The answer first: an undersized meter shows three clear symptoms, and two of them get blamed on something else
One, lights dipping when a large appliance starts, such as the moment an air conditioner compressor cuts in and the lamps dim briefly.
Two, the main breaker tripping when several things run at once, the main one covering the whole house rather than an individual circuit.
Three, the electricity authority declining an application to add new equipment, such as an electric vehicle charger or another air conditioner.
The first two usually get blamed on poor supply in the neighbourhood or a faulty breaker, when the cause may have been the meter size all along.
This article is short and direct: how to read the meter size, how to tell whether yours is undersized, the process for upgrading, and what to do before applying.
Reading the meter size
A domestic meter size is written as a pair of numbers, such as 5(15) or 15(45).
The first is the current the meter is designed to carry continuously, and the number in brackets is the maximum it can take.
A small house that used little electricity often received a small meter to begin with, and the trouble starts when that house adds air conditioning, extends the kitchen, or buys an electric car.
A separate question is single phase against three phase. Ordinary houses are usually single phase, and a house with very high load may need converting to three phase, which is a different matter from simply enlarging the meter.
Check first whether it really is the meter
Dipping lights and a tripping main have other causes, and applying to upgrade when the cause lies elsewhere is money wasted.
Check these three first.
One, loose connections, both in the board and where the supply enters. A loose connection causes voltage drop and localised heating, which mimics an undersized meter.
Two, a main cable that is too small or too long, causing voltage drop along its length.
Three, load concentrated on one circuit, which is solved by splitting circuits without touching the meter at all.
The most direct method is having an electrician measure the voltage at the main board while the house is at its heaviest use and compare it with the voltage when nothing is running. The difference says whether the problem sits before or after the meter.
Tracking down a tripping breaker is in a breaker that keeps tripping.
The upgrade process
Apply to your local electricity authority: the Metropolitan Electricity Authority in Bangkok, Nonthaburi and Samut Prakan, and the Provincial Electricity Authority elsewhere.
What is generally needed:
- Identity card and house registration of the account holder.
- The most recent bill, or the customer number.
- Land title documents or written consent, depending on the case.
- A power of attorney if somebody else attends.
What people do not expect is that upgrading a meter is not simply swapping a box.
A larger meter means more current can enter the house, and the internal system has to be able to take it: the main cable, the main breaker and the board.
If the internal system is unchanged, upgrading the meter alone opens the way for more current to flow into cable that cannot carry it, which is something the authority inspects and imposes conditions about.
So speak to an electrician before applying about what would need upgrading, and budget for both parts rather than only the authority's fee.
For costs and timescales, ask your local authority directly, because both vary with the size requested and site conditions.
Frequently asked questions
How do I know the meter is undersized?
The main symptoms are lights dipping when large appliances start, the main breaker tripping under simultaneous load, and the authority declining applications to add equipment.
How do I read the numbers on the meter?
They come in pairs such as 15(45), where the first is the continuous design current and the bracketed figure is the maximum.
The lights dip whenever the air conditioner starts. Is that the meter?
Possibly, but loose connections and an undersized main cable produce the same symptom. Have an electrician measure the voltage at the board under heavy load and compare it with no load first.
Where do I apply?
To your local electricity authority: the Metropolitan Electricity Authority in Bangkok, Nonthaburi and Samut Prakan, and the Provincial Electricity Authority elsewhere.
Does upgrading mean changing things inside the house?
Usually yes, because a larger meter lets more current in, and the main cable, main breaker and board must be able to carry it.
What does it cost?
It varies with the size requested and site conditions, so ask the local authority directly, and remember to add the cost of upgrading the internal system.
When does a house need three phase?
When total load is high enough that single phase is insufficient, which is a different matter from enlarging the meter, and an electrician should assess the real load first.
Do I need a bigger meter to install an EV charger?
It depends on the house's total load and the charger's size. Have an electrician calculate the load first, because some houses need it and some do not.
Will a bigger meter raise my bill?
Bills are charged on units actually used, not on meter size, so upgrading does not raise the bill by itself if usage stays the same.
In short
- An undersized meter shows as dipping lights, a tripping main, and declined applications.
- Meter sizes are written as pairs: continuous current, then maximum in brackets.
- Loose connections and an undersized main cable mimic the same symptoms.
- Have the voltage measured at the board under heavy load and compared with no load.
- Apply to the local electricity authority with identification and a recent bill.
- A larger meter usually means upgrading the main cable and the board too.
- Budget for both the fee and the internal upgrade.
- Bills follow units used, not meter size.