AC Overcurrent Protection

flowing through it generates a magnetic field) is real and may cause problems with sensitive electronic equipment, as we’ll see in the next chapter.

Marine versus Residential Materials

Many boatowners who want to add AC service to boats that came with only a simple DC setup as standard equipment, head for the nearest residential electrical supply house to get the gear for their new circuit. Some people have undoubtedly seen boats that came right from the factory with this residential gear installed; Square-D switch boxes, panels, and breakers, as well as the solid-copper wire known as Romex, have been used in original-equipment installations by various boatbuilders over the years. Virtually none of this household gear meets current ABYC standards, and it’s definitely not recommended that household AC gear be used on board your boat. In fact, if your boat has household-rated AC equipment, one of your first orders of business should be to remove it and replace it with appropriate marine gear. Remember what happened to the boat and licensed electrician mentioned earlier? You could be next!

Some appropriate exceptions to this rule on marine-grade versus household material are the commonly available plastic outlet boxes, face plates, and plug assemblies that all work just fine on boats as long as they are the all-plastic type. Even with these, however, it’s best to throw away the steel screws that usually come with the equipment and substitute stainless or brass screws instead.

Wire for AC

Most 120-volt AC circuitry on small powerboats will use 12-gauge, tinned triplex boat cable for the entire wiring scheme, regardless of the length of the run or the anticipated load. You can, of course, legally and safely use 14-gauge wire for your AC circuits (as many builders of budget boats do), but since the savings amount to about $30 for a 100-foot roll, you might as well go ahead and use the heavier and slightly safer wire. Tinned triplex with AC colorcoding (black, white, and green) is available from West Marine and most other chandlers.

I’ve said it before, and I’ll say it again: Don’t use Romex or any other solid-copper wire on your boat. If you have any already installed, replace it with proper boat cable.

As for the insulation temperature rating of AC wiring, rated boat cable from a marine supply store will probably have a 105°C rating, although there is still some 90° cable available, and many boats already wired have this lower-rated wire in use.

Figure 11-4on page 163 shows the ampacity for a group of two to three of these triplex cables bundled together. The higher ratings in the tables are specialty cables not readily available, and the lower ratings are not commonly available either. For any new work you’re doing, the 105° cable is more than adequate for typical small-boat installations.

AC Overcurrent Protection

As for rating and location in circuits of overcurrentprotection devices (fuses and circuit breakers), the same basic rules apply for AC circuits as for DC circuits. An exception is the rating and location of breakers for feeder wires from the shore-power inlet on your boat to your main AC distribution panel. Refer back to chapter 4 to refresh your memory regarding the 7–40–72 inch location rule and the 100–150 percent rule. The ABYC allows a run of up to 10 feet between the shore-power inlet and the main circuit breaker on the main feed conductors to your boat’s AC distribution panel. On boats of the size for which this book is intended (up to about 35 feet), this will usually mean the main breaker will be located on the AC distribution panel itself. For larger installations where the distance between the inlet on the boat and the panel exceeds 10 feet, a circuit breaker is required on the feed wire before it reaches the panel. These breakers must be of the trip-free variety, just like those used for DC, so they can’t be held closed by the operator. This means you must use only marine-rated circuit breakers for any replacements to existing services as well as for any new circuits. AC Circuit-Breaker Types

Circuit breakers for use with AC systems must be of the trip-free variety, as already stated. But an addi-

TABLE VIII-C-ALLOWABLE AMPERAGE OF CONDUCTORS WHEN 4 TO 6 CURRENT CARRYING CONDUCTORS ARE BUNDLED

Fig. 11-4. ABYC ampacity table for a bundled run of up to three triplex cables. (© ABYC)

tional consideration is whether or not they need to be of the single- or double-pole configuration. Almost all circuit breakers used with DC systems are singlepole breakers with two terminals on the back designed to be connected in series with the DC positive feed wire. A notable exception is with some DC panel master breakers where two single-pole breakers will be ganged together.

For all AC circuits, the main circuit breaker must be of the double-pole type. With this type, the breaker will have four terminals and be designed to simultaneously trip both the black AC positive conductor and the white AC negative conductor. This added safety measure provides protection even with reverse polarity. So in effect, even if the wiring entering the boat from the dock is set up incorrectly, the breaker will still do its job.

If your boat’s AC distribution panel is equipped with a reverse-polarity indicator, you may use singlepole breakers for any branch circuits on the panel or downstream from the panel. If you’re buying a new distribution panel to add AC to your boat, be sure to get one equipped with this reverse-polarity indicator. Any money saved on the panel by not getting this feature will be false economy in the long run—the incidence of reverse polarity at marina shore-power boxes is just too high! Figure 11-5 on page 164shows both a single- and double-pole breaker of the approved type. Figures 11-6a and 11-6billustrate how these two types of circuit breakers are wired into an AC circuit.