What is minimum size ground do I need for a 200 amp panel? I need to bond the hot and cold in the water heater and already have a ground rod in the ground. I also need to bond the water and gas.
According to National Electrical Code, the water piping must be bonded to the service equipment enclosure, the grounded conductor at the service (service "neutral"), the grounding electrode conductor (where large enough), or directly to one or more of the grounding electrodes. The bonding jumper is to be sized using Table 250.66. For 200 ampere service, you'll probably be using 4 AWG copper or 2 AWG aluminum.
For gas piping, you can bond it to the same things as mentioned for water piping. However, the sizing of the jumper comes from Table 250.122, and is based on the size of the circuit that is likely to energize the piping.
If you had a gas furnace that was fed by a 15 ampere circuit, the jumper could be 14 AWG copper or 12 AWG aluminum. Since the furnace circuit is the most likely to energize the piping, you'll size the jumper based on the ungrounded conductor feeding that circuit. This is a somewhat controversial topic, as some believe the jumper should be larger.
National Electrical Code 2014
Chapter 2 Wiring and Protection
Article 250 Grounding and Bonding
250.104 Bonding of Piping Systems and Exposed Structural Steel.
A) Metal Water Piping. The metal water piping system shall be bonded as required in (A)(1), (A)(2), or (A)(3) of this section. The bonding jumper(s) shall be installed in accordance with 250.64(A), (B), and (E). The points of attachment of the bonding jumper(s) shall be accessible.
(1) General. Metal water piping system(s) installed in or attached to a building or structure shall be bonded to the service equipment enclosure, the grounded conductor at the service, the grounding electrode conductor where of sufficient size, or to the one or more grounding electrodes used. The bonding jumper(s) shall be sized in accordance with Table 250.66 except as permitted in 250.104(A)(2) and (A)(3).
(B) Other Metal Piping. Where installed in or attached to a building or structure, a metal piping system(s), including gas piping, that is likely to become energized shall be bonded to the service equipment enclosure, the grounded conductor at the service, the grounding electrode conductor where of sufficient size, or the one or more grounding electrodes used. The bonding jumper(s) shall be sized in accordance with 250.122, using the rating of the circuit that is likely to energize the piping system(s). The equipment grounding conductor for the circuit that is likely to energize the piping shall be permitted to serve as the bonding means. The points of attachment of the bonding jumper(s) shall be accessible.
- If this is a multifamily dwelling, the sizing might be a bit different.
Having a big ground does not matter if all the other wires are tiny because they will melt before the ground does. As long as the ground is bigger than any of the wires in the house, then it is sufficient. Usually the biggest wire is the one going to the oven or a dryer. The amperage rating of the box does not matter at all. For example, the box is 500 amps and the biggest wire is 100 amps. Then having a 500 amp rated ground is pointless because the 100 amp wire will melt long before the ground will.
The best ground is an 8-foot copper rod you drive into the ground with a sledge hammer. Then you clamp a braided copper grounding strap to it. Good idea to put it in PVC tubing to protect it from rain. For a house, the strap typically is 1/2" wide or larger.