motor SS - Selector switch
CS - Cam switch PB - Pushbutton T - Transformer
CTR - Counter POT - Potentiometer TAS - Temperature activated switch
F - Forward PRS - Proximity switch TB - Terminal block
FB - Fuse block PS - Pressure switch T/C - Thermocouple
FLS - Flow switch R - Reverse TGS - Toggle switch
FS - Float switch REC - Rectifier TR - Time delay relay
FTS - Foot switch RECEP - Receptacle VM - Voltmeter
HTR - Heating element RES - Resistor VS - Vacuum switch
Electrical Formulas
To Find …
Kilowatts, single phase: I × E × PF ÷ 1000
Kilowatts, three-phase: I × E × 1.73 x PF ÷ 1000
Kilowatts direct current: I × E ÷ 1000
KVA (kilovolt amperes), single phase: I × E ÷ 1000
KVA, three phase: I × E × 1.73 ÷ 1000
HP output, single phase: I × E ×%Eff × PF ÷ 746
HP output, three phase: I × E × 1.73 ×%Eff × PF ÷ 746
HP output, direct current: I × E ×%Eff ÷ 746
Amps when HP known, single phase: [HP × 746] ÷ [E ×%Eff × PF]
Amps when HP known, three phase: [HP × 746] ÷ [1.73 × E ×%Eff × PF]
Amps when HP is known, direct current: [HP × 746] ÷ [E ×%Eff]
Amps when kW known, single phase: [kW × 1000] ÷ [E × PF]
Amps when kW known, three phase: [kW × 1000] ÷ [1.73 × E × PF]
Amps when kW is known, direct current: [kW × 1000] ÷ E
Amps when KVA is known, single phase: [KVA × 1000] ÷ E
Amps when KVA is known, three phase: [KVA × 1000] ÷ [1.73 × E]
Where E = volts, I = amperes,%Eff =%efficiency, PF = power factor.
OHM’s Law
I = E ÷ R or E = I × R or R = E ÷ I, Where I = Amps, E = Volts, R = Ohms
Power Equations
P=E×I I=P÷E E = P ÷ I2 R = P ÷ I2 Where:P = Watts
P = I2 × R I2 = P ÷ R E2 = P ÷ R R = E2 ÷ P E = Volts
P = E2 ÷ R I = Amps
R = Ohms
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