What is a CT clamp in site energy monitoring?

A CT clamp is a current transformer sensor fitted around a conductor to measure current for monitoring or control [S1]. Its signal needs a compatible meter or controller. Elexon illustrates a CT ratio as 300/5, describing primary and secondary current ratings [S2]. Correct positioning and configuration matter when your chargers or solar controls rely on that measurement. Checked October 2026.

Last checked 19 October 2026

What is a CT clamp used for?

A CT clamp is a current transformer sensor fitted around an electrical conductor to monitor the current flowing through it. A connected controller can use that information to manage demand, including reducing EV charging when other loads need capacity [S1].

For your site, the measurement matters because the controller can only respond to the load it actually sees. Ask for a drawing identifying the measurement point, the circuits covered and any solar or battery equipment. Read dynamic load management for how the resulting control works.

What does the CT ratio tell you?

The ratio relates primary current to the secondary measurement signal. Elexon’s definition gives the example 300/5: a 300 ampere primary rating and a 5 ampere secondary rating [S2]. It describes measurement scaling, not extra electricity available to your business. Have the installer record the sensor specification and controller settings together.

For an illustration, assume your controller allows 80 amperes at a monitored point and other loads draw 55 amperes. The arithmetic leaves 80 − 55 = 25 amperes before other design constraints. These are assumptions, not a setting recommendation. The principle is demand control using measured current [S1].

What should commissioning prove?

G100, the network recommendation for customer limitation schemes, includes checks of measurement orientation, control response and failure behaviour [S3]. Ask your designer which requirements apply to your arrangement. For solar monitoring, request a demonstration that displayed import and export agree with actual operating conditions. Do not move the clamp yourself to correct an unexpected display.

Keep the test record beside your load profile and site supply assessment. Use the charging cost tool for an energy-cost illustration alongside that assessment.

Next step

Use the site power tool to organise your information, then get matched with an installer that covers your area.

Piggy Pineapple Ltd operates this referral service. We refer enquiries to installer partners and may be paid a fee. Our owner holds shares in at least one partner. Before sharing your details, we name the proposed partner and disclose any shareholding in it.