PV CT monitoring cables

[Greentagger]

The cable or conductors connected to CT’s , what exactly are they called ? Their purpose? CT’s are installed around service conductors at exterior service panel and terminate at inverter. They have been installed in Cat 6( not sure of insulation rating yet) and have been installed in same raceway as load side interconnection conductors. Is this compliant or not? Are these conductors (cable) under 725? Installation is under NEC 2023. Any help please and code references. Lots of PV gurus here.
Thanks,

 

[jaggedben]

"Installed in cat6" sounds weird to me, but I gather you are saying they spliced the CT leads to cat6 to extend them and pulled the cat6 in conduit or EMT with inverter output. That is not compliant because it's mixing a Class 2 wiring method (cat6) with power wiring. However if they had used THHN/THWH, or a class 1 method such as 18awg TFFN, then it would be compliant. See recent discussion here:

0-10 inside the same conduit

Hi, I understand that class 1 and class 2 wiring cannot be installed in the same raceway without a barrier but I'm wondering if I were to use #12 AWG THHN for both the high voltage (120V) and the 0-10 dimming controls if that would be code sufficient? I don't see why not but that's why I'm here

forums.mikeholt.com

(Note changes in code references from 2020 to 2023, but what I said above is true for either code cycle.)

There's a deeper part to your question that's a lot less clear, which is how CT circuits are classified. I've asked and researched this question and never gotten a clear answer. It seems pretty obvious that they are 725 circuits (or 724 under the 2023 NEC) but the code doesn't call out and categorize CT leads specifically and I've never seen them marked. Some CT leads I've seen are marked as insulation types allowed for Class 1 and some aren't. At least one manufacturer recommends extending them with a Class 2/3 wiring method but not in the same conduit. So I've taken to treating them as Class 2, but only using Class 1 methods (or standard power/light methods) where run in the same conduit as other power. And I don't run them in the same conduit as loads unrelated to the solar and/or battery system that the CTs are for.

Honestly if the cat6 outer jacket has a voltage rating (e.g. 300V) that exceeds the other circuits voltage then I don't think what they did is unsafe, but it's pretty clearly not compliant.

 

[Carultch]

The cable or conductors connected to CT’s , what exactly are they called ? Their purpose? CT’s are installed around service conductors at exterior service panel and terminate at inverter. They have been installed in Cat 6( not sure of insulation rating yet) and have been installed in same raceway as load side interconnection conductors. Is this compliant or not? Are these conductors (cable) under 725? Installation is under NEC 2023. Any help please and code references. Lots of PV gurus here.
Thanks,

They are called CT leads. These are the output wires that connect the secondary of the CT, to the metering equipment. What the CT does, is measure the magnetic field around a wire, and produce a corresponding scaled-down output current that is representative of the current in the wire it is measuring. So this would mean for a 400:5 CT, that 400A in the circuit being measured, would deliver 5A to the meter. Every amp the meter measures would correspond to measuring 80A on the circuit being measured.

Since the voltage drop across a CT wire is intended to be insignificant, using a smaller wire than comes standard with the CT, could result in too much resistance for an accurate measurement. If necessary to extend a CT output lead, always expect to use at least the same size as the CT output lead wire. Check with the manufacturer for guidance about the maximum length you can extend these wires, as a function of the wire size. It can be necessary to upsize them for longer extensions.

 

[ggunn]

Since the voltage drop across a CT wire is intended to be insignificant, using a smaller wire than comes standard with the CT, could result in too much resistance for an accurate measurement.

It depends on which type CT is being used. One type is a current source which outputs a current proportional to the current in the wire that is presented to a calibrated resistance in the monitoring equipment, which in turn measures the voltage across it to report the current in the conductors being monitored. Since the current in the leads doesn't depend on the resistance of the leads, you can trim or extend them without affecting the measurement, but the installation should include a bypass switch to short the leads for servicing because an open circuit on the leads can result in dangerous voltages if they are disconnected when there is current in the conductors being monitored.

The other type has a resistor in the CT itself which makes it a voltage source feeding a voltage divider network which includes the calibrated resistance in the monitoring equipment as well as the Vd of the leads; in that case for the sake of accuracy you should not trim or extend the leads because the Vd in the leads is part of the calculation. This type of CT doesn't require the installation of the bypass switch.

 

[solarken]

It depends on which type CT is being used. One type is a current source which outputs a current proportional to the current in the wire that is presented to a calibrated resistance in the monitoring equipment, which in turn measures the voltage across it to report the current in the conductors being monitored. Since the current in the leads doesn't depend on the resistance of the leads, you can trim or extend them without affecting the measurement, but the installation should include a bypass switch to short the leads for servicing because an open circuit on the leads can result in dangerous voltages if they are disconnected when there is current in the conductors being monitored.

The other type has a resistor in the CT itself which makes it a voltage source feeding a voltage divider network which includes the calibrated resistance in the monitoring equipment as well as the Vd of the leads; in that case for the sake of accuracy you should not trim or extend the leads because the Vd in the leads is part of the calculation. This type of CT doesn't require the installation of the bypass switch.

Not necessarily. Most of the CT's I install with PV systems has 333mV AC output at full rated load current. There is a burden resistor in it, but the input impedance of most meters is very high compared to the output impedance of the CT, so the CT lead length is generally negligible and can be trimmed or extended without impacting the measurement. The CT and/or meter manufacturer should provides guidance on lead length.