Choosing the right armored cable (SWA) size essentially boils down to answering three questions:

What appliances will you be using? (High or low power)

How long is the cable? (Distance from the meter box to the shed or garage)

How will the cable be routed? (Buried underground, mounted on the wall, or run in conduit)

Understanding these three things will essentially determine the appropriate size. Below, I'll translate the most common household scenarios into their answers.

First, remember this simple rule of thumb: For 95% of gardens, garages, and sheds:

Just a few lights + one or two outlets (for charging a phone, plugging in a small drill) → 2.5 square millimeters is sufficient.

Lights + common appliances (refrigerator, TV, small heater, computer) → 4 square millimeters.

With kitchen appliances (electric kettle, microwave, small oven) or high-powered tools → 6 square millimeters.

With high-powered equipment (electric heaters above 3kW, small air conditioners, slow chargers for electric vehicles) → 10 square millimeters.

The above applies to distances of no more than 30 meters. Once it exceeds 30 meters, choose a size larger.

How to determine "larger size for longer distances"?

Here's a simple guide:

Within 30 meters: Buy according to the above suggestions.

30–50 meters: Increase the recommended size by one size (e.g., from 4 square meters to 6 square meters).

Over 50 meters: Increase by two sizes (e.g., from 4 square meters to 10 square meters).

Why increase? Because the longer the wire, the more electricity is lost during its journey. Greater loss means your heater will only be lukewarm instead of hot, and the lights will dim. Increasing the wire diameter reduces loss.

Don't worry about the wiring method. In most cases, there are only two:

Many tutorials make the wiring methods sound daunting, but for the average person, you only need to distinguish:

Wall-mounted or overhead: Best heat dissipation, use a smaller wire.

Buried underground: Poorer heat dissipation, it's recommended to handle it as described above for longer distances—burying it underground is equivalent to an extra 10-20 meters of distance penalty. That is, 30 meters buried underground ≈ the loss of 40-50 meters wall-mounted.

The easiest approach: If you're burying it underground, estimate the distance by multiplying the actual distance by 1.2. For example, if the actual buried distance is 25 meters, treat it as 30 meters when choosing the wire size.

How to choose the number of cores? A simple solution:
For powering a shed, garage, or garden house → just buy 3-core armored cable.

For just a few streetlights → 2-core is fine, but 3-core is more versatile and easier to repurpose later.

The colors for 3-core are: brown (live), blue (neutral), and yellow-green (ground). Don't overthink it, you can't go wrong with 3-core.

A real-world example (no calculations needed, just the conclusion):

Scenario: You want to run electricity to your backyard workshop. It will contain: a small refrigerator (150W), a 2000W heater, several LED lights (50W), and a laptop. You'll also occasionally use an electric kettle (2000W).

The workshop is about 35 meters from the house's meter box, and the cable is planned to be buried underground.

Following the method above:

If your electricity usage falls under the category of "Kitchen Appliances," a basic recommendation is 6 square millimeters.

Since the distance is 35 meters (more than 30 meters), you need to upgrade to 10 square millimeters.

Additional impact of burying the cable underground: 35 meters × 1.2 ≈ 42 meters, still recommending 10 square millimeters.

Final answer: 10 square millimeters, 3-core armored cable.

If you insist on saving money with 6 square millimeters: When a heater and electric kettle are running simultaneously, it may cause a circuit breaker trip or low voltage. Therefore, spending a little more for 10 square millimeters will save you trouble later.

The most conservative safety options (if you really don't want to think about it):

Small shed (within 20 meters, only lights and phone charging) → 4 square meters

Normal garden office (within 30 meters, with computer, small heater, and water heater) → 6 square meters

Larger workspace (within 40 meters, with air conditioning, microwave, and multiple sockets) → 10 square meters

Charging an electric vehicle (even slow charging) → 10 square meters or more, up to 16 square meters for longer distances

Three most frequently asked questions:

Q: Will there be any problems if I buy a cable that's too big?

A: No. It's just a bit more expensive, stiffer, and harder to wire, but absolutely safe. Buying a cable that's too small will cause big problems (overheating, tripping, even fire).

Q: I'm really worried, can I have an electrician check it for me?

A: Absolutely. You can even dig the trench and buy the cable first, then just have the electrician do the wiring and testing. This saves money and is compliant with regulations.

Q: Are those "cable calculators" online reliable? A: Yes, it's reliable, but only if you accurately input the length, power, and installation method. If you input it incorrectly, the result will be wrong. It's better to use the "foolproof comparison method" I provided above, which has a lower probability of error.

In summary: For distances under 30 meters, choose 4, 6, or 10 square meters depending on the number of appliances; for distances over 30 meters, automatically increase the size by one; for underground installations, leave some extra margin. If unsure, just go for 10 square meters; it's sufficient for most family garden projects.