Comparisons between AC and DC Circuits

to make the higher voltage, but these systems are used only on larger yachts, and a discussion of them would go beyond the scope of this book. I’ll focus on only the most common, single-phase 120-volt, 20- and 30-amp systems here.

The three colors used with AC in the United States are black, white, and green. The black conductor is used only for the AC ungrounded (positive) lead. Some people refer to this as the hot lead. This explains the trend toward using yellow as a DC negative conductor in accordance with the ABYC’s recommendations for DC systems. There is an obvious risk of confusing the hot AC positive wire with the relatively inert DC negative wire, which has traditionally been black. Increasingly, new-boat builders are moving to the yellow DC negative wire as a means of more clearly separating the AC service from the DC service. Manufacturers are now making the yellow insulated wire available in all gauges including battery-cable sizes.

With AC, the white lead should always be the negative, grounded lead, and green should be the grounding lead, which offers shock protection to the boatowner. This green wire, which does not normally carry current, is one of the most important keys to preventing the “zap” you could receive from electrical appliances on board if they are not properly connected or if a fault occurs anywhere in the system. This, coupled with the fact that appliances will work just fine whether this wire is attached or not, is why ensuring continuity throughout the circuit all the way to the shore-power box is crucial. ISO Color Coding

If your boat is not built to U.S. specifications, there may be some variations in the color-coding scheme described above. The International Standards Organization (ISO), whose standards are used in many countries, prescribes the following colors for use with AC installations:

For hot conductors (ungrounded), either black or brown wires must be used.

For neutral (grounded) conductors, either white or light blue wires must be used. For ground (grounding) conductors, either green (as with the U.S. system) or green with a yellow stripe may be used.

If your boat is wired using the ISO system and you’re making additions or modifications, it’s an excellent idea to stick with the existing color code rather than changing or mixing the codes. However, in the United States you might have trouble finding wire in ISO colors, in which case you should improvise your own coding system as described in chapter 2. Reverse Polarity

Reversal of the black and white leads on an AC circuit creates reverse polarity, where the white wire becomes the hot lead and the black wire becomes the grounded lead. This condition can destroy polarity-sensitive equipment, such as motors, TVs, and microwave ovens, and it creates a serious shock hazard.

The bottom line here? You need to be certain that wiring color coding is matched and appropriately connected through every inch of the AC circuit, from the shore-power source all the way through your boat.

Testing for Polarity

Many newer boats with AC distribution panels have a polarity-test button right on the panel. If you don’t have one of these panels, you can check polarity with your multimeter every time you plug in, or you can buy a simple and cheap circuit tester (see figure 11-9 on page 167) that plugs into any standard outlet. If the indicator lights on the tester don’t light up in the proper sequence, you know you have reverse polarity somewhere in the circuit, and you must shut down the circuit until you find it and fix it.

Figure 11-1on page 160 illustrates typical AC wiring connections from the shore-power inlet on your boat through the AC panel to a standard outlet.

As with the DC circuits discussed throughout this book, things like amperage and voltage are major

Underground Conductor (Black) Grounded Neutral Conductor (White) Grounding Conductor (Green)

Shore Power Cable Connector

Shore Connection

Shore Power Cable Black White Green

Power Inlet (Electrically insulated from the boat if isolator is installed) Main Shore Power Disconnect Circuit Breaker

Branch Circuit Breaker (Typical)

120 VAC Device 120 VAC Grounding Type Receptacle

2 Pole, 3 Wire Grounding Type Plugs & Receptacles Shore Side Boat side Optional Galvanic Isolator Polarity Indicator

To Engine Negative Terminal or its Bus

Fig. 11-1. Typical shore-power wiring diagram through the AC distribution panel to an outlet. (© ABYC)

considerations when wiring for AC. Unlike DC, however, voltage drop through the circuit is really not much of a factor in AC circuits. It’s not that voltage isn’t lost as it finds its way through an AC circuit, but for the lengths of wire runs used on the small boats discussed here and because of the physical considerations cited above, voltage drop is an insignificant factor—so insignificant that the ABYC doesn’t even take it into consideration in its electrical standards for AC circuits. Wire sizing of AC circuits is easier as a result of this disregard for voltage drop, with amperage requirements being the only consideration when designing circuits.

Wattage, however, is an additional element we must take into account here. In chapter 1, you’ll recall, I discussed the Ohm’s law equation and an additional variation for watts, the unit of electrical power. If needed, go back and refresh your memory on this process, because the wattage equation really has some practical use when dealing with AC.

In the United States, as a requirement of the Underwriters Laboratory (UL) rating of AC equipment, each electrical device must be marked with either the operating voltage and amperage, or the operating voltage and wattage. Typically wattage is given. Knowing this, you can easily find out the amperage of a given appliance by dividing wattage by the operating voltage.

This vital information is step one in the determination of overcurrent protection ratings as well as wire sizing. No adjustment, or de-rating, of the ampacity of the conductor for length of wire run is necessary as it is with DC circuitry.

With AC, a different criterion comes into play. Heat generated by whatever electrical resistance is present will require de-rating the wire gauge. Bundling of AC conductors requires that wire sizes be increased. Also, as with DC, wires routed through engine rooms must be larger than those used outside the engine room, to deal with the higher temperatures.

Figure 11-2shows what a typical bundle of AC ca-

Fig. 11-2. An AC wiring bundle. Wire tie bles would look like in your boat. Figure 11-3is a table from the ABYC’s section E-11 showing the ampacity of a typical length of triplex AC boat cable.

Remember that the green conductor does not normally carry current and is therefore excluded from the process.

Another common question that comes up has to do with routing AC and DC wiring in the same bundle. Although the ABYC allows this practice as long as the wiring in question is separated by an appropriate sheath, which can be the outside skin of a typical length of AC boat cable, it’s much better to keep AC and DC wires in separate bundles. The possibility of cross-induction (remember, any wire with current

TABLE VIII-A-ALLOWABLE AMPERAGE OF CONDUCTORS WHEN NO MORE THAN 2 CURRENT CARRYING CONDUCTORS ARE BUNDLED

Fig. 11-3. ABYC ampacity table for a single run of triplex. (© ABYC)