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I'm looking for information / references on when exactly it is required to ground / bond the steel reinforcement in an exterior concrete pad, and how to do so. The pad I'm planning is for a hot tub / spa in the USA / NY. I am not planning a plastic vapor barrier under the slab so it will be directly in contact with the earth (concrete over gravel over sub-soil).

Initially I was skeptical that grounding the rebar/mesh is necessary since concrete on the ground is literally grounded already. But I get that sometimes bonding is needed to avoid a potential difference and I would like to be safe & follow code.

The most detail I've read so far is quoted below but doesn't cite any code references or other qualifications about when / where its needed.

Extract from forum post noted above:

In my area, this is required.

Technically, the metal in the concrete is "bonded" rather than grounded.

The nuts and bolts procedure is to connect, and provide access to, a ground wire to the metal mesh or rebar within the slab - this wire will be connected to the common bonding point within the spa once it is installed.

...

To the common bonding point there are a number of wires from / to various locations / items, including but not limited to:

  • Metal frame of the unit

  • Motor frames within the unit.

  • Metal piping from within the unit.

  • Metal within the pad.

  • Any other metal within 5' of the spa - downspouts, metallic conduit, gas piping, water piping, handrails, etc.

  • A ground wire going to the ground bar of the panel which serves to supply the power for the spa. Could be the SEP or a sub-panel.

(https://www.finehomebuilding.com/forum/concrete-pad-for-spa)

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You need to bond the rebar (note the difference!)

The issue at hand here isn't that the rebar is way out at some voltage far above the "earth potential", whatever that means, but that that the slab and rebar can have a potential on it that's different from the potential of some other nearby conductive object (such as some part of the spa/hot tub). This potential difference means that someone touching the conductive object while standing on the slab will complete the circuit and equalize the potential between the two objects, getting "bit" in the process.

To keep your hot tub setup from growing shocky teeth, all the non-current-carrying metal parts, as well as that in-slab rebar, need to be bonded together and to the hot tub's central bonding point (located at the hot tub pump most likely) with #8 copper. The NEC will let you omit this bonding for self-contained hot tubs that are listed, labeled, and identified for outdoor use without bonding and have rims greater than 28" above all perimeter surfaces within 30" of the perimeter of the hot tub, but I would call it a wise investment to at least have a bonding provision in the slab, in case a future hot tub that gets put in requires the slab to be bonded.

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  • Clarification - is the central bonding point of the tub also bonded to the electrical ground of the power supply? (If so it sounds like it would have a similar effect as installing a second ground rod for your house.) Or is that central bonding point explicitly NOT bonded to your main electrical ground? – StayOnTarget Jul 20 '20 at 0:58
  • @UuDdLrLrSs -- the central bond point is connected to the spa's equipment grounding conductor. (It's not really equivalent to a second grounding electrode unless the slab has a full footing on it) – ThreePhaseEel Jul 20 '20 at 1:00
  • Got it, thanks. I now know what to look for! – StayOnTarget Jul 20 '20 at 1:05
  • Also, UFER grounds are so made-of-awesome that it’s a damned shame to not put one in anytime you are pouring a reinforced slab. If you ever put a subpanel out there, it’d be wonderful to have it. – Harper - Reinstate Monica Jul 20 '20 at 1:54
  • @Harper-ReinstateMonica -- actually, this is one of the cases where we have a big hunk of steel and concrete that shall not be treated as a grounding electrode, see NEC 250.52(B) point 3. (Why? Because treating pool steel as a grounding electrode turns it into a bit of a...magnet for stray utility currents, defeating the purpose of all that carefully-laid equipotential bonding) – ThreePhaseEel Jul 20 '20 at 2:48

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