Voltage and power can be lost from the turbine to the rectifier and from the rectifier to the load. Selecting the right wire size is critical to a good wind turbine system’s performance and safety.
There are 3 considerations in selecting a wire size for a wind turbine system:
Minimize Voltage drop
Undersizing the wire size will result in a voltage drop in the wires that will then require your turbine to spin faster in order to maintain the minimum voltage at the load (battery bank, inverter, …).
Minimize Power loss
Undersizing the wire size will result in excessive power (watts) being lost in the wires rather than delivered to the load (battery bank, inverter, …). A loss of less than 5% is desirable.
Safety
The National Electric Code (NEC) has established the safe operating current (Ampacity) for each wire gauge size, known as the American Wire Gauge (AWG). Various wire types also have defined safe voltages and environments that are important as well. The goal is to minimize accidents and to reduce the potential for fires.
For outdoor applications, use outdoor rated wire that can tolerate years of UV exposure and moisture. Most outdoor extension cords are not designed for permanent outdoor use.
Voltage Drop
Ohm’s law is used to calculate the voltage drop in a wire. The equation is:
Volts = Current (Amps) x Resistance (Ohms)
Here are the common resistance values for copper wire and their NEC “not to exceed” Ampacity:
| Average Wire Gauge (AWG) | Resistance per foot (Ohms) | Max Ampacity (Amps) per the NEC (60C) | Approx. cost per foot ($/ft) | Diameter (inches) | Diameter (mm) |
| 12 | 0.0016 | 25 | $0.25 | 0.0808 | 2.053 |
| 10 | 0.001 | 30 | $0.30 | 0.1019 | 2.588 |
| 8 | 0.0006 | 40 | $0.55 | 0.1285 | 3.264 |
| 6 | 0.0004 | 55 | $1.00 | 0.1620 | 4.115 |
| 4 | 0.00025 | 70 | $1.25 | 0.2043 | 5.189 |
| 3 | 0.0002 | 85 | $2.00 | 0.2294 | 5.827 |
| 2 | 0.000156 | 95 | $2.25 | ||
| 1 | 0.000124 | 110 | $2.50 | 0.2576 | 7.348 |
| 0 | 0.0001 | 125 | $3.50 | 0.3249 | 8.252 |
Using the table above, you can calculate the voltage drop from your wind turbine to the rectifier or battery bank using the following steps. We will use this calculation to eventually determine how much power (Watts) is lost in the wires as heat.
[A] Step 1: Distance from turbine to rectifier/battery?
Example: __100____ ft.
[B] Step 2: DC or AC turbine? _____ (2 for DC, 3 for AC)
Example: __ 3 _____
[C] Step 3: Wire size Ohms/ft from the table ______ Ohms/ft
Example: ___0.0004_ (6 AWG)
[D] Step 4: Maximum sustained rectified DC amps?
Example: ___40__ DC amps
Please note, for [D] use the maximum watts the turbine can sustain divided by your load voltage. In most cases, the load voltage is the voltage of your battery bank (i.e. 12V, 24V, 48V etc.) The maximum sustained watts is generally 50% higher than the rated watts.
Now, let’s calculate the voltage drop using the information we entered above:
Voltage drop for AC wind turbine = A x B x C x (0.67)D
Using the example above = 100 x 3 x 0.0004 x (0.67)40 = 3.22
Voltage drop for DC wind turbine = A x B x C x D
(No sample calculation because our example is for a AC wind turbine)
If you want to calculate the voltage drop from your rectifier to your load (battery), you use the DC wind turbine equation above. This is because after the rectifier, the electricity is DC. For DC calculations, remember to use“2” for Step 2 [B].
Is this voltage drop acceptable in your setup? If not, increase the wire size in Step 3 for [C] until it is.
