CT meter installation for business
Above 100 A per phase a meter cannot carry the load itself, so the supply is metered through current transformers (CTs). Getting it right takes your supplier, its meter operator, the network operator and your electrician, and one number, the CT ratio, decides every bill that follows. We arrange CT metering for new connections, capacity upgrades and replacements, and check the metering details before the first bill.
When a site needs a CT meter
Most CT metering work starts with one of four events. Two come with a bigger supply, which our guide to a new business electricity connection covers end to end. The other two come from the meter you already have.
A new connection above 100 A per phase
A whole-current meter carries the full load through the meter itself, up to 100 A per phase. A new supply sized above that, about 69 kVA on a 400 V three-phase connection, is metered through current transformers instead, so the CT metering is planned as part of the connection.
A capacity increase past whole-current
New machinery, a commercial kitchen, HVAC or EV charging can take an existing site over the whole-current limit. The upgrade to a larger supply brings a CT metering panel with it, and if the old and new services run side by side for more than a day the network treats it as a new connection with a new MPAN.
An ageing meter or its communications
CT meters are read remotely over a mobile modem. The UK's 3G networks are switched off and 2G closes by 2033 at the latest, so older communications need replacing. Often that is a modem change on the existing meter rather than a new meter.
A wrong CT ratio or a missing commissioning record
Every unit billed on a CT meter is the reading multiplied by the CT ratio. If the ratio in the Meter Technical Details does not match the CTs installed, or the network's commissioning record is missing, consumption can be billed at a fraction or a multiple of the real figure.
Whole-current or CT: where the line sits
The limit is current, not kVA, so what 100 A per phase means depends on the supply. Our kVA to amps calculator converts any supply size.
| Supply | 100 A per phase is about | Above that |
|---|---|---|
| Single-phase, 230 V | 23 kVA | Usually a move to three-phase |
| Three-phase, 400 V | 69 kVA | CT metering |
| High voltage, 11 kV and above | Not whole-current | CTs plus voltage transformers (VTs) |
Network operators differ in the detail. NGED, for example, fits a heavy-duty cut-out with CT metering on some sustained loads up to 100 A. Low-voltage metering below 100 kW is built to Elexon's Code of Practice 10 or higher, and 100 kW sites up to 1 MW to Code of Practice 5, both of which require CTs of at least accuracy class 0.5S.
Who does what on a CT meter installation
No single firm fits a CT meter from start to finish, which is why installs stall and ratios go missing. These are the parties on a typical low-voltage supply.
Your electricity supplier
Registers the supply and appoints the meter operator. It stays responsible for the metering being correct, whoever does the work.
Meter operator
Installs and programmes the meter, sets it to the actual CT ratio, checks phase rotation and burden, and sends the commissioning result to the supplier. Under market-wide half-hourly settlement the role is the Metering Service (Advanced).
Distribution network operator
Usually provides the cut-out, the metering panel and the CTs on a low-voltage supply, and commissions the CTs (ratio, polarity, position and burden) before passing its record to the meter operator. CTs an ICP installs pass to the network on adoption.
Your electrical contractor
Fits the main switch and the meter tails on the customer side, ready for the meter operator to connect and commission the meter.
On a new connection the CTs can be part of the contestable works an accredited ICP builds; our directory of UK connection providers lists the firms and the registers to check them on. Moving or disconnecting an existing meter is covered in our guide to commercial electricity meter services.
The CT meter installation process, step by step
- 01
Confirm the capacity and the ratio
Work out the maximum demand in kVA and the current per phase. That decides whether the site is whole-current or CT metered, and the CT size, which must keep the normal current between 1% and 120% of the CT's rating.
- 02
Agree the supply and appoint the meter operator
The supplier registers the MPAN and appoints a meter operator for a half-hourly or CT supply. Getting the supply contract in place early stops a new connection waiting on a meter.
- 03
Fit the CT chamber and the tails
The network operator (or the ICP, on a contestable new connection) installs the cut-out, the metering panel and the CTs. Your electrician fits the main switch and the meter tails.
- 04
Install and commission the meter
The meter operator fits the meter, sets it to the actual CT ratio and commissions it against the network's CT record under the Elexon Codes of Practice. The ratio is recorded in the Meter Technical Details.
- 05
Check the first bills against the meter
Half-hourly data starts flowing to the supplier. Check the ratio, the agreed capacity and the first bills against what is actually installed, before any error has months to build up.
Need to know which form your network operator wants for a new supply or a capacity change? Our electricity connection application tool names it for your postcode and job.
Why the CT ratio decides your bill
A 400/5 CT sends 5 A to the meter when 400 A flows in the cable, so the meter sees one eightieth of the real current and every reading is multiplied by 80. The meter has to be set to the actual ratio of the CTs installed, and the ratio has to come from Elexon's list of valid ratios.
