Where do I put the fuse?

It’s one of the most common questions I get asked by customers installing a new alternator. In order to answer this question, we need to establish a few facts as defined by the ABYC and BMEEA –

• The fuse or circuit breaker is there to protect the cable, not the equipment.
• The fuse shall be installed within 200mm of the source of power (with exceptions).
• The ‘source of power’ will be the battery.
• The alternator is not classed as a ‘source of power’.

“The fuse or breaker protects the cable, not the equipment”.

The fuse or circuit breaker is there to protect the cable, not the equipment. This is basically because it’s more important to stop a wire getting hot enough to cause a fire, than it is to save a piece of equipment.

“The fuse shall be installed within 200mm of the source of power”.

The BMEEA Code of Practice states in section 2.8.1

“A manually reset trip-free circuit breaker or a fuse shall be installed within 200 mm of the source of power…”.

There are exceptions to this, one of which being –

“Over-current protection is not required in conductors from self-limiting alternators with integral regulators if the conductor is less than 1m, is connected to a source of power other than the battery, and is contained throughout its entire distance in a sheath or enclosure.”.

However, as Balmar alternators are ‘externally’ regulated, they are not classed as ‘self limiting’, so that exception does not apply.

The ‘source of power’.

The ‘source of power’ will be the battery, not the alternator (although it is generating power when running, it is not classed as a ‘source of power’).

The alternator is not classed as a ‘source of power’.

Although the alternator is generating power when running, it in itself is not a source of power. If it’s attached to your engine, and that engine isn’t running, then there will be no power coming from it, whereas a fully charged battery sitting next to the engine will be a source of power, as shorting the terminals would demonstrate!

In summary, and other thoughts & considerations.

So, put your properly sized fuse no more than 200mm from the battery terminal (but outside the battery box).

It’s worth mentioning that fuses may ‘work harden’ over time, and vibration causes nuts & screws to work loose, as well as fuses to break. Ensure you maintain clean & tight connections, and perhaps replace the fuse every year or two.

If the fuse does blow, there’s a very high chance that your alternator will pop its diodes, or slightly worse. But your boat should be safe. If you didn’t use a fuse, and there was to be a fault develop, the potentially huge current that would have caused it to blow, will now be flowing through the cable getting hotter and hotter, until the insulation melts. This will then expose the bare conductors, which may cause a short circuit, or set light to something. Costly and inconvenient as it may be, if the fuse blows, you’re alternator will probably need repairing, but at least your boat shouldn’t be on fire!

So expect the worst and take some spare fuses and ideally, a spare alternator, regulator and/or service/repair kit.