CT meter
What is a CT meter?
Once a business draws enough power, a normal electricity meter can no longer cope with the current running through it. A CT meter is the answer: instead of the full current passing through the meter, current transformers scale it down to a level the meter can read safely, then multiply it back up. If your site has a CT meter, it is a sign you are a substantial energy user, and that a few details on your bill deserve closer attention. Here is how CT meters work and what they mean for you.
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A CT meter is a metering arrangement for larger electricity supplies. It does not change what you pay per unit, but it usually sits alongside half-hourly settlement and a higher capacity, which is where the money is.
What a CT meter is
A CT meter, short for current transformer meter, is a type of electricity meter used on sites with a large power demand. Rather than passing the full electrical current through the meter itself, it uses current transformers to scale that current down to a small, safe level the meter can measure. The meter then multiplies the reading back up to give the true consumption.
You will find CT meters on larger commercial and industrial premises, factories, big retail units, warehouses and anywhere the load is too high for an ordinary meter to handle directly.
How a CT meter works
Current transformers are clamped around the main supply cables. They produce a small secondary current that is precisely proportional to the large current flowing in the cable, typically scaled down by a fixed ratio. The meter reads this small current, applies the ratio, and records the actual energy used. This means the delicate metering electronics never carry the full site load, which keeps them safe and accurate.
Why large sites use them
A standard direct meter has a limit to the current it can safely pass through it, usually around 100 amps. Above that, running the full current through a conventional meter would be unsafe and impractical. CT metering removes that limit by measuring a scaled-down signal instead, so there is no ceiling on the site’s demand. That is why any high-demand business site ends up on a CT meter.
CT meters vs direct meters
The difference comes down to how the current is measured:
| Meter type | How it measures |
|---|---|
| Direct (whole-current) meter | The full supply current passes through the meter. Used on standard and smaller sites. |
| CT meter | Current transformers scale the current down; the meter reads the smaller signal and multiplies it up. Used on high-demand sites. |
Most small businesses have a direct meter. A CT meter is a marker that you are a larger user, which usually brings half-hourly settlement and a capacity agreement with it.
The CT ratio and multiplier
The key figure on a CT meter is the CT ratio, written as something like 200/5. It means the current transformers scale a primary current of 200 amps down to 5 amps for the meter to read, a ratio of 40 to 1. That figure of 40 is the multiplier: the meter’s raw reading is multiplied by it to give your real consumption. If the multiplier recorded against your meter is wrong, every unit on your bill is wrong by the same proportion, so it is a number worth getting right.
A worked example
Say your CT meter has a ratio of 200/5, giving a multiplier of 40, and the meter’s raw register moves by 500 units over a period:
500 units × 40 = 20,000 kWh of actual consumption billed.
Example only. Your CT ratio and readings will differ, so treat this as illustrative.
Now imagine the multiplier was mistakenly recorded as 80 instead of 40. Your bill would show 40,000 kWh, double your real usage, until someone spotted it. That is exactly the kind of error a bill check is designed to catch on a CT-metered site.
CT meters and half-hourly settlement
CT meters almost always go hand in hand with half-hourly metering, where consumption is recorded every 30 minutes and sent automatically to your supplier. That combination means your supplier prices against your actual time-of-use consumption, and your bill will show network and capacity charges based on your kVA demand. In other words, the CT meter is often the point at which a business moves into the more detailed, more negotiable end of the market.
Accuracy and getting it right
CT metering is highly accurate when installed and configured correctly, which is why it is trusted for the largest supplies. The risks are practical rather than technical: a mis-recorded CT ratio, transformers installed on the wrong phase, or data gaps that get filled with estimates. On a large site even a small error scales up into a large sum, so accurate metering data is worth the attention.
How to check your CT meter
Two quick checks catch most CT problems. First, confirm the CT ratio physically marked on the transformers or meter matches the multiplier your supplier is billing against, a mismatch is the classic error. Second, check your half-hourly data is complete rather than peppered with estimates. Both come out in a bill validation, which is the simplest way to be sure a CT-metered site is billed correctly.
