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Re this answer regarding wiring a garage door power point from a light is this legal I thought it wasn't but just checking.

Edit: Legality will depend on location as mentioned in answers, I worded my question wrong the answers have shown that. What I should have asked is what are the dangers of doing this.

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Legal where? Australia? –  Vebjorn Ljosa May 6 '12 at 10:53

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up vote 3 down vote accepted

As ChrisF says, check your local codes on this one. The purpose of separate circuits is to ensure that you have lights when you plug in the vacuum cleaner, hair dryer, or some other high load device. If the lights don't have any outlets, and the circuit has been properly sized, the odds of them tripping and leaving you in the dark are slim.

When adding to an existing circuit, the important question is to be sure you're not overloading that circuit. You typically want high load devices on a dedicated circuit (e.g. sump pump, AC, refrigerator). For general purpose outlets, the rule of thumb is 8 outlets on a 15 amp circuit, or 10 on a 20 amp. With lighting, I think it's limited around 10-12 per circuit, but there's a big difference if you're putting low wattage fixtures (CFL/LED) vs high wattage incandescent.

The calculation for capacity of a breaker is a target max of 80% of the breaker capacity. So 15A breaker x 80% utilization = 12A target. 12A x 120V = 1440VA. The assumption is 180VA per receptacle, so 1440VA / 180VA = 8 receptacles.

Now, if you're adding a garage door opener to the circuit, and the label says 8A, then 8A x 120V = 960VA. 1440VA target on a 15A circuit - 960VA = 480VA remaining for other receptacles. 480VA remaining / 180VA per receptacle = 2.6 receptacles allowed if all you have is a 15 amp circuit, so you shouldn't branch off a circuit that already has more than two outlets. But if one of those receptacles is you running some power tools when someone opens the garage door, there's a good chance the breaker will trip anyway.

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Good calculations –  ChrisF May 6 '12 at 11:52

The legality, or otherwise, will largely depend on where you live. In the UK up until the 1960's it was legal and common to have power drawn from lighting circuits. This was because a lot of older homes - built before domestic electricity was common - had only been retrofitted with lighting circuits.

However, the main reason for not running appliances from lighting circuits is that usually (again I'm speaking primarily for the UK) the lighting circuit is rated at a lower current than the power circuit as it has a lower load. They are protected by a 5A fuse/breaker as opposed to a 30A fuse/breaker, so if your device (in this case your garage door opener) draws more than 5A you will be overloading the circuit which will be constantly blowing the fuse or tripping the breaker. In extreme cases it could lead to fire.

It would be far safer to wire into to the higher rated power circuit.

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