Why do I need such thick wires in household electrical installation?

Question

I am running electricity (for light and a couple of outlets) into my garden. I am trying to decide on which wire thickness I need to choose. FWIW, I am in Europe.

The whole house is wired with two types of wires: outlets with 2.5 mm² (roughly 13 AWG) and ceiling lights and corresponding switches with 1.5 mm² (roughly 15–16 AWG). All breakers are rated at 16 A, and there are two groups of them, each protected by a 20 A breaker with 30 mA RCD.

Since 2.5 mm² wires are much stiffer and hence difficult to run through conduits (and it's not a common practice to use stranded wires for wall installations in Europe), I am inclined to run 1.5 mm² wires into my garden.

However, as the existing installation and googling show, it is a common practice to use 2.5 mm² or thicker wires for everything but lighting, and 1.5 mm² for lighting. I wonder why? Aren't these standards outdated?

My logic is as follows. A 1.5 mm² wire is rated for 16 A (I used this tool, and entered 30 °C, PVC insulation, 3 elements in confined bundle). At 230 V, this gives the maximum power capacity of 3680 W. The highest power device I was able to think of for a garden is Kärcher K7, which is rated at 2.8 kW. Thus, 1.5 mm² wire should be more than adequate.

Obviously for modern LED lighting, even 1 mm² is ridiculously thick, based on such calculation.

Another aspect to consider is the voltage drop. Copper's resistivity is 17 nΩ·m. Hence a 1.5 mm² 20 m cable has a return-path resistance of R = rho × L / A = 0.45 Ω. It will cause, at full current, a voltage drop of 7.2 V, which is probably perfectly fine for all modern devices. A connected LED bulb might be more picky, but it consumes only 10 W, so the voltage drop will be much less, 10 / 230 × 0.45 = 0.02 V.

Where am I wrong in my consideration?

Answer 1

TLDR: Because they said so.

I can't speak for Europe but I bet they arrived at their standards the same way the US did. The standards are not outdated. Much the opposite: they are extremely well-refined.

It doesn't make an sense to someone with an electrical engineering background. Code seems to ignores a bunch of basics. True. The authors of the electrical code care about a whole bunch of other things. They are motivated by a mountain of fire investigation reports and personal injury reports, and trying to improve safety in both new construction and hundred of millions of existing structures with a mishmash of old or bad work. And electricians and homeowners whose skills and intelligence vary and need to be able to work on others' stuff. They have pressure from builders to cheapen work, from manufacturers to make things cheaper or old-work-compatible, from the insurance industry who want fewer fires, and lawyers looking to cover their clients' tails.

For instance in the USA, loop wiring to common lamps and receptacles is always done with 14 AWG (2 mm²) and 12 AWG (3.3 mm²), with 15 or 20 amp breakers, and the minimum size for any architectural wiring is 14 AWG. Why? Because they said so. Because they did the math and shrinking down to 16 AWG (1.3 mm²) just doesn't save enough money in copper to be worth the risks, e.g. of someone using it in receptacle circuits and/or putting the wrong breaker on it. These are the kinds of considerations that dominate the writing of electrical code. They make no sense and will drive you crazy until you get the rhythm of it... and then it'll make a lot of sense. Bear with it.

I don't know what Europe is like with all the different standards... but America is thick with idioms. You can't have ungrounded large appliances (except NEMA 10!) You have to splice inside electrical boxes (except this!) NM is allowed in homes but not in businesses. Wirenuts and stabs are allowed in USA but not in Europe. On and on it goes.

Answer 2

However, as the existing installation and googling show, it is a common practice to use 2.5 mm² or thicker wires for everything but lighting, and 1.5 mm² for lighting. I wonder why? Aren't these standards outdated?

Home wiring has been standardized over the last 100 years. More and more loads are being added to receptacle circuits while lighting is actually becoming more efficient. This is why the lighting is normally done with smaller wire. This is reflected in your minimum size breaker at 16 amps for 1.5 mm² wire. Ours here in the US is a 15 amp breaker with a #14 wire.

Obviously for modern LED lighting, even 1 mm² is ridiculously thick, based on such calculation.

However, this is sized for the entire circuit not for individual loads. So, since LED lighting does draw so much less power it just means you can now wire a house with many more lighting outlets on a single circuit. This frees up more circuits for the appliances that modern humans loves.

There is a method to the madness and if you spend enough time in the industry you will overcome the learning curve.

Have fun in your garden!

Answer 3

A bundle in confined air is not the same as a bundle in e.g. fibreglass insulation. The latter will get much hotter.

I don't know about Europe, but for 2x1.5mm^2 conductors (you don't have to count ground), ASNZS3008.1.2 (assuming 30C air, 15C ground, 75C insulation) allows only 10A if they are completely surrounded by thermal insulation (which they may well be if it's going through your house to get to the DB. 16A if only partially surrounded). If buried in a wiring enclosure underground, that more than doubles to 23A.

There are legal limits on voltage drop; here it is 2.5% to the distribution board, then 2.5% for the final subcircuit. 2.5% of 230V is only 5.75V. Might be different in Europe.