Transfer Switch Symbol
Definition: The Transfer Switch symbol represents an electrical switching device—drawn as a three-terminal block or double-throw switch with terminals labelled Utility (normal supply), Load (output to building), and Gen (generator or alternate supply)—that safely transfers an electrical load between two power sources without paralleling them, used in standby power system diagrams per IEC 60947-6-1 and NFPA 70 (NEC) Article 702/700.
Also known as: ATS, automatic transfer switch, manual transfer switch, MTS, generator transfer switch, source transfer switch, changeover switch.
What the Transfer Switch symbol means
The Transfer Switch symbol denotes a device that connects a load to one of two available power sources — the utility (normal mains supply) and an alternate source (standby generator, second utility feed, or UPS output) — while ensuring the two sources are never connected in parallel (anti-paralleling interlock). In wiring diagrams, the three-terminal symbol shows the Utility input at the top, the Load output and Gen (generator) input at the bottom, with the internal switching mechanism providing the make-before-break or break-before-make transfer between sources.
Transfer switches are classified as automatic (ATS) — transferring automatically when the utility fails and the generator starts — or manual (MTS) — requiring an operator to manually change the switch position. The symbol is used in residential standby generator installations, critical facility power distribution, and industrial backup power systems.
How to identify the Transfer Switch symbol
The transfer switch symbol is a three-terminal block: Utility input (top terminal), Load output (bottom-left terminal), and Gen input (bottom-right terminal). The internal switching element is typically shown as a double-throw contact or a two-position changeover switch inside the block body. Automatic transfer switches may show a coil or solenoid symbol inside the block indicating the motorised actuator. The symbol body may be labelled 'ATS', 'TS', or 'Transfer Switch'. In one-line diagrams, the transfer switch appears as a standard changeover (DPDT or 3-position) switch symbol with source labels.
Function in a circuit
The transfer switch continuously monitors the utility supply voltage and frequency. When utility power is normal, the switch connects Load to Utility and the generator is off. When utility power fails (voltage drops below threshold, typically 80 % of nominal, or frequency deviates beyond ±3 Hz), the ATS signals the generator to start. After the generator reaches stable voltage and frequency (typically 10–30 seconds), the ATS transfers the Load from Utility to Gen. When utility power is restored and stable for a delay period (typically 1–5 minutes), the ATS transfers the Load back to Utility and signals the generator to cool down and stop. The critical safety feature is the mechanical or electrical interlock that prevents both sources from being connected simultaneously, which would cause a phase fault or damage the utility transformer.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60947-6-1 specifies automatic transfer switching equipment (ATSE) for low-voltage applications. IEC 60947-6-2 covers control and protective switching devices. The schematic symbol follows IEC 60617 conventions for changeover switches and multiple-position switches. IEC 62040 covers UPS systems that incorporate internal transfer switches. |
|---|---|
| ANSI/IEEE 315 | NFPA 70 (NEC) Article 702 (Optional Standby Systems) and Article 700 (Emergency Systems) govern transfer switch installation in North America. UL 1008 (Transfer Switch Equipment) governs safety requirements. ANSI/IEEE 315-1975 defines the changeover switch schematic symbol. IEEE 446 (Recommended Practice for Emergency and Standby Power Systems) provides design guidance. |
| Key difference | IEC 60947-6-1 defines ATSE as 'automatic transfer switching equipment' and classifies it by utilisation category (AC-31A, AC-33B, etc.). NEC/UL 1008 classifies transfer switches by application (emergency, legally required standby, optional standby) and load type. IEC symbols show a qualified switch symbol; ANSI/IEEE symbols show a double-throw or changeover contact, often with coil annotation for automatic versions. |
Terminals / pins
| Pin | Name |
|---|---|
| utility | Utility |
| load | Load |
| gen | Gen |
Typical values
Current ratings: 30 A to 4000 A (residential to utility-grade). Voltage: 120/240 V AC single-phase, 208/240/480 V AC three-phase (North America); 230/400 V AC (Europe). Transfer time: open-transition ATS: 0.1–30 s; closed-transition ATS: <100 ms (brief parallel). Generator start delay: 3–10 s. Retransfer delay: 1–30 min (allows generator warm-up/cool-down). Neutral switching: some ATS designs switch the neutral conductor; others use a solid neutral.
Where the Transfer Switch symbol is used
- Residential standby generator systems — ATS automatically transfers home circuits to a natural gas or propane standby generator during utility outages
- Critical facility (hospital, data centre) power distribution — ATS ensures sub-second transfer to emergency generator or UPS output on utility failure
- Industrial facility emergency power — mandatory emergency lighting and fire pump circuits transferred to standby generators per NEC Article 700
- Commercial building legally required standby — elevator, exit lighting, and HVAC smoke evacuation systems transferred to standby power per NEC Article 701
- Marine and mobile applications — transfer switch on recreational vehicles (RVs) and boats selecting between shore power and onboard generator
- Telecommunications and broadcast facilities — ATS on critical server rooms and transmitter sites providing continuous power through utility interruptions
Example
In a residential standby power installation, the ATS symbol appears between the utility service entrance and the main panel, with Utility connected to the utility service entrance conductors, Load connected to the main panel bus, and Gen connected to the standby generator output. On utility failure, the ATS transfers the main panel to the generator within 30 seconds, supplying all house circuits; on utility restoration, it retransfers after a 5-minute stabilisation delay.
