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Install tip: Avoid DC faults by properly wiring

May 02, 2023

By SPW | March 12, 2018

This installation tip was provided by Richard Baldinger, head of marketing for Fronius USA

A PV system electrical fault often results from improper wiring. Specifically, points of connection, tension and friction are particularly notorious for being a fault source. Proper planning, materials and installation mitigates the risk of serial and parallel arcs, short-circuits and ground faults along the entire PV circuit.

The PV system designer and materials procurement team must call out and provide the appropriate wiring and related materials for the job. The installation crew should review equipment installation instructions and note any unusual connections, required lugs, termination methods and crimping tools. Check with equipment guides on acceptable wire size, stranding, conductor metal and temperature rating.

Conductor joins should occur in a raceway, transition box, enclosure or a connection device approved for the wire type and environment. Connections must be solid to avoid risk to performance, pay-off, and property. Under-production disappoints end-customers, while faults reduce installer pay-off by requiring extra truck rolls. Worst, the customer's property is at risk for electrical or fire damage.

PV source circuitToday's modular push-fit, locking connectors such as Multi-Contact MC4 and Amphenol Helios H4 are easy and fast. PV source circuits can be extended and/or combined with manufactured extension cables (straight through or Y-connector), but if making your own, use OEM connection parts and approved crimping tools, methods and, if necessary, certification. Again, poor connections are a hazard. So make sure connecting cables click and can't be pulled apart. This is especially true for MLPE devices (optimizers and microinverters) because there are a lot of connections.

Keep PV source circuit wires from dangling by using wire clips. This not only looks nicer but eliminates ground fault and shock risk due to wire abrasion. It also hides the wire from sharp-toothed rodents looking for something to chew. If using metallic zip ties, be careful not to over-tighten and risk insulation damage. Likewise, don't over-bend the wiring in an attempt to make a tight coil.

Transition to PV output circuitIf the transition is in an empty junction box using twist-on connectors (wire nuts), consider water-resistant wire nuts. Use twist-on connectors suitable for the wire sizes and use the ones that have a securing spring integrated for better wire hold and conductivity. Loose connections risk serial arcs. Make sure to have properly rated connectors to avoid risking parallel arcs (wherein the 300-V connector insulation melts off the 470-V source circuit wire). Do not use wire nuts for bonding equipment ground conductors. Use a listed method for a properly bonded connection. This is typically a copper sleeve crimp and appropriate tool or ground marked terminals in an enclosure.

For AHJs requiring a more secure current conductor connection, consider water-tight pressure connectors (insulated tap connectors) instead of wire nuts. Shielding against water reduces chances of corrosion which increases resistance. Common brands include Polaris, Ilsco and PennUnion.

For pass-through and combiner transition boxes that provide conductor connection points (terminals), the ones that are spring loaded are easiest. With screw-type compression terminals, use a torque driver and follow manufacturer's instructions on how tight to fasten them. Under-torqueing risks eventual loosing from thermal cycling and vibrations. Again, loose connection risk is easy to avoid. Over-torqueing risk splaying of wire connector and potential cracking of termination housing.

Whether a simple pass-through box or a fused combiner box with AFCI and rapid shutdown, the box should include wire glands and rain-tight conduit connectors to prevent water intrusion (i.e., short circuits). The wire glands also provide strain-relief for connections inside to avoid serial arcs and ground faults.

PV output circuitAvoid damaging the nylon sheath of conduit conductors when unfurling, handling and pulling DC homeruns. Provide ample pull boxes if there are a lot of bends or high conduit fill. Take the time to cleanly ream smooth metal tubing cuts and make a secure connection to the fish tape before pulling. Reduced abrasion means less chance of ground fault or short-circuit. Don't pull damaged or used wires.

DC disconnect / inverter input circuitConnection to a string inverter usually involves installing a wiring box in addition to the inverter itself. So there are really two sets of connections to be made. An exception is the Fronius SnapINverter which includes the terminal connection block within the inverter mounting bracket.

Read installation instructions before first-time installation. Inverter wiring compartments are not all the same. Terminal connection types differ. Keep the following in mind:

The fewer the number of connections, the better. Use devices listed for the application. Consider equipment that has easy connections (module connector whips or spring clamps). While DC homeruns end at the inverter or wiring box, remember that the equipment grounding conductor still must be bonded to the grounding system. Finally, be safe out there with proper personal protective clothing. Consider DC wires to be live unless voltage is measured (both to each other and to ground) because power comes from the solar panels and potentially DC capacitors. Now go get wired!

PV source circuit Transition to PV output circuit PV output circuit DC disconnect / inverter input circuit