Accu-CT Split-Core Sensors Explained: 333mV CTs
Reading time: ~7 min read
📋 Key Takeaways
- A current transformer (CT) is how you measure amps without touching them. It clamps around a conductor and outputs a tiny, safe, proportional signal — 333.33mV at full rated current on this family.
- Split-core = retrofit-friendly. The core hinges open and snaps over an existing wire — no shutdown, no disconnecting conductors, one-handed install even with gloves.
- 333mV output is the safe standard. Unlike 5A-output CTs (which are dangerous open-circuit), these have a built-in burden resistor: the output is a low voltage that meters, data loggers and inverter monitoring inputs accept directly.
- Accuracy is two-tier: standard ±0.75% from 1–120% of rated current, or revenue-grade Option C0.6 at ±0.50% with a calibration certificate for billing-adjacent applications.
- Size by amps and window: the ACTL-0750 family covers 5–250A in a 0.78" window; larger feeders step up to the 1.25" AcuCT-125 series at 400–600A.
Every solar monitoring system, every home energy dashboard, every "why is my bill so high" investigation starts with the same unglamorous component: a current transformer clipped around a wire. The Accu-CT line — originally developed by Continental Control Systems and now the sensor behind Onset's HOBO loggers, eGauge meters, SolarEdge monitoring and SMA systems — is the industry's default answer for one reason: it measures accurately at 1% of its rating, not just near full scale. This guide explains what the numbers on the label mean and how to pick the right one.
What "333mV split-core" actually means
Three facts packed into the product name:
- Split-core: the magnetic core opens on a hinge, clips around the conductor, and latches shut — the patented Accu-CT mechanism is genuinely one-handed and glove-friendly. No pulling conductors out of lugs, no shutdown on most retrofit monitoring jobs.
- 333mV output: at full rated primary current, the CT outputs 333.33 millivolts AC. Half of rated current = ~167mV. The meter does simple math to display amps, watts and kWh. A precision burden resistor is built in, so the output stays a safe low voltage even with the secondary open — no shorting blocks, no arc risk. (Old-school 5A CTs can produce lethal voltage when open-circuited under load; that is the problem this design eliminates.)
- Revenue-grade lineage: the family meets IEEE C57.13 and IEC 60044-1 accuracy standards across its full temperature range, with a 100:1 turndown ratio — roughly ten times better than commodity CTs at the bottom of the range. That low-end accuracy is exactly what solar and motor applications need, since inverters and variable-speed drives swing current constantly.
The T-ACT-0750-100: the one in the searches
The T-ACT-0750-100 — Onset's part number for the 100A Accu-CT — is the single most-searched CT in our catalog, and at $50.69 against a $105 list price it is also one of the best sensor deals we stock. The label decodes as: 0.75-inch-class window (0.78" / 20mm actual), 100A primary rating, 333mV output. Accuracy is ±0.75% from 1% to 120% of rated current with ±0.5° phase angle — good enough for energy monitoring, submetering and solar production/consumption splits. It ships with 8-foot leads, runs from −30°C to 55°C, is UL recognized, and is assembled in the USA. One CT per conductor: for a 240V split-phase service you clip one on each hot leg; for consumption-vs-production monitoring, a pair goes on the service entrance and another on the inverter output.
Choosing the rating and the window
| Model | Rating | Window | Best for | PES price |
|---|---|---|---|---|
| T-ACT-0750-100 | 100A | 0.78" | HOBO loggers, residential submetering | $50.69 |
| SolarEdge AcuCT-075-200-333 | 200A | 0.75" | SolarEdge consumption monitoring | $58.58 |
| CCS ACTL-0750-200 (250A class) | up to 250A | 0.78" | SMA systems, revenue-grade monitoring | $103.04 |
| eGauge Accu-CT C0.6 250A | 250A | 1.25" | eGauge revenue-grade (±0.5%, cal cert) | $86.13 |
| Accuenergy AcuCT-125-400 | 400A | 1.25" | Large feeders, commercial panels | $70.08 |
| AcuCT-125-600:333 | 600A | 1.25" | Service entrances to 600A | $61.38 |
Two sizing rules. First, match the amp rating to the conductor's normal load, not its theoretical maximum — accuracy is specified from 1% to 120% of rating, so a 100A CT on a 30A circuit lives in its most accurate band, while a 400A CT on the same circuit wastes resolution. Second, check the window against the conductor with insulation: 4/0 THHN fills most of a 0.78" window; parallel runs or large service conductors need the 1.25" series.
