AE 868
Commercial Solar Electric Systems

Conductor/Wire sizing

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Conductor Sizes

Conductor sizes are expressed in American Wire Gauge (AWG). Usually larger diameter conductors have smaller AWG numbers. There are two types of conductors, either solid or stranded. Solid conductors consist of one solid core metal conductor that is usually ridged. On the other hand, stranded conductors consist of multiple smaller conductors stranded together and are usually more flexible. Stranded conductors are ideal for PV source circuits, facilitating module removal for servicing, or junction box access.

Reflection

Since larger conductors have greater current-carrying capacity, are they stranded or solid?

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ANSWER: Larger conductors can be stiff and difficult to work with during installation. Therefore, larger conductors are stranded, which makes them more flexible.

Note:

Conductor sizes and corresponding diameters, area, and resistance can be found in NEC Chapter 9 on Table 8. You can also review the reprinted version in Chapter 11 of the textbook.

Conductor Ampacity

Conductor sizing is based on a conductor’s ampacity rating.

Ampacity is the current that a conductor can carry continuously under the conditions of use without exceeding its temperature rating.

According to the NEC, nominal conductor ampacity at 30°C is determined by the conductor material (copper or aluminum), size, insulation type, and application (direct burial, conduit, or free air). In NEC, table 310.15(B)(16) (formerly 310.16) is used to look up the ampacity rating for any conductor.

Ambient Temperature Derating

Temperature affects conductor’s ampacity; nominal ampacity is derated (reduced) when ambient temperature are higher than the nominal 30°C. Temperature-based ampacity correction factors can be found in NEC table 310.15(B)(2)(a). The correction factor is then multiplied by the nominal ampacity (found on table 310.15(B)(16) to calculate the derated ampacity. Therefore, for a certain current-carrying capacity rating, the size of the required conductor must be increased to account for higher temperature deratings.

Number of Conductors Derating

Since most PV systems consist of multiple PV source circuits that run to the combiner box, more than three current-carrying conductors can be run together in a conduit or a raceway for longer than 24′′. In this case, conductor ampacities must be further derated by an additional correction factor that can be applied to the temperature-corrected ampacity using NEC table 310.15(B)(3)(a), where designers can look up the corresponding ampacity correction factor based on the number of current-carrying conductors.

Reflection

Why is it needed to derate the ampacity when more that three current-carrying conductors are installed together?

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ANSWER: Because bundling several current-carrying conductors together affects their ability to dissipate heat.

For example, if USE-2 conductors are used for three PV source circuits and are run through a conduit, each with positive and negative conductors, the total is six current-carrying conductors. The correction factor from the NEC table is 0.80.

Note:

The neutral conductor in a three phase AC system is not considered a current-carrying conductor. We touched base on the conductor's insulators on the electricity basics in the orientation of this class, and we noticed that insulators can be different in types and materials. As a recap, insulation protects the bare conductor from coming into contact with personnel or equipment. Conductor insulation types used in PV systems must be compatible with the environmental conditions and ratings of the associated equipment, connectors, or terminals.

Reflection

What are the properties of insulating material for a conductor?

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ANSWER:

  • Maximum operating temperature,
  • Application and Environmental resistance (such as to sunlight, oil, or moisture),
  • Permissible installation locations (such as direct burial, in conduit, or exposed).

PV module electrical connections are usually installed with full exposure to extreme temperature, sunlight (UV), and precipitation. PV conductors need to be rated for outdoor applications with high temperature, moisture, and sunlight resistance. Insulation types UF, SE, USE, and USE-2 are permitted in PV source circuits, provided they have the necessary weather resistances. Single-conductor USE-2 is recommended because it has high temperature, moisture-resistance, and sunlight-resistance ratings, and is widely available. In the NEC, table 310.15(B)(2)(c) is used for ambient temperature increment due to conduits being exposed to sunlight on or above rooftops.

Conductors and color codes

Since a PV system is a mixture of both AC and DC systems, conductor color codes are similar to any other electrical system with each side of the system. For example, when we look at the DC side of the PV array, we should consider applying the DC color code. Once we exit the inverter, the color code of conductors should comply with the AC color code of electrical systems. Below are main considerations for the color codes of conductors.

  • Grounded conductor must be white, gray or have three stripes, not green.
  • Module frame ground must be bare or have green or green with striped insulation or identification.
  • Grounded conductor is the negative conductor (white). No NEC requirement for ungrounded conductor. Use AC convention Black and Red.
  • In two wire negative-grounded PV system, the positive conductor could be red or any color with red marking, except green or white.
  • In a three-wire, center-tapped system, Positive conductor Red, center tap conductor must be white and the negative conductor could be black

Note:

NEC articles 250.119 article 200.6(A)(2) are used to find the conductor color codes.

PV Circuit Current Requirements (sizing, protection)

As instructed in NEC 690.8 Circuit Sizing and Current, the wires from the PV modules to the inverter must be able to carry 156% of the short circuit current (Isc). This 156% comes from multiplying 125% twice; the first 125% accounts for the continuous duty that is used as a safety factor whereas the second 125% is a factor that accounts for the extra current that PV modules might deliver. This current, that is higher than the rated short-circuit current of PV modules, can be generated when the solar irradiance exceeds 1000 W/m2. Therefore, the maximum output current is derated by a factor of 125% x 125% or 156%.

Circuit current is the sum of the parallel source circuit's maximum current as calculated in 690.8(A)(1). This applies mostly for central inverter configuration.

PV Source Current [690.8(B)]

The maximum PV source circuit current is 125% in addition to 125% of the short circuit current. In our example, if a PV module has Isc of 8.60 A
I max = I sc ×125%×125%=8.6×1.56=13.44A

Conductor Sizing [690.8(B)(1)]

Conductor size is chosen to be the smallest size that can safely conduct the maximum conductor current of a circuit (Imax). As discussed previously, the maximum current that the PV system can generate is 156% of Isc. That means the conductor must carry this amount of current.