Is it required to electrically ground rebar in an exterior concrete slab?

Question

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)

ThreePhaseEel · Answer

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.

Is it required to electrically ground rebar in an exterior concrete slab?

One Answer

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

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