Key facts
- The Transfer Switch symbol is a three-terminal block with Utility (utility supply input), Load (building load output), and Gen (generator/alternate supply input), with an internal changeover switching mechanism shown as a double-throw contact.
- The critical function of a transfer switch is its anti-paralleling interlock — it ensures that the utility supply and the generator output are never connected simultaneously, preventing a dangerous back-feed condition.
- IEC 60947-6-1 governs automatic transfer switching equipment (ATSE) in low-voltage applications; UL 1008 governs in North America; NEC Articles 700, 701, and 702 govern installation requirements.
- An open-transition ATS breaks the connection to one source before connecting to the other (brief power interruption of 0.1–30 s); a closed-transition ATS briefly parallels both sources for <100 ms during transfer to achieve a seamless transfer.
- Automatic transfer switches start the standby generator automatically on utility failure, wait for generator stability, transfer the load, and retransfer to utility after a delay when utility is restored.
- The transfer switch must be rated for the full connected load current and must have appropriate interrupting capacity for fault currents in the installation.
- Generator-to-utility back-feed through a transfer switch without proper isolation is illegal and potentially fatal — it energises utility lines that lineworkers may assume are dead during an outage.
- Neutral switching: some transfer switch designs switch the neutral conductor (required for separately-derived generator systems under NEC 250.30); others use a solid (non-switched) neutral.
Diagrams that use this symbol
- circuit diagram generator
- dc generator diagram
- generator connection diagram
- generator wiring diagram
- ats diagram
- ats wiring diagram
- transfer switch installation diagram
- ats circuit diagram
Frequently asked questions
What does the transfer switch symbol look like in a wiring diagram?
The transfer switch symbol is a three-terminal block with three labelled connection points: Utility (normal mains supply input, typically at the top), Load (building load output, bottom-left), and Gen (generator or alternate source input, bottom-right). Inside the block, the switching mechanism is shown as a double-throw contact or changeover switch. Automatic versions may show a coil or motor actuator symbol inside the block.
What is the difference between an ATS and an MTS?
An ATS (Automatic Transfer Switch) monitors the utility supply and automatically transfers the load to the standby generator when utility power fails, then retransfers automatically when utility is restored. An MTS (Manual Transfer Switch) requires an operator to physically change the switch position — it provides the same anti-paralleling safety interlock but without automatic monitoring or actuation. ATS is required for emergency systems under NEC Article 700; MTS is permitted for optional standby per NEC Article 702.
Why is it dangerous to connect a generator without a transfer switch?
Without a transfer switch, the generator output can back-feed through the building wiring into the utility grid via the main service entrance, energising supposedly dead utility lines at lethal voltages. This is extremely dangerous for utility workers repairing the outage. It can also damage the generator when utility power is restored (out-of-phase re-closure). NEC 702.12 and utility regulations prohibit generator connections without proper transfer switching.
What is the difference between open-transition and closed-transition transfer switching?
Open-transition ATS breaks the connection to one source before connecting to the other, causing a brief power interruption (0.1–30 seconds depending on settings). Closed-transition ATS briefly parallels both sources for less than 100 milliseconds during transfer, providing a seamless (no-interruption) transition. Closed-transition is used for sensitive loads (servers, medical equipment) that cannot tolerate even a brief interruption; it requires synchronisation of both sources before paralleling.
What NEC articles govern transfer switch installation?
NFPA 70 (NEC) Article 700 governs emergency systems (required by law for life-safety loads such as exit lighting and emergency egress). Article 701 covers legally required standby systems (elevators, fire pumps). Article 702 covers optional standby systems (residential standby generators, commercial backup power). Each article has specific requirements for transfer switch type, rating, and installation location.
What current rating do I need for a residential transfer switch?
For a whole-house transfer switch (supplying the entire main panel), the ATS must be rated at least equal to the main service ampacity — typically 100 A, 150 A, or 200 A for residential services in North America. For a sub-panel (critical loads only) transfer approach, a smaller ATS (30 A or 60 A) is used with a separate critical-loads panel. The ATS must be rated for the generator's maximum output current as well.
What standard governs transfer switch safety and construction?
In North America, UL 1008 (Standard for Transfer Switch Equipment) governs the safety and construction of automatic and manual transfer switches. In Europe and internationally, IEC 60947-6-1 specifies requirements for automatic transfer switching equipment (ATSE) in low-voltage applications. IEC 60947-6-2 covers combined functions including transfer switching for control and protective applications.
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