Buying guide · 2026

Automatic Transfer Switch (ATS) for Shop, Clinic & Small Office DG Backup

If your shop, clinic or small office runs on a DG for backup, a manual changeover knob is a liability — the cashier shouldn't be wrestling with a lever during a power cut. An ATS does the switchover automatically. This guide covers sizing, wiring and what not to do.

Published Apr 2026 · By Santosh Electricals team

First: ATS vs AMF panel — not the same thing

ATS (Automatic Transfer Switch) physically switches the load between mains and DG. Think of it as a motorised changeover.
AMF (Auto Mains Failure) panel controls the DG — starts it when mains fails, stops it when mains returns.

Most ready-made DGs (Kirloskar, Mahindra, Cummins) ship with AMF built in. You still need an ATS on the downstream load side. Some sites combine both into one panel; for smaller shops and clinics, a standalone Techno ATS near the load is the clean setup.

2 Pole vs 4 Pole — how to decide

Your supply	Recommended ATS	Techno SKU
Single phase (phase + neutral)	2 Pole ATS	TMCB ATS 2P
Three phase without neutral switching	3 Pole ATS	On request
Three phase + neutral (safer for mixed loads)	4 Pole ATS	TMCB ATS 4P

For most shops and clinics on single phase: TMCB ATS 2P. For apartments, small factories or any 3 phase premises: TMCB ATS 4P.

Sizing the ATS current rating

Add your total load (lights + fans + AC + fridge + equipment) in kW.
Divide by voltage (230V single phase or 400V three phase) × power factor (0.8 assume).
Add a 25% safety margin for inrush and future load growth.

Example: 8 kW single phase clinic → 8000 / (230 × 0.8) = 43A → pick 63A ATS. Don't undersize — nuisance tripping under AC startup is a common complaint.

Wiring sequence — what goes where

Incoming: Mains supply from meter → ATS mains input terminals.
Incoming #2: DG output (after AMF / breaker) → ATS DG input terminals.
Outgoing: ATS load output → sub-distribution board → final load.
Sensing: ATS senses mains voltage; when it drops, ATS waits for DG voltage to stabilise (usually 2–5 second delay), then transfers.
Neutral: On 4P ATS, neutral also switches — prevents neutral circulating current between sources.

Delay settings most installers get wrong

Transfer delay (mains → DG): 2–5 seconds. Long enough for DG to stabilise, short enough to minimise outage.
Return delay (DG → mains): 30–60 seconds. Lets mains voltage settle before transferring back; prevents flapping on unstable mains.
DG cooldown: After return, let DG run 1–2 minutes unloaded before stopping (handled by AMF, not ATS).

Common mistakes that burn equipment

Wrong phase rotation on DG — 3 phase motors spin backwards on DG. Always cross-check phase sequence before first load transfer.
Undersized ATS — engineer assumes "shop runs 5kW" but ignores inrush. AC compressor kicks in, ATS contactor welds. Size +25% over calculated.
No manual bypass — during ATS service, the load is stranded. Good installs include a manual bypass changeover.
Same-cable load and sensing — ATS sensing should come from a dedicated tap, not downstream of the load, otherwise voltage drops confuse the logic.

When you might skip ATS

Very small load (< 2 kW) with portable genset — a manual changeover knob works fine, and ATS cost isn't justified.
Load tolerates a break — warehouse or godown where a short outage doesn't hurt.
Inverter-fed critical load already backed up — computers / billing machines on UPS; the DG can start up at leisure.

Indicative setup for a typical Chennai shop

Shop load: 6 kW single phase (lights, 2 ACs, fridge, billing PC).
DG: 7.5 kVA single phase genset with built-in AMF.
ATS: Techno TMCB ATS 2P, 63A.
Enclosure: Wall-mount panel near main DB.
Manual bypass: One 63A changeover switch for service access.