What it means for your business
If your site has a CT meter, you are a significant energy user, and the details matter more than they would for a small shop. Make sure your CT ratio is recorded correctly, that your half-hourly data is complete rather than estimated, and that your capacity (kVA) is sized to your real demand. Getting these right, alongside a competitive business electricity contract, is where larger sites find real savings. A periodic bill validation is the simplest way to catch a metering or multiplier error before it costs you.
Frequently asked questions
What is a CT meter?
A CT meter, or current transformer meter, measures electricity on high-demand sites. Current transformers scale the large current down to a safe level the meter can read, and the meter multiplies it back up to record the true consumption.
Why is it called a CT meter?
CT stands for current transformer. These transformers are clamped around the main supply cables and produce a small current proportional to the large one, which the meter reads. The meter is named after the transformers that make it work.
Why do large sites need a CT meter?
A standard direct meter can only safely pass a limited current, usually around 100 amps. Above that, running the full load through a conventional meter is unsafe. A CT meter measures a scaled-down signal instead, so there is no ceiling on demand.
What is the difference between a CT meter and a normal meter?
A normal, direct meter passes the full supply current through itself and is used on smaller sites. A CT meter uses current transformers to scale the current down, reads the smaller signal, and multiplies it up. It is used on high-demand sites.
What is a CT ratio?
The CT ratio, written like 200/5, is how far the transformers scale the current down, here from 200 amps to 5, a ratio of 40 to 1. That 40 is the multiplier applied to the meter reading. If it is recorded wrongly, your whole bill is wrong by the same proportion.
How do I work out my actual consumption on a CT meter?
Multiply the meter’s raw register movement by the CT multiplier. As an illustration, a 200/5 ratio gives a multiplier of 40, so a raw reading of 500 units becomes 20,000 kWh. Your own ratio and readings will differ.
Do CT meters record half-hourly data?
Almost always. CT meters typically go hand in hand with half-hourly metering, recording consumption every 30 minutes and sending it automatically to your supplier, which prices against your actual time-of-use consumption.
Can a CT meter cause billing errors?
Yes, usually through a mis-recorded CT ratio, transformers on the wrong phase, or data gaps filled with estimates. A wrong multiplier can double or halve your bill until spotted, which is why it is worth checking during bill validation.
Are CT meters accurate?
Yes, CT metering is highly accurate when installed and configured correctly, which is why it is used for the largest supplies. The risks are practical, such as a wrong ratio or estimated data, rather than the technology itself.
How do I check my CT meter is billed correctly?
Confirm the CT ratio marked on the transformers or meter matches the multiplier your supplier bills against, and check your half-hourly data is complete rather than estimated. A bill validation covers both.
Does a CT meter change what I pay per unit?
Not directly. The CT meter simply measures your consumption. What changes for CT-metered sites is that they are usually half-hourly settled with capacity charges, which is the more detailed and negotiable end of the market.
How do I know if I have a CT meter?
CT-metered sites are larger commercial or industrial premises, and the meter is usually connected to current transformers on the main cables rather than carrying the full current. Your bill will typically show half-hourly data and kVA capacity charges.
What is the link between a CT meter and kVA?
CT-metered sites almost always have a capacity agreement measured in kVA, the maximum demand the connection can deliver. The meter records the actual demand, and network and capacity charges on your bill are based on that kVA figure.
What happens if my CT multiplier is wrong?
Every unit on your bill is out by the same proportion. If a multiplier of 40 is recorded as 80, your billed usage doubles until corrected. It is one of the most expensive metering errors and a key thing a bill validation checks.
Should I get a CT meter checked?
If you have one, it is worth confirming the CT ratio is recorded correctly and that your half-hourly data is complete rather than estimated. A periodic bill validation catches multiplier and data errors before they cost you.
Elexon Balancing and Settlement Code guidance (elexon.co.uk) · Ofgem (ofgem.gov.uk)