A.Cliff Popejoy, a licensed electrician in Sacramento, Calif., replies:
I agree with your electrician. The smaller-gauge grounding wire that was installed in your house should serve just fine to clear a short or a fault. But to do so, the wiring needs to have been correctly installed, and it needs to be in good condition.
I infer from his comments about the new panel that your electrician is installing arc-fault circuit interrupter (AFCI) breakers for most circuits. AFCI breakers, which are required by the National Electrical Code for new construction, will trip under the same three conditions as regular breakers:
- If there's a short circuit (unintentional contact between an energized or "hot" wire or part and the return "neutral" conductor);
- If there's a ground fault (contact between something that's hot and something that's grounded); and
- If there's an overload (more current flowing than the breaker and wire are rated to safely carry).
But an AFCI breaker will also trip in two other situations: 1) With ground fault current of 30 milliamps or more; and 2) with certain current patterns indicative of an arcing fault. An arcing fault can create a lot of heat (think arc welding on a smaller scale), but the current flows in spikes of such short duration that it usually won't trip a standard breaker. With AFCI breakers, the 16-gauge ground wire should be adequate.
In a conventional circuit equipped with a ground wire, if there's a problem with something connected to the circuit where the hot wire is touching an exposed metal part, the ground wire is there to carry a fault current back to the panel. In doing so, it keeps the metal parts of the tool, table lamp, or appliance at zero volts, and therefore safe. If enough ground fault current flows, the circuit breaker will trip.
Even without AFCI breakers, the 16-gauge ground wire should be adequate in most instances to clear a short, a fault, or an overload. The 16-gauge wire is more fragile than 14-gauge wire and is more apt to break either while making connections, or later, from being flexed over time as things are plugged and unplugged in an outlet that is loose. So there is some advantage to replacing all of the wiring, but the advantage of having a more robust ground wire has to be weighed against the cost of rewiring the house.
What I'd be more concerned about with the 16-gauge ground wire is whether the connections ("splices," in electrician-speak) in the ground wire are all good. Every circuit that serves receptacles and lights is made up of segments of wire (inside cable in most houses) that run from box to box. And in each box the grounds are spliced together (as are the hot and the neutral wires).
With a hot wire or a neutral wire, a bad splice can give users some warning before it fails by causing flickering lights, a buzzing noise, or a burning smell. But the ground wire is different because it only carries current when there's a ground fault. A bad splice in the ground wire keeps it from carrying the fault current adequately, so the breaker won't trip as quickly as it is designed to, which can lead to a fire.
To check the condition of the circuit, you need a specialized tester that measures voltage drop in the circuit when a controlled load is applied. Running a voltage-drop test will tell you the condition of the wiring and the integrity of the splices without having to open every box in the house to check the connections. Plus, the tester can tell you if there are bad splices that you can't see, such as in a hidden junction box, or worse, not in any box at all. These testers are expensive, but a good service electrician who does troubleshooting and repair work should have one and should also have enough experience to interpret the results of the test.