Short answer: size the current transformer so the site’s maximum demand sits between roughly 50% and 100% of the CT primary rating — never below about 30% — and match the secondary to the meter’s own current input. A CT-type DIN rail meter such as the DTSU666-CT is rated 0.015–1.5(6) A on the current input, which means a 5 A secondary CT, not a 1 A one. Then check three more things before the order goes out: accuracy class, total burden in VA including the leads, and whether the class carries an “S”. Get the ratio wrong and the meter is not inaccurate by a few percent — it reads a fraction of the real energy and nobody notices until the bill and the sub-meter disagree.
This is the part of sub-metering that fails silently. A board is built, the meter is wired, the display shows a plausible number, and the number is wrong by a factor nobody questions because there is nothing to compare it to. We write as an independent trading company — we are not a manufacturer and not an appointed distributor of any brand — so what follows is the selection logic we apply when a buyer sends a feeder current and a destination.
Why the ratio is a band, not a maximum
A CT is a transformer with a specified accuracy range. Below that range the core is under-excited, the phase error grows, and the meter’s calculation drifts; above it the core saturates and the secondary current stops tracking the primary. Both ends are failure modes, and both are invisible on a display that only shows a totalised kWh figure.
That is why the ratio is chosen from the load profile, not from the breaker frame. A 250 A frame with a 60 A running load does not want a 250/5 CT. It wants the smallest standard ratio that covers maximum demand with headroom for growth — typically 1.2 to 1.5 times the highest sustained current, and above the starting surge if the load is a motor.
What “1.5(6) A” tells you about the secondary
The bracket notation is the meter’s basic current and maximum current, not a range of CT secondaries you may choose from. On the DTSU666-CT the input is written 0.015–1.5(6) A: the meter resolves current down to tens of milliamps for light-load accuracy, is calibrated at 1.5 A, and will take 6 A without damage. Those numbers are built around a 5 A secondary CT, which drives 5 A at rated primary current — inside the 6 A ceiling with margin for overload.
| Meter current input | CT secondary it expects | What happens if you mismatch |
|---|---|---|
| Direct connected, 5(60) A or 10(100) A | No CT — feeder passes through the meter | Fitting a CT in front of a direct meter multiplies the reading by the ratio a second time |
| 1.5(6) A, CT type | 5 A secondary (rated primary / 5) | A 1 A secondary CT reads one fifth of true current; the ratio setting cannot repair a five-fold scale error |
| 0.015–1.5(6) A, class C / 0.5S | 5 A secondary, class matched to the meter | A class 1.0 CT in front of a 0.5S meter caps the whole system at class 1.0 |
| Panel meter with 5 A CT input (PD666 type) | 5 A secondary | Same rule; the difference is form factor, not measuring principle |
The general principle: the weakest element in the chain sets the accuracy of the chain. A class 0.5S meter behind a class 1.0 CT is a class 1.0 installation, and paying for the better meter buys nothing.
Choosing the primary ratio from the load
Start with the highest sustained current the feeder will carry — not the breaker rating, not the cable capacity, and not the transformer size. Then apply the band.
| Maximum sustained load | Ratio to quote | Reason |
|---|---|---|
| Up to about 45 A | 50/5 | Load sits in the upper half of the range; smallest standard ratio that still covers growth |
| 45–90 A | 100/5 | Same band, one step up; avoids a 150/5 running at 50% |
| 90–180 A | 200/5 | Common on sub-mains; check the surge if a motor is involved |
| 180–360 A | 400/5 | Typical feeder ratio; verify burden once lead runs pass a few metres |
| 360–700 A | 800/5 | Large feeders; consider a split-core CT if the cable cannot be disconnected |
| Load that will never exceed 20% of a ratio you already hold | Step down one standard ratio | A CT at the bottom of its range is the most common cause of under-reading |
Two constraints cut across the table. Standardisation: a buyer who keeps 200/5 spares for every board will sometimes accept a 200/5 on a 60 A feeder — a decision with a measurable accuracy cost, not a free simplification. And growth: a ratio that covers today’s 60 A but not next year’s 150 A means a new CT and a re-calibration, so pick the band with the expansion plan in view.