Standard vs revenue grade (C0.6)
Standard Accu-CTs are ±0.75% — plenty for dashboards, diagnostics and solar monitoring. The Option C0.6 models are calibrated and verified to IEEE/ANSI C57.13 class 0.6 and IEC 60044-1 class 0.5S (±0.50% from 1–120%), ship with a per-unit certificate of calibration, and use tighter phase-angle specs (±0.25°) for accurate power-factor and watt readings. If the measurement ever feeds money — tenant submetering, demand-response verification, performance guarantees — spend the difference on C0.6. The family is also UL 2808 listed, which is what the 2017 NEC requires for field-installed CTs inside panels.
Install rules that keep you honest
- One conductor per CT. Clipping both hots of a 240V circuit (or hot and neutral) cancels the fields and reads near zero. Two CTs for split-phase, three for three-phase.
- Mind the arrow. CTs are directional — the arrow or "this side toward source" marking determines whether power reads as import or export. Backwards CTs are the number-one cause of "negative consumption" support calls.
- Keep leads tidy and twisted. Millivolt signals are robust but not magic — route away from high-current parallel runs where practical, and land on the meter terminals the manufacturer specifies.
- De-energize when you can. Split-core install on live conductors is common practice for qualified electricians with the right PPE, but the safe default is a locked-out panel.
Need the math on the meter side? Our amps-to-watts calculator converts what the CT sees into power, the kWh-to-amps calculator goes the other direction for three-phase, and the power consumption calculator turns kWh into dollars.
Frequently Asked Questions
What does a 333mV output CT do?
It measures AC current without breaking the circuit. The CT clamps around a conductor and outputs a small AC voltage proportional to the current — 333.33 millivolts at its full rated current, scaling linearly below that. An energy meter, data logger or inverter monitoring input reads that voltage and converts it to amps, watts and kWh. A built-in burden resistor keeps the output safe and low-voltage, unlike older 5A-output CTs.
Why split-core instead of solid-core CTs?
Installation. A solid-core CT requires threading the conductor through a closed ring — meaning a shutdown and disconnecting the wire. A split-core hinges open and clips over the existing conductor in seconds, one-handed, even wearing gloves. On retrofit monitoring jobs that is the difference between a ten-minute install and a scheduled outage. Solid-core still wins on ultimate accuracy in lab settings; for field energy monitoring, split-core accuracy (±0.5–0.75%) is more than sufficient.
What size CT do I need for a 200A residential service?
A 200A or 250A CT per hot leg — the SolarEdge AcuCT-075-200 (200A, 0.75" window) or the eGauge Accu-CT C0.6 (250A, 1.25" window) are the usual picks. Match the rating to what the service actually carries, verify the window closes over your conductor with insulation (2/0–4/0 service cable fits the 0.75–0.78" window; larger parallel runs need 1.25"), and use one CT per leg with the arrows oriented the same way toward the utility.
What is the difference between standard and C0.6 revenue-grade Accu-CTs?
Accuracy and documentation. Standard Accu-CTs are ±0.75% from 1–120% of rated current — ideal for dashboards and diagnostics. C0.6 models are calibrated to ±0.50% (IEEE C57.13 class 0.6, IEC 60044-1 class 0.5S) with tighter phase-angle error and a per-unit calibration certificate. If the reading ever touches money — tenant billing, performance contracts, incentive verification — the C0.6 is worth the premium.
Can I install an Accu-CT myself?
The mechanical part is genuinely easy — hinge open, clip on, latch. But the clip goes on inside an electrical panel, next to live service conductors that never turn off. If you are not qualified and equipped to work inside a panel (or your jurisdiction requires a licensed electrician for it), hire one — it is typically a 15-minute job per CT. Either way: one conductor per CT, arrow toward the source, leads landed on the correct meter terminals.
Measure First, Then Optimize
PES Supply stocks the Accu-CT and AcuCT line from 100A to 600A — standard and revenue-grade, HOBO, eGauge, SolarEdge and SMA variants — all parcel-shippable and priced for contractors. Monitoring hardware ships fast.
Shop CT Sensors & Monitoring Contact Us for Bulk PricingRelated Resources
- Amps to watts calculator: 120V, 240V, 12V
- kWh to amps calculator: 120V, 240V and 3-phase
- Power consumption calculator: kWh and cost
- Electrical load calculation: NEC 220 demand factors
- CT sensors and monitoring hardware