If the ratio in the Meter Technical Details is wrong, so is every bill, and fixing it means rescaling the data for every day it was wrong. Check the ratio a supply your size should have with our free CT ratio checker, and if you are paying for headroom you never use, read whether a kVA review would cut your capacity charges.
Sized to the load. The CTs must keep the normal current between 1% and 120% of their rating, so a capacity increase can need larger CTs.
Commissioned twice. The network commissions its CTs and passes the record on; the meter operator then checks the meter against it. A missing CT record is one of the highest-risk errors in Elexon's guidance.
Read remotely. CT meters send half-hourly data over a modem, which is why the 2G and 3G switch-off matters for older installations.
CT meters and half-hourly settlement
A site must be half-hourly metered once the average of its three highest monthly maximum demands over 12 months exceeds 100 kW, and profile class 5 to 8 supplies with an advanced meter have been settled half-hourly since April 2017. Elexon's market-wide half-hourly settlement programme is now moving every meter across: advanced meters began migrating in October 2025, every market participant had qualified by 28 October 2026, and migration is due to finish on 7 May 2027.
Half-hourly data is also what makes a CT meter useful beyond the bill. Our guide to half-hourly electricity meters explains the settlement side, and Purely Insights half-hourly data turns the readings into a profile you can act on.
How we arrange your CT metering
We work for you, not the suppliers. On a new connection the CT metering is scoped with the capacity; on an existing site we start from the meter you have.
- Confirm the capacity, the current per phase and the CT size the supply needs.
- Put the supply contract in place so the supplier appoints a meter operator in time.
- Coordinate the network operator or ICP, your electrician and the meter operator.
- Check the commissioning, the CT ratio and the agreed capacity against the first bills.
Building a new site? Start at our new energy connections hub, then see what drives the price in our breakdown of new connection costs.
CT meter installation: common questions
- What is a CT meter?
- A CT meter is an electricity meter connected through current transformers (CTs) rather than carrying the full load itself. Each CT scales the current in a supply cable down to a small secondary current, usually 5 A or 1 A at full rating, and the meter multiplies its reading back up by the CT ratio. Supplies above 100 A per phase are metered this way.
- At what size does a business need a CT meter?
- Above 100 A per phase. On a 230 V single-phase supply that is 23 kVA and on a 400 V three-phase supply about 69 kVA. Network operators differ in the detail: NGED, for example, fits a heavy-duty cut-out with CT metering on some sustained loads up to 100 A. Your network operator confirms the arrangement for your connection.
- Who installs a CT meter?
- Several parties share the job. On a low-voltage supply the network operator usually provides the metering panel and the CTs and commissions them, your electrician fits the main switch and tails, and the meter operator appointed by your supplier installs, programmes and commissions the meter itself. We coordinate the parties so the meter is commissioned to the right ratio first time.
- Does a CT meter have to be half-hourly settled?
- Not because it is a CT meter. A site whose average of its three highest monthly maximum demands over 12 months exceeds 100 kW must be half-hourly metered, and since 1 April 2017 profile class 5 to 8 supplies with an advanced meter have been settled half-hourly under Elexon modification P272. Market-wide half-hourly settlement is now moving every meter to half-hourly settlement, with migration due to finish in May 2027.
- What happens if the CT ratio is wrong?
- The bills are wrong by the same factor. A meter recorded as 200/5 behind 400/5 CTs bills half the real consumption, and the reverse doubles it. When the error is found, the data collector has to rescale the metered volumes for every day in the affected period, so the correction can arrive as a large back-bill or credit. Our free CT ratio checker estimates the ratio a supply of your size should have.
- Do I need a new CT meter because of the 2G and 3G switch-off?
- Not always. CT meters send their readings over a mobile modem, the UK's 3G networks are already switched off, and Ofcom expects 2G to close by 2033 at the latest. Where the meter itself is sound, replacing the modem is often enough. Ask your meter operator, or ask us to check which your site needs.
More on capacity, contestable works and who pays is in our new connections FAQs.
Sources
Checked 2 November 2026.
- NGED, ST SD5D/4: LV metering arrangements (CT metering above 100 A per phase)
- Elexon, Code of Practice 10: metering via low voltage circuits
- Elexon, Code of Practice 5: maximum demand up to 1 MW
- Elexon, guidance on commissioning measurement transformers (Code of Practice 4)
- Elexon, change of measurement class and profile class (100 kW sites, new MPAN on upgrade)
- Elexon, current and voltage transformer ratios
- Elexon, P272: mandatory half-hourly settlement for profile classes 5 to 8
- Elexon, market-wide half-hourly settlement
- Ofcom, the 3G and 2G switch-off
CT metering, commissioned right first time
Tell us the site, the supply and the date. We confirm the capacity and ratio, line up the meter operator, the network and your electrician, and check the bills match the meter.