Sparky, Are you using low-voltage lighting? If so, either Hadco or Seagull(I don't remember which, maybe both)make an accessory to mount their tree lights. It's like a bracket with a stretchable strap that wraps around the tree. I guess as the tree grows(a slow process anyway), the strap stretches. As far as line voltage, maybe stainless steel screws. A marine supply store probably has them. If not, McMaster-Carr does.
There appears to be two factors to be considered here. One is the upward growth of the tree, and second is the outward or ring growth of the tree. The upward growth can be accounted for by allowing an unstapled loop in the cable, which would allow the cable to "stretch" in an upward fashion as the tree grows taller. The increasing diameter of the tree can be allowed for by installing the conduit riser several inches from the tree trunk as trees grow very slowly in diameter. Refer to 300-5 (a) and (d). Anyway, a new owner will probably have you take it all out and install something else in its place.
I thought that trees only grow from the top, so the loop would be unnecessary. I've always sleeved UF in Pvc conduit strapped to the tree with a piece of flexible at the top that goes to the box. I'd usually try to run the conduit up the back side of the tree (to be unseen) and come around the front with the flex to the fixture.
Guys; thanks! I usually will shy away from a 'live-vegetaion' install, but you know how it is, the customer is king!!! I will probably run UF( only due to line V) and provide a few expansion joints. Will the avearge oak outgrow a few joints in my carear??
I don't think the tree grows "up" so you don't have to worry about that. You haven't seen any tree swings up in the stratosphere have you? The UF inside the PVC will have a little flexibility because the cable itself is not fastened to the tree. Some slack up near the fixture would be good though. It's likely that the tree would grow aroung anything that's fastened to it over time.
I like the idea of the expansion joints in the PVC, I've never installed a tree light but I did see one where the electrician came up the "back" (side facing neighbor's property) of the tree with PVC then wrapped around the tree to the light. There was a good 2" between the box and conduit where the tree's growth had snapped the TA in the light box right off.
I did a tree install last year. I had PVC underground, used a 2' piece of flex at the base of the tree, emerging from the ground, and then coupled that to 15' of Rigid going up the tree. This went into a WP box. I used #10x 1 1/2" Stainless "Sheet Metal" screws. fasten the rigid. I did not fully tighten them to the tree, and informed the ownere that they would probably have to loosen the screws every couple of years, or the pipe would soon dissapear. I painted everything flat black, so you can't see it unless you're within 5' of it.
The only crummy part was trying to bend a 15' length of rigid to match the subtle bends of the tree trunk.
The “loosen the screws every coupla years” approach is probably the best compromise. I had to fix a bunch of these where the tree growth had destroyed the original installation. I used long SS lags and stood off the PVC a couple of inches from the trunk. I’ll go back and sell the owner some maintenance during a slow period.
I was careful to seal all openings, because the worst part was that the fixtures were full of ants that bite or sting when disturbed. [Ouch!]