I install low voltage exterior lighting. I have a customer who asked me if I would look at her post light which is 120v in line and controlled only by a photocell on the post itself. She said the post light only worked for a couple of weeks after they moved in and no longer works. I figured it was a bad photocell however when I removed the top of the post and checked the main wire, I was only reading 12 volts.
Could a damaged wire be a possible cause for such a low voltage reading? The post is about 75' from the house.

 

I install low voltage exterior lighting. I have a customer who asked me if I would look at her post light which is 120v in line and controlled only by a photocell on the post itself. She said the post light only worked for a couple of weeks after they moved in and no longer works. I figured it was a bad photocell however when I removed the top of the post and checked the main wire, I was only reading 12 volts.
Could a damaged wire be a possible cause for such a low voltage reading? The post is about 75' from the house.

I think the clue might be in "low voltage lighting" - but most up to date ones use switch mode transformers, the output AC frequency might be too high for a typical multimeter to make sense of. The optoelectronics part most likely runs off a rectified, smoothed and possibly even regulated 12V DC supply - although I have seen purely mains rated control in the form of an LDR rated for mains, that fed a number of turns of resistance wire wound round a bi-metal strip that operated the contacts.

 

When it was working, what was the light bulb? If it was a 120V bulb, then the problem might be the triac circuit that the photocell controls. These can be taken out by the current surge that causes the bright flash when a bulb blows.

ak

 

Some post lights also have the transformer and sense unit in the base and run up to the lamp with 12v.
Max.