A simple question that’s surprisingly complex. Why? Because there are two answers, not just one.
If we’re talking about typical 110-120 volt AC wiring as commonly found in homes, shops, offices and so on, the black wire is “hot” and white wire is “neutral”. There is also a third wire, usually bare without its own insulation, which is a safety ground wire. These types of wires are found in BX and Romex /NMD90 cables. Without splitting too many hairs over terminology, that can roughly translate into black meaning “positive” and white meaning “negative”. Remember, we’re talking about AC (alternating current) here.
However, if we’re talking about DC circuits and electronics, quite often you’ll find a red white and a black wire. (Often, but not always …) In the world of DC electronics, the accepted wiring convention is that the red wire carries the positive voltage, and the black is circuit ground. Usually the red is marked as + (plus) and the black is marked as – (minus).
Notice that in both AC electricity and DC electronics, there is a black wire. But in AC electricity the black is hot (“positive”) whereas in DC electronics black is for negative.
Why? Because the worlds of AC power distribution and DC electronics developed somewhat independently, but the fact that both used black wires for different meanings was rarely an issue.
But now that low voltage LED lighting is commonplace, the question of “where to connect the black wire” has become confusing. Essentially, LEDs are a form of DC electronics – another name for LED lighting is SSL lighting, which means Solid State Lighting. As DC electronics, wires attached to LED devices usually (should) conform to DC wiring colour-code conventions.
The issue can arise when installing LEDs using an external power supply/driver, such as with LED strips (tape lights) and LED bars or even pendant lights. On the AC input to the power supply, you need to connect the hot/positive wire to the corresponding input of the power supply. But on the low voltage DC output side (12 – 24 V), the black wire means negative, and the red wire is hot/positive. If the power supply has terminals on the output side instead of coloured wires, the temptation would be to connect the black wire to the + (positive) terminal, and the red to the minus. But, LEDs being polarity-sensitive devices, the light won’t turn on! What’s more, there are even some special LED strips that can be damaged by reverse polarity connection.
AND NOW – we’ve noticed a rather disturbing trend of LED products coming out using white and black wires, with the black wire connected to positive. That’s really confusing. (China, I’m looking at you!) What’s going on?
My best guess is that there are some product engineers out there who want to “do it right” and so they go to trusty google (ok, it’s Baidu in China, as Google is blocked …) to find out what color the wires should be, and where they should be connected. And … they find some article that says black is positive/hot and white is neutral negative. But what’s missing is the fact that info is about AC WIRING only! It doesn’t apply to electronic devices like LEDs.
For LEDs, the red wire should be positive and the black wire negative. That’s it. Black wire should never carry positive in DC wiring. Got it?