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Excessive Voltage Drop & Your Boats Electrical System

Posted By Ed Sherman , Tuesday, January 30, 2018

One of the things we are concerned with on boats is excessive voltage drop. This is especially true with your boat’s DC electrical system, typically 12 volts. With only 12 volts to start out with, a loss in voltage can have a really profound effect on how your DC appliances operate.

This is most important for things like your electronic equipment, navigation lights and bilge blower circuits on gasoline fueled boats. In fact, the ABYC dictates that a maximum drop of only 3% of nominal voltage is allowed.

Excessive voltage drop is caused by things like loose connections, corrosion, or sometimes by wire used in a circuit that is too small a wire gauge to handle the amount of amperage the appliance in the circuit needs to function properly.

Symptomatically, excessive voltage drop will show up as an engine starter motor that is not cranking your engine fast enough, or lights that are glowing too dimly or a fish finder that is blinking diagonally lines and other forms of mishmash on the display screen. Remember, if voltage drop is excessive, inadequate amperage is flowing through the circuit in question. Both volts and amps are closely related.

The third key player in this mix is resistance, measured in ohms. We’ll talk more about ohms in a later installment, but understand that all three of these components must play together well, and the truth is, on a boat, they often don’t. I’ve used the diagram below, which was first used in one of my very old Cruising World articles on this topic, to explain the interaction between volts ohms, and amps.

So, looking at the diagram above. Think of volts a pressure, amps as rate of flow, and ohms, well you can see what he’s always trying to do, especially on your boat. With more pressure, you can actually distribute high amperage through a smaller pipe (wire), but if voltage is a constant, and too low, amperage is just not going to get through the ohm guy. OK, so now lets see how to use your voltmeter to measure something besides how much voltage your boat’s battery has available.

In the diagrams below, I’m showing you how to measure voltage drop at different points in a starter motor circuit. In this case the symptom is slow cranking. You’ve already tested the battery and you know it is in good shape, and up to charge.

In the above example, the meter is set to the DC volts scale. The black lead from the meter is connected to your battery’s negative terminal and you are tracking through the circuit with your red lead. You are looking for a voltage reading of something in the order of 0.1 or 0.2 volts at any given point in the circuit. The measurement is the amount of voltage drop in that particular leg of the circuit. It is cumulative, so if you want to truly isolate a given leg, put your negative lead on one side of the starter button and the positive lead on the output side of the starter button and depres the button. A voltage reading of greater that 0.2 volts tells you that the starter button switch has too much resistance through it, causing excessive voltage drop. Your slow cranking problem could be caused by the switch itself. A well connected starter circuit with everything in good order should not have a cumulative voltage drop of more than about 1 volt with the engine cranking. This methodology will work on any electrical circuit.

Measuring Amps

With amperage, its nice to know ahead of time what the normal current draw is for a given circuit. Some electrical appliances will give you this information, it’ll be on a label on the appliance somewhere. (Actually required on AC appliances). The appliance might only tell you the operating voltage and the wattage of the appliance. That’s OK, because with those two bits of information, you can easily calculate the third bit you need. A water heater for example might be rated at 120 volts, 1500 watts. By dividing the watts by the volts you can derive the amps needed to run the appliance at its maximum potential. 1500 divided by 120 equals 12.5 amps. OK, but what if you turn on the heater and clamp the AC hot wire (black wire) and measure less than 12.5 amps? Whats implied?

Here’s where your thorough understanding of all of this becomes important. There are several possibilities here and you will need to decide which it is. The water heater above may only be rated to 500 watts, or the cable supplying the unit is too small to carry the amperage required. If its neither of those two things, it is possible the heater element is faulty, in which case the element will need to be removed for further inspection. The tables below show some typical requirements for popular on board appliances, both AC and DC. Use these as a rough guide. These tables come from two of my books, The Powerboater’s Guide to Electrical Systems and Advanced marine Electrics and Electronics Troubleshooting. 

Tags:  AC and DC electrical system  Electrical systems  voltage drop 

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