Wire & Connectors

Use the right wire for each part of the job. The wire needs to last as long as the other equipment. A common mistake is to use what ever is available at the hardware store to wire solar panels and batteries. A few years later when the insulation starts flaking off the wire, the whole job has to be redone.

Comparing Common Wire types for Solar:

Wire Type
Outer Insulation
Outdoor Rated
Common Uses
Available from:
Indoor House wiring
Hardware store, etc.
Outdoor circuit wiring
Hardware store, etc.
Buried power lines
Electrical supply
In conduit
Hardware or Electric supply
Solar module wiring
Solar dealer

outdoor rated wire is rated to be buried directly in the ground, or to be exposed to direct sunlight. Any wire exposed to possible physical damage should be in conduit (generally up to 6 ft out of the ground).

Wire Size: wire is sized by American Wire Gauge AWG. Smaller numbers indicate larger wire up to 0000 or 4/0 ( pronounced "Four ought"). Larger wire is listed in MCM, which is normally only used in commercial work. Other countries use metric sizing which measures wire by its cross sectional area of copper in sq. millimeters.

Comparing Common Wire Sizes:

American Wire Gauge
Metric (Sq. MM)
Max. Current
(example only!)**
Example Uses
House circuits, solar interconnects
pump wiring, portable generator
solar array to house
small Battery cables
small house service
0 (1/0)
00 (2/0)
Battery Cables
Large Inverter cables
250 MCM
Main feed to building

** Choosing the right wire size depends on numerous factors: Continuous Amps, surge amps, volt drop, cost, insulation rating, temperature, # wires in conduit, wire type, breaker or fuse type, etc. See our wire selector spreadsheet for guidance, use the latest NEC book to get closer, and consult with your electrician and inspector for the final say on what wire size is correct.

Copper Vs. Aluminum (Cu, Al)

In solar work, most wiring is done in copper. Aluminum cannot carry as much current for its size, has greater resistance, and most importantly is prone to corrosion. Aluminum is however cheaper, so for very long runs ( over 150 ft) of very heavy gage wire (2/0 or larger) Aluminum can be a good choice. It comes in twisted 3 wire or 4 wire bundles known as URD. It must be connected using Al rated connectors, completely isolated from copper wires or connectors, and all exposed Al should be coated with special aluminum oxide inhibitor (NoAlox).

Flexibility: wire flexibility is important when wiring solar modules, especially if using a moving tracker. Stiff wire could prevent the tracker from moving properly, and over time strain harden and break. Also, generators or any location subject to movement or vibration should use flexible wire. Standard Stranded THHN wire is still so stiff it can strain the posts on batteries and inverters. Flexible wire is also easier to pull and work with, but it can be harder to make good connections. Solid core wire works much better for wiring household outlets for instance. Wire flexibility is rated in Strand Count - the number of separate strands of copper that make a single cable.

Strand Count
Good Use
Proper Connectors

Bad Use
Improper Connectors

It is also important to use the right connector with the stranded wire. Stranded wire doesn't work well with household outlets for instance, and very finely stranded cable (welding cable) can over compress and tear in some mechanical lugs (like in normal AC load centers).Solid core wire on the other hand, is very difficult to properly crimp.

Connectors. Remember, a circuit is only as safe as its weakest link. All connections need to be rated for the same high DC amperage and extreme conditions that the wire is rated for. A single weak connection can disable an entire system with dozens of connections.

Many ways of connecting one wire to another exist. Some are permanent like soldering, or crimping, some are reversible like mechanical connectors or wire nuts. Good design allows connectors to be accessed later (inside junction box, with removable cover) for inspection, testing, and retightening.

Wire Nuts:

are cheap, and work well for #6 and smaller coarse stranded wire.

Mechanical Connectors: consist of a metal barrel or bar with holes in it. A screw tightens and pinches the wire in the barrel. A screw driver or hexhead driver are used to tighten the connection to its rated torque.
Examples include the Main lugs and neutral bar in Load centers, the connectors on breakers, insulated blocks, Euro strips, etc.

