ASK THE EXPERTS: Aluminum or Copper Wire?

Intermediate

I need to run a cable between my MidNite Solar combiner box (with breakers) and my EPSolar 60 A PV controller. The cable run is about 150 feet. I would like to use ACWU cable (aluminum) since it is much cheaper than NMWU cable (copper). Can I use the ACWU with DIN rail combiner terminals, and the controller terminals?

If not, could I use ACWU for the majority of the run and then, at each end, use copper wire in junction boxes at the combiner box and at the inverter? The Cu and Al wires would connect in the junction boxes.

Pierre Levasseur • via email

Aluminum wire has one big advantage over copper, and a few minor ones. It also has major disadvantages; in most solar applications, these negatives outweigh the positives.

The primary reason people consider aluminum wire for PV systems is the significantly lower cost compared to copper. Other minor advantages include lighter weight for long wire pulls, flexibility, and price stability—copper prices fluctuate widely on the commodity markets.

However, the disadvantages of aluminum wire include that:

  • It must be sized larger than copper wire, commonly one or two American Wire Gauge (AWG) sizes larger. As such, it requires a larger raceway and more space inside electrical boxes. The additional cost of upsizing wire size is also a factor.
  • Aluminum oxidizes more quickly than copper, especially when exposed to moisture and humidity, and in contact with dissimilar metals like copper. This appears as a white, powdery substance on the metal and will result in a high-resistance connection, potentially causing overheating and posing a fire hazard. All aluminum wire connections must be treated with an anti-oxidization compound during installation.
  • Splices joining aluminum to copper wire must be made with special (and expensive) connectors rated Al/Cu, such as Alumiconn, Burndy, and Polaris. These connectors are filled with antioxidant compound. Filling standard wire nuts with antioxidant is not safe, and does not meet National Electrical Code requirements. Crimping lugs to aluminum wire must be done with special crimps and a special crimping tool.
  • Thermal expansion and contraction of aluminum wire is significantly higher than copper, so connections can gradually loosen over time. These connections need to be inspected and retorqued on a regular basis.
  • Aluminum wire is not as malleable as copper, and is susceptible to breakage from repetitive bending. Any nicks to the wire made while stripping the insulation can cause breakage very easily, so great care or special strippers must be used.
  • Most PV equipment lugs are not rated for aluminum wire, so an Al/Cu pigtail (terminating in an enclosure) must be used.

MidNite Solar products are not rated for use with aluminum wire. Equipment terminals must be specifically labeled Al/Cu to be compatible with aluminum wire.

To avoid the hassle and potential risks of using aluminum wire, first reduce the amperage (and therefore the wire size) of your PV home-run by wiring the modules into strings of the highest voltage possible for your controller—in this case, an open-circuit voltage (Voc) of 150 VDC. Apply PV module temperature corrections before choosing the string voltage; most likely your maximum will be between 130 and 135 volts per string, but, in cold climates, it could be as low as 120 V. Then, you could run standard THWN copper wire in conduit. The reduced wire size may make the copper more affordable, especially considering the extra maintenance, the cost of special connectors, and the extra junction boxes you’d need to transition from copper to aluminum and then back to copper. I would not recommend NMWU wire for your application, as buried wires not in conduit or metal sheathing are vulnerable to damage from burrowing animals, even when at proper depth.

Aluminum wire does have its place in PV systems, including long PV home runs at high amperage; long well-pump runs; and utility service entrances. But, in most cases, the extra installation hassle, inspection, maintenance, and cost of special connectors don’t add up to much cost savings.

Dan Fink • Buckville Energy Consulting

Comments (1)

tyme2par4's picture

Your fourth bullet is incorrect. Yes, aluminum does have a higher thermal expansion rate, but your connections do not require periodic retorquing. Repeated tightening of a mechanical connector will actually damage the wire over time and cause a failure.
Connectors that are rated for aluminum wire should be properly torqued as per the manufacturers specs, and left alone. Unless there is significant vibration in the area, the connector will perform fine without further maintenance.

Show or Hide All Comments

Advertisement

X