Measuring Current Flow in a DC Circuit Is a Basic Troubleshooting Skill for Boaters

All About Amperes in a Boat’s Electrical System Measuring current flow in a DC circuit is a basic troubleshooting skill for boaters.

By Ed Sherman December 7, 2015 We’ve recently talked about 12-Volt basics for boaters, and covered the major “players” involved in electrical circuitry – volts, amps, watt, and ohms. And we’ve homed in on the causes of electrical resistance, which in almost all cases is something to be avoided and reduced aboard boats.

Now it’s time to take a closer look at one of those key players -- the ampere. What exactly is it, and is it a friend or foe? One thing’s for sure -- unless you’re an electrical engineer, you’ll probably be left with more questions than answers if you go by the dictionary definition. Here’s what Merriam-Webster has to say:

Ampere - the practical meter-kilogram-second unit of electric current that is equivalent to a flow of one coulomb per second or to the steady current produced by one volt applied across a resistance of one ohm.

That makes it all clear right? Probably not. So, let’s take a swipe at this.

Picture amperage as the rate of electrical flow past a given point in an electrical circuit. If you think of voltage as pressure, then think of ampere as volume. Fig. 1: The Blue Sea 8110 is a self-ranging multimeter with a clamp that makes it easy to measure ampere flow in a DC circuit. Photo courtesy of Blue Sea.

Of all the players in our triad of key electrical values -- volts, ohms and amps -- I think amperage is the one I worry most about because it’s the thing that trips circuit breakers, blows fuses, and melts wire insulation. In a nutshell, amperage needs to be carefully controlled because at the end of the day this is the stuff that gets electrical fires started.

When we discuss correct selection of wire gauge for an electrical circuit, one of the terms we come across is “ampacity,” which basically describes how much electrical amperage a wire is capable of carrying without creating a dangerous situation wherein the insulation starts to melt. Fuses are rated in amperage, and it’s important to realize that the nominal rating, say 10 amps, is not the point at which the fuse will blow. In the US, the actual trip point will be somewhere between 125 and 150 percent of the nominal rating. In Europe, that trip point will often be at 160 percent of the rating. The reasoning behind this trip rate versus nominal rating disparity helps to explain what actually goes on within an electrical circuit as it functions. The bottom line is that amperage flow may not be constant, so fuses and circuit breakers, or “circuit overcurrent protection devices” as the ABYC describes them, need to provide a bit of latitude in their ratings to accommodate this fluctuation without nuisance tripping, inadvertently opening the circuit and shutting down all electrical current flow.

So now you may wonder why amperage, or current flow, is not always constant in a circuit. The answer is that some appliances, motors, and even conventional lighting will often have what’s referred to as “start-up” current. A motor at rest is going to need some extra “umph” to get it rotating. Once it gets going, things will stabilize and current draw will be fairly constant in most cases. A circuit supplying power to an incandescent light will have a variation in current from initial startup until the light filament heats up, and the heat in the filament of the bulb will add resistance. If voltage remains constant, the additional resistance will actually reduce current flow. This is a simple application of Ohm’s Law.

Measuring Amperage

Measuring amps has gotten significantly simpler over the last 15 years or so. Today we use inductive amp clamps that simply encircle the power conductor for the circuit in question and give us a direct reading in either AC or DC amps. For this task it’s best to use a meter that’s described as “self-scaling,” so that you don’t need to concern yourself with trying to guess how many amps you’re about to try and measure. Let the meter do the work there.

However, you will need to concern yourself with the direction the current is supposed to be flowing in DC circuits. One of the clamps I use is shown in Fig. 1.The BlueSea model 8110 is really all that most boaters will ever need in a multimeter. It comes with a simple pictorial instruction chart that clearly illustrates how to make all the measurements discussed in our series here at boats.com. It’s sensitive enough to measure AC or DC amps as low as 0.01 amps and up to 400 amps -- plenty of range for most all modern boats. Note that two “A” scales are shown, one with a wavy line under the letter “A,” designating alternating current, and the other with a straight line designating direct current. Fig. 2. The side of the clamp with the + sign should lie in the direction of the current flow.

The 8110, and for that matter all meters used for this purpose, will have a clamp that provides an indicator of where DC positive and negative are located relative to the meter clamp. In Fig. 2 you see my meter’s clamp with the + sign and arrow that points in the presumed direction of current flow in the circuit. In this case, the plus sign on the clamp needs to be on the side of the circuit where the DC power source is located, typically the battery side of the circuit. The arrow shown on the clamp jaw points toward the load in the circuit. Also important in this case is to only clamp the DC positive conductor to take the measurement. Fig. 3, courtesy of the Blue Sea manual, shows the proper arrangement. Fig. 3: The Blue Sea manual shows the correct alignment of the clamp to measure current moving from the power source to toward the load.

What do the ampere readings tell you?

Finding out how much current you should be measuring when you clamp your meter around the positive conductor supplying a circuit should be fairly easy. Most devices will come with an info sheet that tells you the operating voltage and either wattage or the amperage draw.

Armed with the specification when you measure for yourself, you need to know what’s implied if your actual reading is lower than specified. Think resistancehere. Either the wiring in the circuit is too small a diameter to meet the task at hand, or there’s a faulty connection at one of the termination points, or there’s corrosion somewhere in the circuit. The reading you get won’t tell you where in the circuit the problem lies, but it will definitely let you know there is indeed a problem. To pinpoint the where part of the equation, use the information in Electrical Resistance on Boats: Keep That Voltage Drop in Check.

What happens when excessive amperage flows?

Excessive amperage flow will always manifest itself as damage due to excessive heat build-up. The bottom line is that if a fuse doesn’t blow or a breaker trip, then heat damage will occur. As already mentioned, the most obvious examples of this will be burned or melted parts in the circuit. A recent report from BoatUS indicated that DC electrical fires were the cause of 43% of their insurance claims between 2009 and 2013. Fig 4: When amperage runs wild in a short circuit, the result will often be melted insulation -- and fire.

A significant percentage of those fires were caused by undersized wiring, loose or corroded connections, over-fused circuits, or circuits with no fuse at all, such as starter motor circuits where fuses are not required. Fig. 4 shows what can happen when a short circuit occurs, essentially taking the resistance out of the circuit. Again, applying Ohm’s Law in this case, if voltage is a constant and resistance goes down in value, then amperage must go up. In the case of a starter motor circuit where no fuse or breaker is required, the result will be burnt wires and melted insulation.