What Is My CT Ratio?

Frequently asked questions

What is a CT ratio?

A CT ratio describes a current transformer (CT): the primary current it is rated for and the current its secondary winding delivers to the meter at that load. A 400/5 CT sends 5 A to the meter when 400 A flows in the supply cable, so the meter sees one eightieth of the real current. The ratio, 80 in that example, is either programmed into the meter or applied as a multiplier when the readings are processed, which turns the scaled-down reading back into the site's true consumption.

How do I find my CT ratio?

The ratio is stamped on the rating plate of the CTs themselves and recorded in your Meter Technical Details (MTDs), which your Meter Operator (MOP) holds. No public database returns the CT ratio for a specific meter, and the MPAN does not contain it. The CTs sit in sealed metering equipment connected to live conductors, so never open it yourself: only an authorised meter operator should. The calculator on this page estimates the likely ratio from your supply size, and our team can confirm the recorded one for you.

What is the difference between CT metering and whole-current metering?

A whole-current meter carries the site's full load current through the meter itself, which works up to 100 A per phase. On a 400 V three-phase supply that is about 69 kVA (100 A × 400 V × √3 ÷ 1000), and on a 230 V single-phase supply it is 23 kVA. Above that, the supply cables pass through current transformers and the meter measures a scaled-down secondary current instead, usually 5 A or 1 A at full rating. So supplies above about 69 kVA three-phase are normally CT metered.

What is a VT and when does a site have one?

A voltage transformer (VT) steps a high voltage down to a level a meter can measure, most often 110 V. High-voltage supplies such as 11 kV are metered through both CTs and VTs, and the readings are scaled by both ratios. An 11000/110 VT presents 110 V to the meter, a ratio of 100, so a site with 30/5 CTs and an 11000/110 VT has an overall multiplier of 6 × 100 = 600. Low-voltage supplies at 400 V or 230 V do not need VTs because the meter connects to the supply voltage directly.

Does my MPAN tell me if I have CTs?

Not directly. The first two digits of the full 21-digit MPAN are the profile class: 00 means the supply is half-hourly settled, and 01 to 08 are Elexon's non-half-hourly profile classes. Half-hourly meters can be whole-current or CT-operated, so the profile class shows how the supply is settled, not what hardware is fitted. Whether CTs are installed, and their ratio, is recorded in the Meter Technical Details. Enter your MPAN in the tool above and we will read what it can tell you.

Why does the CT ratio matter for my bill?

Every unit on a CT-metered bill is the meter's reading multiplied by the ratio, so the ratio recorded in the Meter Technical Details has to match the CTs actually installed. If a site has 400/5 CTs recorded as 200/5, the multiplier is 40 instead of 80 and every half-hour of consumption is billed at half its real level; the reverse doubles it. The ratio also needs checking when you change Meter Operator, when a meter is replaced and when you raise your agreed capacity, because CTs are rated for a maximum primary current and a larger supply can need larger CTs.

Is this CT ratio tool free?

Yes. The calculator, the MPAN check and the searchable copy of Elexon's valid CT and VT ratios are free to use with no sign-up. If you want the ratio recorded for your meter confirmed, leave your details and our team will check it for you, also free and with no obligation. Purely Energy is a B-Corp certified consultancy that works for you, not the suppliers.