Wiring and Cabling – SS1 Solar Photovoltaic Installation & Maintenance Lesson Note (Second Term)

Wiring and cabling in a solar system means choosing and fitting the right cables to carry electricity between the panels, the battery, the inverter and the appliances.

• DC cables: run from the solar panels to the controller, battery or inverter.
• AC cables: run from the inverter to the appliances.
• Cable size: depends on current, distance and material.

Behavioural objectives

1. State the difference between DC and AC cabling in a solar setup.
2. Give the colour codes used for DC and AC cables.
3. Explain why cable size matters and define voltage drop.
4. List the factors that affect the choice of a cable.
5. Name the common cable types and fittings used in a solar system.
6. Describe safe and neat cabling practices.

Where do the cables run in a solar system?

Cables join every part of a solar system, and each run carries either DC or AC electricity.

• Panels to controller: DC cable, usually outdoors on the roof.
• Controller to battery: DC cable, short and fairly thick.
• Battery to inverter: DC cable, very thick because the current is very high.
• Inverter to appliances: AC cable, the same type used in house wiring.

What is the difference between DC and AC cabling?

DC cabling carries direct current from the solar side, while AC cabling carries alternating current to the house side.

• Direct current (DC): electricity that flows in one direction, as from panels and batteries.
• Alternating current (AC): electricity that changes direction many times each second, as from the inverter and the grid.

What is DC cabling?

DC cabling is the wiring from the solar panels to the inverter or battery.

• UV resistance: the wires are outside on the roof, so the cover must be UV-resistant. Sunlight will not then bake and crack the plastic.
• Colour code: red for positive (+) and black for negative (-).

What is AC cabling?

AC cabling is the wiring from the inverter to the TV, fans and bulbs.

• Standard: it is the same type of wire used in normal house wiring.
• Colour code: usually brown or red for live, blue or black for neutral, and green/yellow for earth.

How do DC and AC cabling compare?

Important rule: never use regular indoor AC wire for solar panels on the roof. The cover can peel and cause a fire.

What cables and fittings are used in a solar system?

A solar system uses PV cable, battery cable and AC cable, joined with connectors, lugs and protective devices.

Cable types

• Solar (PV) cable: UV-resistant, weatherproof DC cable for the panel strings on the roof.
• Battery cable: thick, flexible copper cable between the battery and inverter.
• AC cable: ordinary house-wiring cable from the inverter to the distribution board and sockets.

Fittings and devices

• MC4 connectors: plug-in connectors that join solar panel cables safely.
• Cable lugs: metal ends crimped onto thick cables so they bolt firmly to battery and inverter terminals.
• Fuses and circuit breakers: cut the supply if the current becomes too high.
• Conduits and trunking: plastic pipes and covers that protect the cables.

How do we choose the size of a cable?

The size of a cable is the cross-sectional area of the copper inside it, measured in square millimetres (mm²).

• Choosing a wire size is like choosing the size of a water pipe.
• Forcing a lot of water through a tiny straw is difficult, and the straw might burst.
• Forcing a lot of current through a thin wire makes the wire very hot and wastes power.

What is voltage drop?

Voltage drop is the loss of voltage along a cable because of the cable’s resistance.

• Cause: a thin or very long cable has more resistance.
• Effect: the voltage that reaches the inverter or appliance is lower than the voltage sent out.
• Result: lost power, a hot cable and poor performance of the equipment.
• Cure: use a thicker cable or a shorter run.

Why do some cables need to be thicker?

Current is found by dividing power by voltage: current (A) = power (W) ÷ voltage (V).

• Example: a 1200 W load on a 12 V battery draws 1200 ÷ 12 = 100 A.
• Same load at 24 V: it draws 1200 ÷ 24 = 50 A, so the current is halved.
• Meaning: low-voltage battery cables carry high current, so they must be very thick.

Which wire size suits which job?

What factors affect cable choice?

Three main factors affect cable choice: the current, the distance and the material.

1. The current (amps): the more amps, the thicker the wire must be. At the same battery voltage, a 5 kVA system needs much thicker wires than a 1 kVA system.
2. The distance (length): the longer the wire, the more voltage is lost. For panels far from the batteries, use an extra thick wire.
3. The material: most solar cables are copper because it carries electricity very well.

• Avoid CCA: copper clad aluminium wire is cheap, but it does not last and it gets too hot.

How do we keep cabling safe and neat?

A safe installation is a neat installation, because poor workmanship is a common cause of solar fires.

• Conduits and trunking: never leave cables hanging or lying on the floor. Plastic pipes and covers protect them from rats and from people tripping.
• Tight connections: a loose wire in a terminal will spark. Sparking creates heat and can start a fire. Always tighten your screws.
• Labelling: put small stickers on wires, for example “From Panel 1” or “To Battery”. This helps whoever repairs the system later.
• Correct polarity: join positive to positive and negative to negative. Wrong polarity can damage the controller or inverter.
• Isolate first: switch off breakers and open the DC isolator before touching connections. Panels give voltage whenever there is light.

Cable safety checklist

• Is the cable thick enough for the load?
• Is the solar cable UV-resistant (for the roof)?
• Are all the connections tight and firm?
• Are the cables protected from rats and rain?
• Are the cables labelled and the colours correct?

Evaluation

1. Define voltage drop and state one way to reduce it.
2. Why must DC cables on the roof be UV-resistant?
3. Compare the cable on a phone charger with the cable on an electric pressing iron. Which is thicker, and why?
4. A battery is 20 metres from the inverter instead of 1 metre. Do you need a thicker or thinner wire to keep the power strong?
5. You see a red wire and a black wire from a solar panel. Which is likely the positive (+) wire?

Assignment

In your notebook, list the three factors that affect cable choice and explain each in one sentence. Then write three safe cabling practices from this lesson.