Power Loss
Ohm’s law is also used to calculate the power loss in a wire. The equations we use are:
Equation 1 (All wind turbines):
Power Loss (watts) = Voltage Drop^2/Resistance (Ohms)
Equation 2A (DC wind turbines):
Power Loss (watts) = DC Operating Current (Amps)^2 x Resistance (Ohms)
Equation 2B (AC wind turbines):
Power Loss (watts) = 0.67 x (DC Operating Current (Amps))^2 x Resistance (Ohms)
Where Resistance = A x B x C (from above)
Since you know the voltage drop from our earlier calculation, you can calculate the power loss using the first equation. Note that we are calculating the power lost in the wires when the wind turbine is operating at its maximum sustained DC current output (i.e. maximum power):
Power Loss (watts) = Voltage Drop^2/Resistance (Ohms)
and
Power Loss (watts) = Voltage Drop^2/(A x B x C)
Example = 3.22^2/[100 x 3 x 0.0004] = 10.37/[0.12] = 86.4 Watts of power lost in wires at maximum wind turbine power
Repeat this calculation for the rectifier-to-load section of your system (if you are using a 3-phase AC wind turbine), and then add up the total power loss in watts. Compare this power loss to the power rating of your turbine. For instance, on a 1000 Watt turbine, a wire loss of 86 watts is a 8.6% loss! That is not the fault of the wind turbine! By increasing the wire size in this example to #3 wires, the power loss would be reduced to 4.8% at maximum sustained conditions. Alternatively, if it is possible, you can shorten the wire run from the wind turbine to the rectifier to 60 feet from the 100 feet we used in our example calculation.
You will have to decide how much wire power loss is acceptable to you but most people strive to stay under 5%, and many commercial systems are required to achieve no more than a 3% loss.
Increase the wire size for [C] until the total voltage and power loss is acceptable to you.
At this point, I think it is important to note that we calculated the power lost in the wires when the wind turbine is operating at MAXIMUM POWER.
Now, let’s calculate the power lost in the wires when this same wind turbine we have been using in the examples above is operating at 15 amps (not 40 amps). 15 amps is a much more realistic rating because wind turbines only operate at peak power when it is very windy (~28 mph wind). For this calculation we will use the following power loss equation from above:
Equation 2B (AC wind turbines):
Power Loss (watts) = 0.67 x (DC Operating Current (Amps))^2 x Resistance (Ohms)
Where Resistance = A x B x C (from above)
Example = 0.67 x (DC Operating Current (Amps))^2 x (A x B x C) = 0.67 x 15^2 x (100 x 3 x 0.0004) = 18 Watts of power lost
What does 18 Watts of power loss really mean? If your wind turbine is charging a 24 volt battery bank (actual battery voltage would be about 27 volts) at 15 amps then the wind turbine is producing:
Power (Watts) = volts x amps = 27 volts x 15 amps = 405 Watts
If 18 Watts is lost in the wires as heat, then only 4.2% of the power is lost in the wires. So, under normal operating conditions the power lost in the wires is very small. It is something you can and will have to live with.
Safety
There is a good chance that if you used reasonable wire sizes above to minimize power losses, then you have selected a wire size that also has not exceeded the maximum Ampacity level specified in the chart above. You’d better double-check it though!
If you are running only 2-wires from a DC wind turbine, then the value used in Step 4 [D] is the amps flowing in each wire. For instance, 40 Amps requires you use at least a #8 AWG wire size.
If you are running a 3-wire (3-phase “wild” AC) wind turbine, then the value used in Step 4 [D] can be reduced by 33% since 3 conductors are used rather than 2. For example, a maximum of 40 DC amps would be reduced to 26.64 DC amps, which now permits a #10 AWG wire to be used. This illustrates the advantage of a 3-phase/3-wire set-up.
Summary
Unfortunately, power loss and voltage drop is a fact of life. Wind turbines, extension cords, the wiring in your house’s wall all waste power as heat. Making a good wire size choice will not only be safer, it will allow you to get the best long term performance out of your system. It is one of the most beneficial areas to put your green energy dollars.