There are a few decisions to be made when selecting current transformers:
- Type: opening (split-core) or non-opening (solid-core)
- Accuracy: monitoring or revenue grade
- Size: must be large enough to fit around the conductor being monitored
- Rated Amps
In most cases, opening or split-core CTs are the preferred choice, because installation is much easier. CCS does sell solid-core CTs in sizes up to 1.25 inches (31.75 mm) and with rated currents up to 400 amps.
CCS offers monitoring grade CTs with typical accuracies in the 1% to 1.5% range and phase angle errors of less than 2.0 degrees. These generally have accuracy specifications from 10% to 100% (or 120%) of rated current with increasing errors below 10% of rated current. This grade of CT is generally sufficient for monitoring the relative power consumed by different loads within a building, or for comparing the power savings of energy efficiency upgrades. CCS also offers revenue grade CTs with a typical accuracy of 0.5% and phase angle error of less than 0.5 degrees. These generally have accuracy specifications from 1% to 100% (or 120%) of rated current, so they are accurate over a much wider range of operation. These are recommended for use with the WattNode Revenue meters and for any situations where higher accuracy is desired or better accuracy at low currents (such as monitoring standby power).
It is obviously critical that the opening in the CT is large enough to fit around the conductor being monitored. Generally, if a CT has a rated amps high enough for the conductor, then it should fit, but this is not always the case. If you are monitoring multiple parallel conductors (common above 400 amps) or bus bar conductors, consider measuring the required CT size before placing the order. It is tempting to order the largest possible CT to ensure it will fit, but CCS recommends against this for a few reasons:
- Larger CTs may be difficult to fit in the panel.
- Larger CTs may be difficult to fit between the other wires coming out of adjacent breakers.
- For best accuracy, the diameter of the conductor being monitored should be more than half the opening size of the CT. For example, a 4/0 AWG conductor is typically 0.64 inches in diameter and commonly carries 200 to 250 amps. This will fit in a 0.75 inch CT and largely fill the opening (best case for accuracy). It would be about one-half the opening of a 1.25 inch CT, which should be fine for accuracy. But it would be less than one-third the diameter of a 2.0 inch CT and accuracy could suffer.
See CT Opening Size vs. Conductor Size for some common conductor sizes for different currents and recommended CT sizes.
The rated full-scale current of the CTs should normally be chosen somewhat above the maximum current of the circuit being measured (see Current Crest Factor for more details). In some cases, you might select CTs with a lower rated current to optimize accuracy at lower current readings. Take care that the maximum allowable current for the CT can not be exceeded without tripping a circuit breaker or fuse. With revenue grade CTs (Accu-CTs), the accuracy is excellent at very low currents, so the easiest approach is to just select a CT with the same rated amps as the rating of the circuit (generally the rating of the breaker or fuse) being monitored. CTs can measure lower currents than they were designed for by passing the conductor through the CT more than once. For example, to measure currents up to 1 amp with a 5 amp CT, loop the conductor through the CT five times. The CT is now effectively a 1 amp CT instead of a 5 amp CT. The effective current rating of the CT is the labeled rating divided by the number of times that the conductor passes through the CT. If you are using the individual phases (ØA, ØB, and ØC) of the WattNode meter to measure different circuits, you can use CTs with different rated current on the different phases. Instead of setting one CtAmps value for all phases, you can use different values for each phase: CtAmpsA, CtAmpsB, and CtAmpsC (Note: not all WattNode models support different CT rated amps for different phases).
CCS only offers CTs that measure AC current, not DC current. Significant DC current can saturate the CT magnetic core, reducing the AC accuracy. Most loads only have AC current, but some rare loads draw DC current, which can cause measurement errors. See the article DC Current and Half-Wave Rectified Loads for more information.