Class: 0.5 or 0.5S, and where the S earns its money
IEC 61869-2 defines the accuracy classes and, crucially, where they apply. A class 0.5 CT is accurate to 0.5% between 5% and 120% of rated current. A class 0.5S CT holds its accuracy from 1% of rated current upward. The difference matters only when the load spends real time at low current — and that is exactly the shape of modern loads.
| Class | Accurate from | Fits these loads | Where it disappoints |
|---|---|---|---|
| 1.0 | 5% of rated current | Internal cost allocation, trend monitoring | Billing, tenant sub-metering, any figure that gets reconciled against a utility invoice |
| 0.5 | 5% of rated current | General commercial sub-metering with a steady base load | Seasonal sites where the load drops to a tenth for months |
| 0.5S | 1% of rated current | Solar self-consumption, EV charging, night-heavy profiles, anything with a wide turndown | Nothing, other than cost and the need for a matching meter |
The DTSU666-CT is offered with class C / 0.5S accuracy and carries MID B+D certification to EN 50470-1/-3 — the combination that lets a measured value stand up in European tenant billing or an efficiency audit. Where MID is required, it is a property of the meter module and its certificate, checked per ordering code: a datasheet line is not a certificate.
Burden: the part that is usually wrong
Every CT is rated for a burden in VA, and that budget has to cover the meter plus the wiring. The wiring is not a rounding error. Take 2.5 mm² copper leads on a 10 m one-way run: the loop is 20 m, copper conductor resistance at 2.5 mm² is about 7.41 Ω/km per IEC 60228, so loop resistance is roughly 0.15 Ω. At 5 A secondary current the leads alone dissipate about 3.7 VA — before the meter draws anything.
Put that against a 2.5 VA CT and the arithmetic is over before it starts: the CT saturates, secondary current flattens as the primary rises, and the meter under-reads precisely at peak. Three fixes, in order of preference: a CT with a larger burden rating; shorter or thicker leads; or a lower secondary current, provided the meter’s input is specified for it.
Budget the burden as a sum: CT rated burden ≥ meter burden + lead burden, with headroom for contact resistance and temperature. The meter’s own burden is in its manual and is usually a fraction of a VA per phase; the leads are the variable the installer controls.
Ratio setting and the double-multiply trap
A CT-type meter has to be told the ratio, because it only sees secondary current. Set it once — in the meter, in the Modbus register map, or in the head-end software — and never in two places. The classic failure is a meter configured for 400/5 while the billing platform also multiplies by 80: the reported energy is 6,400 times too high, and it surfaces only when someone compares against a utility invoice.
The second trap is polarity. P1 faces the supply and S1 goes to the meter’s current input for that phase; reversed, the phase angle reads 180° out, and a three-phase total can come out below the sum of its parts or read export instead of import. A 3P4W meter needs three CTs and does not instrument the neutral; a 3P3W arrangement uses two, and the meter configuration has to state which wiring it is in.
What to send with the enquiry
Maximum-demand current and the load profile; system voltage, frequency and whether it is 3P4W or 3P3W; the accuracy the figure must stand up to, and whether MID is required at destination; CT ratio and class if you already hold stock, or the load figure if you want a ratio recommended; lead length and cross-section from CT to meter; and split-core versus solid, which decides whether the feeder must be disconnected. Destination matters more than buyers expect: the certificate file, not the ratio, usually sets the timeline.
FAQ
Can I use a 100/5 CT on a feeder that only draws 25 A?
It will work, and it will be the least accurate option available. 25 A is 25% of the primary rating, where phase error and low-load drift dominate. A 50/5 CT puts the same load at 50% of range, which is where the class is specified to hold. If 50/5 cannot cover a planned expansion, class 0.5S is the better compromise than a large ratio.
What is the difference between 1.5(6) A and 5(60) A on a DIN rail meter?
The first is a CT-input meter: it expects a 5 A secondary from an external transformer. The second is direct connected: the feeder passes through the meter’s own current path and no CT is used. A direct meter cannot go on a 200 A feeder, and a CT meter without CTs reads zero.
Do I need class 0.5S or is 0.5 enough?
It depends on load shape, not site size. If current stays above 20% of the CT rating for most hours, class 0.5 holds. If the site has long low-load periods — solar, overnight EV charging, a seasonal plant — the 0.5S band from 1% is what keeps the monthly total honest.
How do I know the CT burden is enough?
Add the meter burden from its manual to the lead burden calculated from loop length and conductor resistance, then require a CT rated above that sum with headroom. Long runs and thin leads are the usual culprits; 3.7 VA of leads against a 2.5 VA CT is a real and common mismatch.
Does the ratio need to match on all three phases?
Yes. Three phases with different ratios produce a total that is arithmetically correct and physically meaningless. Summation metering with mixed ratios is possible, but it needs a meter configured for it and a note on the drawing.
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