What happens when you have a bad connection: If not enough metal is touching metal, or the connection is loose, or corroded, you have a Bad connection. Bad connections will not allow electricity to flow freely. Because they cause resistance, some voltage is lost in the connector as heat. This voltage drop can cause poor system performance, and other equipment to malfunction.
Bad connections do not improve with time. The heat and possible arcing, along with moisture and corrosion cause the resistance to increase. If the connector continues to overheat, the insulation on the wire can melt back and even catch on fire.

The Difference between a short and a bad connection: A short actually reroutes electricity, like a power wire touching grounded metal, or a wrench connecting the positive and negative battery terminals together. Conversely, a bad connection restricts or stops electricity. A bad connection can then come loose and touch something it shouldn't and thus become a short too!

Good Connection Bad Connection
Wire Nut Each wire is close to equal length in connection, Wires twisted until they begin wrapping around each other past the connector. Enough copper is stripped off to make a couple of bare twists without sticking bare metal outside the wire nut. Some bare wire wrapped around the insulation of the other wire, wire not twisted enough, wrong size connector used, wire nut loose, bare wire sticking beyond the wire nut. Wires can be pulled out
Mechanical Lug Correct wire is inserted completely, insulation is trimmed visibly back from connection, connector is tightened to listed torque. All strands inserted completely through connector Pulled loose, not enough metal in connection, insulation pinched in connection, not tightened, over tightened and damaged connector or wire. Too finely stranded wire used or some strands straying out of connector.
Crimp Lug Same rules as above, but lug must match wire size, and the proper crimp tool is used. Wrong sized lug, crimp made with pliers, hammer, or mismatched crimp tool, wire is loose or can pull right out of connection
Split Bolt

Wires are stripped back for entire surface of split bolt, Tightened with two wrenches,(hard to use torque wrench)
Insulated completely with soft cold shrink tape first, then wrapped with regular electrical tape after

Wires not stripped enough, Aluminum and copper touching without metal separator in between, Not tightened enough(bad connection), not taped enough (short to metal box)


Finding Torque specs for different lugs can be tough, since they change depending on the wire size too. Try not to guess at torque; manufacturers use different alloys of metals for greater conductivity that are much softer than a regular bolt, and take far less torque. Don't wait till you have broken a main lug off from overtightening, Check that Torque!

Example Torque Spec.s:

Ft-lb.s Inch- Lb.s Tool Lug Type
screw driver
small screws on light switch
3/8" drive Inch-Lb. Torque wrench
Main lugs Outback Inverter
3/8" drive Inch-Lb. Torque wrench
Main lugs Xantrex Inverter
3/8" drive Inch-Lb. Torque wrench w/ hex attachment
Main Lugs in Large AC load Center

You'll need a torque wrench that reads in Inch- Lb.s.
1 in-lb. of torque is 1 lb. of force applied to 1" of leverage.
1 foot-lb. then is 1 lb exerted against a 1 ft lever (like a wrench) around a pivot point (the screw). Finally, 12 inch-lbs = 1 ft-lb.


Crimp Lugs: are copper tubing or alloy that is pinched permanantly onto the wire by a press or hammer jig. Use the right size crimp with right wire, and the right crimping tool. If you don't have the right tool get one, or use a different type of connector.

Steps to a Good Connection:
1) Start with the right connector and wire.
2) Strip insulation from wire so that bare wire is just visible outside of the connector. For example if a lug is 1/2" deep, strip about 5/8" of wire so that 1/16" of bare metal is visible on either side of the connector. Some connectors have a strip guide marked on them to show proper length.
3) Once the insulation is removed, protect the bare metal from damage, make sure all strands of copper are in the connector.
4) Tighten the connector to the proper torque, use the proper crimp tool crimped in the middle of the lug, or twist wire nuts until the wires wrap around each other.
5) heat shrink over the connection, or wrap it in electrical tape if using uninsulated crimps.

Switches need to be rated to the same conditions as the rest of the circuit.
Warning: Regular AC light switches should NOT be used for DC! The 50 cent silent switches from the hardware store eventually will burn in the "ON" position and cannot be turned off! They are marked "AC only" for a reason.
DC rated switches are much more expensive and only available through solar dealers.