Just bought an older home that has a 3 prong dryer outlet. It is right above an unfinished crawl space so it would be easy to run a good grounding cable to a grounding rod and/or an iron water pipe in the crawl space. Unfortunately, the service wire does not have 4 conductors all the way back to the panel and would be difficult to replace. Is this a good idea and would this be the equivalent of modern wiring assuming the proper installation of the grounding rod? Thanks
Yes, most definitely. A few years ago NEC (the electrical code) was revised to allow this type of retrofit grounding. There is no need for the ground wire to follow the same route as the conductors.
The ground wire needs to be 10 AWG and go to any of:
- The ground bus on the same panel which the conductors come out of
- The "grounding electrode system" out of that panel (i.e. the bare braided ground wire going from the panel to the water pipes or ground rods)
- Any other circuit which itself has a 10AWG or larger ground wire going back to that panel
That last rule is important if you have other ungrounded outlets you aim to retrofit. It means, plan this grounding run so those other circuits can tap it, because this run can be their path back to the panel.
Different circuits are allowed to share ground (in retrofit scenarios) because current only flows in the moment a device is having a ground fault (for as long as it takes for the breaker to trip, milliseconds in the case of a bolted fault) -- and nobody expects two different devices to fault at the same time.
By no means an expert, but avoid using the water pipe to ground equipment. I could easily imagine a poor connection existing and a section of the water pipe being energized by the dryer in a bad day. I would put in the extra effort and run a ground wire through the crawlspace back to the service panel or ground rod (that too may not be allowed - but still better than the water pipe).