CT clamp installed wrong
A backwards or misplaced CT clamp is one of the most common solar installation faults — and the one most likely to be blamed on something else. It causes battery charging failures, incorrect portal readings, and export limiting problems.
You can diagnose it from your monitoring data without opening any electrical enclosures.
CT clamp errors are often misdiagnosed as battery faults or portal bugs. A remote diagnostic session can confirm the cause and walk you through the fix.
Book a Remote Diagnostic — from £75 → What's includedWhat does a CT clamp actually do?
A CT (current transformer) clamp measures the current flowing through a cable by clamping around the outside of it — no electrical contact required. In a solar battery system, the CT is almost always installed on the main grid feed cable at the point where the house connects to the grid.
This reading is the inverter's primary source of information about what the house is doing. It uses the CT data to decide: should the battery charge or discharge? Is the house importing or exporting? Should the export limit be applied?
If the CT clamp is giving wrong data — because it is backwards, on the wrong cable, or has a loose connection — the inverter makes the wrong decisions. The battery appears not to work. The portal shows nonsense figures. And none of it is the battery's fault.
Symptoms of a CT clamp fault
Any one of these should prompt a CT clamp check.
Timed charge is set correctly but the battery stays flat through the cheap-rate window. The inverted CT reading is suppressing the charge command.
Your portal shows grid export figures late at night when no solar is generating and no battery is discharging. The clamp is reading import as export.
Battery discharges when it shouldn't — for example, draining overnight when demand is low. The reversed reading is triggering unnecessary discharge.
The animated power flow in your monitoring portal shows energy moving in implausible directions — house consuming solar while simultaneously exporting more than the panels produce.
Your DNO has an export limit but you are breaching it, or solar generation is being curtailed to zero even when export headroom is available.
On installations where the CT monitors the solar circuit, a backwards clamp shows negative generation or figures that don't peak at midday on clear days.
How to diagnose it from your monitoring data
Work through these checks in order. You don't need to open any enclosures.
Open your monitoring portal late at night when the battery is idle and no solar is generating. Your house is drawing power from the grid. The portal should show a grid import figure. If it shows export, the CT clamp is backwards.
If you have timed charge enabled, open the portal during the charge window. Grid import should rise noticeably as the battery charges. If it stays flat or shows export, the inverted CT reading is blocking the charge command.
Turn on a large known load — a kettle draws about 2,000–3,000W. Watch the grid reading in your portal in real time. It should rise by roughly the wattage of what you turned on. If the grid reading falls, stays flat, or moves by an unrelated amount, the CT clamp is backwards or on the wrong cable.
If the CT monitors the solar circuit rather than the grid feed, look at your historical generation data. It should peak at midday on clear days and drop to zero at night. If it shows the opposite pattern — high at night, flat during the day — the clamp is backwards on the solar feed.
Plug a smart plug or energy monitor into a known appliance and compare its reading to what the portal shows. If the portal's house consumption figure doesn't move when you turn appliances on and off, the CT clamp is either on the wrong cable, has a bad connection, or has failed entirely.
What the fix looks like
The CT clamp needs to be removed and refitted so the direction arrow on the clamp points the correct way (usually towards the consumer unit, away from the grid). In many installations the clamp is accessible at the inverter end without opening the consumer unit.
The CT cable itself carries no mains voltage — only a millivolt signal. The work is low-risk if the clamp location is accessible. Some inverter brands (including GivEnergy) also have a software inversion option in the configuration settings, which can correct the polarity without physically moving the clamp.
If the clamp is on a sub-circuit rather than the main grid incomer, it needs to be physically relocated. This may require accessing the consumer unit — notifiable work that must be carried out by a qualified electrician.
In some properties, the main incomer cable is in a separate meter box or at a different location from the inverter. This can make correct CT placement genuinely difficult, and is something we regularly correct during system recovery visits.
Important: Do not open your consumer unit unless you are a qualified electrician. Even with the main switch off, the cables entering the consumer unit from the meter remain live. The CT clamp cable itself is safe, but working near live incoming cables is not.
Brand-specific notes
GivEnergy portals display grid power flow clearly, making a reversed CT immediately visible. The app shows a CT direction correction toggle in advanced settings on some firmware versions. See the GivEnergy CT clamp guide for full steps.
Growatt systems use a separate meter device (ShineWiFi or ShineLan) which includes a CT clamp. The clamp orientation arrow must face the grid side. Firmware allows software polarity correction via the ShineServer portal.
Sunsynk uses CT1 and CT2 clamps — CT1 is typically the grid clamp, CT2 is the load or solar clamp. Each has its own polarity setting in the inverter menu. Backwards CT on Sunsynk is a common cause of "battery not charging on cheap rate" complaints.
Related faults and guides
Frequently asked questions
A CT clamp measures the current flowing through a cable without making electrical contact. In a solar battery system it is used to measure the grid feed — the data the inverter uses to control battery charging, discharging, and export limiting.
A backwards clamp inverts the polarity of the reading — import becomes export and export becomes import. This causes battery charging failures, incorrect portal power flow, and export limiting problems. It is one of the most common installation faults we diagnose remotely.
If the clamp is accessible outside the consumer unit, a competent person can flip it. The CT cable carries no mains voltage. However if the clamp is inside a consumer unit or distribution board, this is notifiable electrical work — only a qualified electrician should access it. Some inverters (GivEnergy, Growatt) also offer software polarity correction, which avoids physical work entirely.
Run a live load test: turn on a kettle (about 2–3kW) and watch the grid reading in your portal in real time. It should rise by roughly 2,000–3,000W. If it doesn't move, the clamp is on the wrong cable, has a loose connection, or has failed.
CT clamp direction errors affect all brands that use external CTs — GivEnergy, Growatt, Sunsynk, Solis, Fox ESS, and SolarEdge all use them. GivEnergy and Growatt are the brands where we see this fault most frequently reported.
In most cases, yes. Once the CT direction is correct the inverter's power flow logic works correctly and battery charging resumes as configured. Check your system mode settings after the fix — some systems reset settings when power is cycled.
Want us to confirm the diagnosis?
A remote session includes a live load test walkthrough, monitoring data review, and a clear written report on the cause — including whether the repair is DIY-safe or needs an engineer on site.