Voltage Sags and Surges at Home — Causes, Fixes, and When to Upsize the Meter or Transformer
Lights dimming when the air-con starts, the TV rebooting by itself, bulbs flickering in the evening, a PC dying after a storm — this page separates sag from surge from flicker, tells you which side of the meter the cause sits on, what to check first, and when the real answer is a bigger meter, three-phase, a complaint to the utility, or just an SPD/UPS. Every figure cites PEA and ERC standards documents.
A voltage sag (ไฟตก) is voltage below normal — PEA's standard at a home's supply point on the 220 V system is 200–240 volts — while a surge (ไฟกระชาก) is a brief spike above normal. Frequent sags come from two camps: your side of the meter (overload, several air-cons starting together, a main cable/meter too small, old wiring or loose joints) and the utility's side (end of a long line, a community transformer running full, a lost phase). To tell them apart: if the neighbours see it too, it is the utility's → complain on 1130/1129, and under ERC standards they must inspect and contact you within 5 working days; if it is only your house, it is yours → an electrician checks the main run first, and only then think about upsizing the meter. As for a "transformer", an ordinary home does not own one; the utility assesses that after you file.
What causes a voltage sag — and how do sag, surge and flicker differ?
Sag / undervoltage (ไฟตก)
Voltage below normal, momentarily or for hours; dimming, hot motors, compressor appliances at risk if it recurs.
Surge / transient (ไฟกระชาก)
A spike above normal lasting a fraction of a second — lightning, system switching, a large load dropping off — electronics boards are the usual casualty.
Flicker (ไฟกะพริบ)
Rapid, repeated voltage wobble from pulsing loads nearby (welders, pumps, big motors) — PEA's standards file it in the same complaint class as voltage problems.
Lost or weak phase (ไฟไม่เต็มเฟส)
PEA's own term for a supply abnormality caused by "overload, faulty equipment or too long a supply distance", giving sags, weak supply or partial blackouts — unplug everything immediately if it happens.
| System | Minimum | Maximum | PEA control target |
|---|---|---|---|
| 220 V (typical single-phase home) | 200 V | 240 V | 230 V under the transformer at peak load · end of line ≥200 V |
| 380 V (three-phase) | 342 V | 418 V | 400 V under the transformer at peak load · end of line ≥342 V |
These are PEA's figures (74 provinces). The MEA area (Bangkok, Nonthaburi, Samut Prakan) runs the same 220/380 V 50 Hz system, but we found no public MEA document stating its band in checkable text, so we print none — call 1130 to confirm. Have an electrician measure at the consumer unit while the symptom occurs; repeated readings below 200 V or above 240 V are the evidence to attach to a complaint.
Why does the voltage keep sagging? Six common causes — and whose side they are on
| Cause | Typical sign | Whose side | What to do |
|---|---|---|---|
| In-house overload — several air-cons, a pump and a water heater running together | Dimming when a compressor starts; breaker warm or tripping | Yours | Stagger use; have an electrician size the main run; then consider a bigger meter |
| Old or undersized wiring, loose joints, cheap power strips | Dimming on one circuit only; hot socket/switch; burning smell | Yours (fire risk) | Stop using that circuit; call a licensed electrician at once |
| Meter or service cable smaller than the house's total load | Sags every time a big load runs, even with new wiring | Yours (filed with the utility) | Apply to upsize the meter / go three-phase to match real load |
| End of a long line, far from the transformer, community transformer running full | Whole street sags at set times (evenings, hot season); neighbours complain too | Utility | Written complaint via 1130/1129 — must be inspected and contacted within 5 working days |
| Unbalanced three-phase, a lost phase, or a loose/broken neutral | Some circuits dim while others glow too bright; bulbs blow often | Either side | Unplug everything immediately (PEA advice), then call an electrician and the utility |
| Lightning, storms, grid switching, a utility equipment fault | Surge — electronics in several homes fail at once | System / nature | Earthing + SPD, unplug during storms (MEA advice), UPS for sensitive gear |
How do you fix sags and surges? A 5-step self-check before spending money
Work through these in order and within a few days you will know which side the problem is on — most "bought a whole-house stabiliser and it did nothing" stories skipped steps 1 and 4.
Log the pattern: one house or the street, what time, what was on
For 3–5 days note when it sags, what was switched on, and ask two or three neighbours whether they see it too — that one answer splits "my side vs the utility's" more reliably than any gadget.
While it is happening: switch off / unplug sensitive gear at once
PEA's first official tip is "switch appliances off immediately when the voltage drops", and for a lost phase "unplug all electrical equipment at once" — a motor straining at low voltage and a circuit board meeting the return surge are the two most common casualties.
