Q I am providing a service for a 75-ft Christmas tree with a 6 ft star on top. This tree is on residential property and sets about 30-ft from a garage. The service consists of a six-circuit outdoor panel with a 3-ft trough and four GFCI 20A receptacles mounted on pressure-treated 3/4-in. plywood. I will be running out No. 6 conductors from the garage through PVC piping.
The 6-ft star is made up of re-rod and I am concerned about lightning strikes. The star probably should have been made out of nonconductive material. I planned on attaching a grounding device to the star and installing a No. 4 copper insulated conductor down the tree to one or more ground rods. Unlike a building, it's not possible to set up perimeter ground rods.
Here are my questions:
1. Because of potential lightning strikes, should I attach the service to the tree or tree stand?
2. Should the service and tree grounding be tied together or each on its own?
3. If the tree grounding should be separate, would one ground-rod be sufficient?
4. I will be running out a ground conductor with the feeder conductors. Are there any other circumstances that I should be aware of?
A First, a few issues should be clarified. Since your power source is the garage, the supply to the tree is most likely a feeder rather than a service. I cannot determine the proper treatment of your proposed lightning protection scheme because the scheme does not comply with NFPA 780, which is the Standard for Installation of Lightning Protection Systems. Appendix F of this standard provides recommendations for protection of trees, and the No. 4 copper downlead conductor does not meet the requirements of Chapter 3. The star should be equipped with a listed air terminal.
Question 1: Section 410-16(h) specifically permits trees to be used for the support of fixtures and "associated equipment." To the extent that the panelboard and receptacles are considered to be associated equipment, I suppose they could be mounted to the tree. However, my personal preference would be to mount the panel on the garage and extend the branch circuits and GFCI receptacles to the tree. Either way, a direct lighting strike to the tree could damage some of your electrical equipment.
Question 2: Lightning ground terminals and power grounding electrodes must be separate systems. This requirement can be found in Section 250-60. However, bonding of electrical equipment on the tree to the lightning system and bonding of any separate electrodes is required by Section 250-106.
Question 3: One ground rod for the lighting protection system is probably not sufficient, at least not according to NFPA 780. Under NFPA 780, the number and type of electrodes depend on the soil conditions. Ground terminals are covered in Section 3-13 of NFPA 780. Again, Appendix F makes specific recommendations for trees.
Question 4: The grounding conductor run with your feeder (or branch circuits) should be bonded to the lightning downlead. In addition, any metal bodies or wiring methods used in the tree should be bonded to the lightning system to reduce the damage that may occur due to sideflash or currents induced by a lightning current. These requirements are generally stated in Section 250-106 and the fine print notes found there. Specific requirements are found in Sections 3-20 and 3-21 of NFPA 780. I hope this is helpful.