Understanding basic electrical circuitry and having a clear picture in your head of what the electrical components on board actually do could save you money and time – even if DIY repairs are not on the cards. Previous 12 Volt system guides on boats.com covered the major players involved in electrical circuitry – volts, amps, watt, and ohms and considered many of 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? 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.

Measuring amperes in a DC circuit with a multimeter

A self-ranging multimeter with a clamp that makes it easy to measure ampere flow in a DC circuit.

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. 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 melting. Fuses are rated in amperage, but it’s important to realise that amperage fluctuates, so 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.

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 “oomph” to get it rotating. Once it gets going, things will stabilise 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 adds resistance. It is a simple application of Ohm's law to deduce that if voltage remains constant, the additional resistance will actually reduce current flow.

Measuring Amperage

Measuring amps has become easier 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.

Amp clamp

The side of the clamp with the + sign should lie in the direction of the current flow.

All relevant multimeters should 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, as the image below illustrates.

How to measure amperage with a multimeter

The Blue Sea manual shows the correct alignment of the clamp to measure current moving from the power source toward the load.

What do the ampere readings tell you?

Most devices will come with an info sheet that tells you the operating voltage and either wattage or the amperage draw. You need to know what’s implied if your actual reading is lower than specified. Think resistance here. Is the wiring in the circuit too small a diameter to meet the task at hand? Or is there a faulty connection at one of the termination points, or is there 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 problem, use the information in Voltage drop on boats: understanding resistance.

What happens when excessive amperage flows?

Excessive amperage flow will always manifest itself as damage due to excessive heat build-up: if a fuse doesn’t blow or a breaker trip, then heat damage will occur. The most obvious examples of this will be burned or melted parts in the circuit. A recent report from BoatUS (the American equivalent of the BMF) indicates that DC electrical fires were the cause of 43% of their insurance claims between 2009 and 2013.

Understanding and controlling amperage on board boats

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. Again, applying Ohm’s Law, 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.

For a more in-depth look at fire prevention on board, see: Onboard fire prevention: 7 safety essentials. Or for practical advice on boat electrics, see: Onboard fire prevention: 7 safety essentials.


Written by: Ed Sherman
Ed Sherman is a regular contributor to boats.com, as well as to Professional Boatbuilder and Cruising World, where he previously was electronics editor. He also is the curriculum director for the American Boat and Yacht Council. Previously, Ed was chairman of the Marine Technology Department at the New England Institute of Technology. Ed’s blog posts appear courtesy of his website, EdsBoatTips.