Check your side: consumer unit, breakers, main run, joints (electrician)
Have a licensed electrician open the consumer unit: any hot or blackened joints, is the main cable sized for the load, is any breaker unusually warm, is there real earthing — a loose joint or a hot cable is a fire emergency, not just a voltage issue.
Measure the actual voltage during the symptom and compare with 200–240 V
An electrician measures at the consumer unit with a multimeter (or leaves a logging meter if it happens when nobody is home); photograph the reading with the time — repeated values under 200 V or over 240 V while your own big loads are off are first-class evidence that the problem comes from the network.
Utility side → file a written complaint and count 5 working days
Call 1130 (MEA — the MEA Smart Life app also takes "ไฟฟ้าตก" reports) or 1129 (PEA) and file in writing with your name, address and a contact channel, attaching your notes from steps 1 and 4 — under the ERC regulation of 2558 and PEA's 2567 standards the utility must inspect and contact you within 5 working days (100 %) and resolve the complaint within 4 months (≥95 %); missing the 5-day mark pays 200 THB/day, capped at 2,000 THB.
Several air-cons make the voltage sag — do I need a bigger meter, three-phase, or a transformer?
| Situation | The answer that matches the cause |
|---|---|
| Sag only in my house, when the air-con / pump / water heater starts | Have the electrician check the main cable and joints first, then apply to upsize the meter to match real load — the A(B) sizes, fees and steps are on apply for an electricity meter. |
| Very high total load, a shop/large house, or three-phase equipment | Consider applying for three-phase to spread the load over three lines — the per-unit tariff is no different from single-phase; conditions and application cost are on three-phase electricity cost. |
| "Do I need my own transformer?" | For an ordinary home, no — the utility assesses that after you file and tells you; it normally arises only for large loads or where the existing network cannot cope. (For factories adding solar, transformer kVA sizing lives on solar grid interconnection.) |
| The whole street sags in the evening / hot season; neighbours see it too | This is a distribution problem (end of line / community transformer) — do not buy equipment; file the written complaint via 1130/1129 as in step 5 above. The utility is obliged to upgrade the network within the 4-month window. |
| Brief, occasional sags/surges, but you run a PC / CCTV / aquarium / medical device | Point protection beats whole-house: a UPS for the PC/router/medical device (PEA: a UPS keeps supplying through outages, surges and sags) plus an SPD + earthing at the consumer unit per MEA's advice — a voltage stabiliser (AVR) goes on individual sensitive appliances. |
Three-phase supply — must each phase read the same voltage, and how much difference is abnormal?
How do you protect a home from surges? Earthing, SPD, RCD, UPS — what each one stops
- Earthing — the safe path for fault current into the ground, and the foundation of every protective device; without it an SPD cannot do its job properly (and a 3-pin plug means nothing).
- SPD (surge protective device) — fitted at the consumer unit by an electrician; it clamps spikes from lightning/switching before they reach appliances. "Surge-protected" power strips help only at the point of use and also need a real earth.
- RCD (residual-current device) — trips automatically on leakage current and protects *people* from shock, especially in damp spots; it does not stop surges, but MEA recommends it alongside earthing every time.
- UPS — PEA's description: it "keeps supplying power through an outage, surge or sag, controlling the voltage so the surge does not damage the appliance"; right for the PC, router, CCTV and medical devices — size it for the load you will actually connect.
- The free habits — from PEA's infographic: switch off at once when the voltage drops, always unplug after use (switched off is not disconnected), pick appliances with built-in protection; and if a surge does kill something, the claim procedure is on power outage: can you claim.
This page is the home/shop lane — factory and solar questions live on two other pages
Who wrote this page
Compiled by the CapSolar team, a commercial solar provider in Thailand, and reviewed by Frank Lee (Founder). Voltage bands, complaint deadlines and compensation cite PEA's 2567 standards manual and the ERC regulation of B.E. 2558; protection advice cites official MEA/PEA pages whose accessibility we verified as of Aug 2026 (links at the bottom). Where the documents are silent — an imbalance percentage, MEA's own voltage band — we say so plainly.
FAQ
Keep reading
Electricity Meter Size & Cost
5(15) / 15(45) / 30(100) sizes, fees, and upsizing.
Three-Phase Electricity Cost
When to go three-phase, and whether you need your own transformer.
Power Outage: Report & Claim
Who to call, outage notices, and claiming for damage.
Solar Grid Interconnection (Transformer kVA)
For factories: transformer size vs solar capacity.
Sags and surges tripping your factory's machines?
For businesses and factories, solar engineered for power quality cuts the bill and steadies the supply — CapSolar assesses free of charge, no obligation.