Comply with NEC 2020 690.11 AFC protection for Central Inverter

[jouz.lim]

Hi all,

I have a question about AFCI functionality in combiner boxes. Almost all string inverters are currently integrated AFC function, with supported cable length up to 200m (three-phase). So, can I ask what solution we use to resolve AFC in combiner boxes?

I am not a US citizen, and in my country, we do not apply AFCI functionality so I don't understand the solution in reality. If any, can you recommend to me some devices and brands you used to resolve AFC?

Another question, UL 1699B-2018 mentioned that cable length supported is 50μH + 0.7μH per meter above 80 meters. What does it mean? And in case that the cable length in the actual site is longer than standards. So what can I do?

Thank all.

 

[pv_n00b]

We don't do anything differently if there is a DC combiner in the system. If the inverter has DC AFCI then it protects all the DC conductors connected to it, in theory anyway.
We don't typically use the UL standards in system design. The equipment manufacturer should provide you with cable length limits that you cannot exceed. They will have had that cable length tested to the UL Standard. If your design requires a longer cable length then you need to modify your design so it is in compliance with the manufacturer's instructions.

 

[jouz.lim]

We don't do anything differently if there is a DC combiner in the system. If the inverter has DC AFCI then it protects all the DC conductors connected to it, in theory anyway.
We don't typically use the UL standards in system design. The equipment manufacturer should provide you with cable length limits that you cannot exceed. They will have had that cable length tested to the UL Standard. If your design requires a longer cable length then you need to modify your design so it is in compliance with the manufacturer's instructions.

Hi @pv_n00b,

Many thanks for your reply. Normally we will use DC combiners in case we use central inverter, not string inverter. As I know, currently only string inverters are integrated AFCI protection, not any central can. Am I wrong?

So in case, we use DC combiners with central inverter. Can you introduce me to some AFCI devices manufacturers you used in DC combiners?

I have seen the AFCI devices from Siemens. They mentioned that their products only cover series arc-faults, cannot cover all cases of parallel arc-faults. So, any recommendations for me?

Have a good day!

 

[Carultch]

Another question, UL 1699B-2018 mentioned that cable length supported is 50μH + 0.7μH per meter above 80 meters. What does it mean? And in case that the cable length in the actual site is longer than standards. So what can I do?

H stands for Henries, which are a unit of inductance. The Greek letter mu indicates micro, as in millionth. This is a property of the geometry of a circuit that encloses a region of space, and generates a magnetic field. Ferrous materials also contribute to this. Think of inductance as an "electrical inertia", that generates a voltage to oppose a change in current. Unlike resistance where voltage is proportional to current, inductance means a voltage (or rather, emf) is proportional to a rate of change in current. A helical coil of wire called a solenoid, and a torus coil of wire called a torroid, are two geometries we use for increasing inductance.

This is an important parameter for transformers, electromagnets, and inductors in electrical circuits, where we want this property to be high. But for general wiring, it is a nuisance. Particularly with signal wiring, because it can mean your signal wiring unintentionally acts as a low pass filter against high frequency signals. There are likely look-up tables that tabulate this property for the wire sizes and the condition of whether it is in a ferrous (i.e. iron-based) conduit, or a non-ferrous conduit like aluminum or PVC.

 

[pv_n00b]

Many thanks for your reply. Normally we will use DC combiners in case we use central inverter, not string inverter. As I know, currently only string inverters are integrated AFCI protection, not any central can. Am I wrong?

So in case, we use DC combiners with central inverter. Can you introduce me to some AFCI devices manufacturers you used in DC combiners?

I have seen the AFCI devices from Siemens. They mentioned that their products only cover series arc-faults, cannot cover all cases of parallel arc-faults. So, any recommendations for me?

All the major DC combiner manufacturers like SolarBOS, ConnectPV, etc., in the USA offer combiners with AFCI for 1,000V and below circuits. I don't know of anyone offering a DC combiner at a higher voltage. Unfortunately, large central inverters and 1,500V arrays tend to go together. I also don't know of a central inverter that has DC AFCI, probably because most are 1,500V.