Changeover Switch and ATS for a Generator: Types, Sizing, Neutral and Safe Connection

The short answer

• A transfer device is essential. A generator feeding a building’s wiring needs a changeover switch or ATS, so the load is never on both sources.
• Manual or automatic. A manual I-0-II switch suits a hand-started set; an ATS detects a mains failure, starts the set, transfers the load and switches back.
• Size by current. Take the larger of the generator’s full-load current and the main breaker rating, then round up.
• Poles follow the earthing. Switching the neutral depends on the earthing system and the generator neutral; the electrician decides under the local code.

Why a transfer device is essential

Without a transfer device, a generator on the building wiring can push power back into the supply cable (backfeed). The local transformer works in both directions and steps the voltage up to thousands of volts, so a line worker who believes the line is dead can be killed.

If the mains returns while the generator is connected, the supplies meet out of step: the generator can be badly damaged and the wiring can catch fire.

Manual changeover switch or ATS

Manual changeover switches

A manual changeover switch has three positions: I (mains), 0 (off) and II (generator). It is break-before-make, so the two sources can never be joined. The 0 position isolates the load from both and, on many models, can be padlocked; a pause there lets running motors slow down. Avoid switches with exposed live parts.

Automatic transfer switches (ATS)

In a contactor pair, each coil is fed through the other’s normally closed auxiliary contact, with the maker’s mechanical interlock. Size each contactor by its AC-3 current when motors are connected, AC-1 only for purely resistive loads (see our contactor sizing guide), and add a lockable switch-disconnector for isolation.

What the ATS controller does

1. Detects mains failure: it compares the voltage on every phase, often frequency too, with set limits.
2. Starts the generator: after a few seconds (often 1–3 s) to ride through brief dips, it switches a volt-free start contact.
3. Transfers: once generator voltage and frequency are within limits, it opens the mains side, pauses at off and closes the generator side.
4. Retransfers: it switches back once the mains has stayed healthy for a set time, often a few minutes.
5. Cools down: the set runs unloaded for a few minutes, as its maker advises, then stops.

With large motor loads, a longer pause at off (delayed transition) lets the motors’ residual voltage decay. Some small units have no start output and transfer once a hand-started set comes up. An AMF (auto mains failure) panel combines generator control and transfer.

How to size a changeover switch

The switch carries mains current in position I and generator current in position II, so it must be rated for the larger, and at least as high as the breakers or fuses that protect it.

• Single-phase: I = S ÷ V (S in VA, V = 230 V).
• Three-phase: I = S ÷ (√3 × V) (V = 400 V between lines).

Use the highest kVA on the nameplate (the standby rating, if given); for any set rated only in kW, divide by the nameplate power factor (usually 0.8 for diesel sets). Round up to the next standard rating. Ratings apply at a stated ambient, often 40 °C, so check the derating for a hot panel.

Worked example: home or shop generator

1. 7.5 kVA single-phase set: I = 7,500 ÷ 230 = 32.6 A. Main breaker (assumed): 40 A.
2. The larger is 40 A: choose at least 40 A, or 63 A for headroom in a hot enclosure.
3. The switch does not limit the load: on generator, keep it under 32.6 A or feed only an essential-loads board.

Worked example: factory set

1. 250 kVA, 400 V three-phase set: I = 250,000 ÷ (√3 × 400) ≈ 361 A.
2. Main incomer MCCB (assumed): 400 A, so choose a device with an AC-33 rating of 400 A at 400 V.
3. With a 630 A incomer, transfer the whole site at 630 A and shed load on generator, or feed essential loads through a 400 A switch to a sub-board.

Poles and the neutral

This is the electrician’s decision, under the local code and the utility’s rules. Two rules are fixed: the generator neutral is earthed in one place only, and a combined neutral and earth (PEN) conductor is never switched or broken.

With two neutral-earth links, neutral current flows in earth conductors and metalwork, and RCCBs can trip without a fault. With none (a floating generator neutral), earth faults can go undetected. Some generators have an internal neutral-earth link: check the manual or test it. In a TT system, where the earth comes from the building’s own electrode rather than the supply, the neutral counts as live and is normally switched.

Ratings to check on the datasheet

Utilisation category

IEC 60947-6-1 (transfer switching equipment) rates duty as AC-31 for non-inductive or slightly inductive loads and AC-33 for motor loads or mixed loads including motors; suffix A means frequent operation and B infrequent, as in AC-33B. Switches to IEC 60947-3 use AC-21 (resistive), AC-22 (mixed resistive and inductive) and AC-23 (motors); AC-20 is off-load only. With motors connected, the AC-33 or AC-23 current must cover the load.

Short-circuit rating

The switch must withstand the prospective fault current where it sits, usually highest on the mains side near a large transformer. Class PC (switch-based) devices make and withstand a fault but rely on an upstream fuse or breaker to clear it: check the short-time withstand current (Icw), making capacity (Icm) or conditional rating with a named fuse or breaker. Class CB (breaker-based) devices clear it themselves; see our MCB vs MCCB vs RCCB guide.

Enclosure and IP rating

Many switches and ATS units have an IP20 front and must sit in an enclosure: IP4X or better indoors, IP54 or higher outdoors or in dusty rooms (5 = dust-protected, 4 = splashing water). Keep it out of direct sun and label it: I = mains, II = generator.

Generator, UPS and solar inverter together

• UPS or battery inverter: it has its own input and output and is not a changeover switch. Feed its input from the changeover output; its output feeds only its own circuits and must never be joined in parallel with the mains or generator supply to those circuits. Any bypass switch you add must be break-before-make, unless the UPS maker supplies and specifies a make-before-break maintenance bypass. Size the generator for the loads plus the charger.
• Grid-tie solar inverter: it is designed to stop when the mains fails, but a running generator looks like mains to it. On the load side of the changeover it can run on the generator, and when solar output exceeds the load, power flows back into the generator, which can trip or be damaged. Check before connecting. Common answers are an inverter connection on the mains side of the changeover, so it stays off on generator, or a controller made for solar with generators.
• Hybrid inverter: use its generator input or settings so it cannot export into the generator; see our solar accessories.

Checklist before you buy and install

1. Agree the earthing system, generator neutral and number of poles with the electrician.
2. Work out the generator current, compare it with the main breaker and round up.
3. Choose manual or ATS; check sensing, delays, start signal and testing.
4. Check the utilisation category (AC-33 or AC-23) and the short-circuit rating.
5. Choose the enclosure and IP rating, a lockable off position and clear labels.
6. Size the cables and the generator output breaker (see our cable sizing guide).
7. Check the UPS and solar connections.
8. On three-phase, match the generator’s phase rotation to the mains; test transfer and retransfer on load.

Browse our switches and distribution panels, plus circuit breakers, contactors & motor starters and cables (brands on the site include ABB, Schneider Electric and Chint). Send us the generator nameplate, your main breaker rating and a photo of the board on WhatsApp and our team will help you choose.