Fusing, Cables & Mains Power.

See disclaimer on the introductory / index page here.


If this is not done in the right way there is a risk of fire or serious damage or, probably less dramatically, a single fault could take down more systems than is should do, perhaps everything.

Protection can be with  traditional fuses of various types or with circuit breakers (CBs) or more often a combination of both with CBs often for circuits for medium currents and fuses for low and very high currents, CB's can be less effective for the former and expensive for both.

Battery Fuses:

Batteries, or battery banks, should have primary protection from shorts on major cables etc, they will normally be of the "ANL" type, be placed as close to the battery as possible and be in a dedicated fuse holder, it is tempting to put the fuse directly onto the battery but the fuses are vulnerable to vibration or stress and if it fails you are without power so is rather risky. Holders can be for a single fuse or several, the later may be useful for a master fuse plus the individual fuses on several branches (see below) but may not be appropriate for multiple batteries (e.g. the starter and service battery) as one will inevitably require an over long cable which would be unprotected. 

In the picture below (click to enlarge) FB2 (left above the battery) is the fuse for the main service / domestic bank (for reasons explained elsewhere I have two service banks, the 2nd bank has the same type of fuse and holder located by the battery).

Sancerre's main power distribution panel. Not as neat as I
would like but not bad given growth, space etc.
The service battery can be used to start the engine if there is a problem with the engine start battery so it has to be capable of taking the current needed to start the engine plus whatever other electrics are switched on. The starter on my Yanmar 2GM20 is rated at 1.0 KW / 12V = 83.3 Amps, some head room is required and main cables are rated well above that so a 150 or 200 Amp fuse is appropriate.

It is tempting to take a cable from this fuse directly (or via a selector switch) to a bus bar on the switch panel and to other services, but consider what happens if you should short that bus bar to earth or if the cable chafes through at some point, that cable will be significantly smaller than that to the fuse and on to the starter motor and would likely melt before the master fuse blows. 

So the best thing to do is to protect cable runs with their own fuses, so on my panel above fuse MTA 2 (a smaller version of an ANL fuse which can take less current but more than standard fuses) protects the unswitched power supply to the main distribution and switch panel (used for items such as the gas alarm that should stay on regardless of the battery selection switch) and MTA 1 protects the switched supply to various places. Due to space and other factors some cables go to fuses or CB's not shown here but close by, e.g. the bilge pump fused switch and the electric toilet pump CB on the other side of the bulk head.

On the main distribution and switch panel items should be fused as appropriate, if a switch includes a CB, as most of mine do, it may also be necessary to add a fuse if the CB is rated to high, mine are rated at 20 Amp much higher than some cable so a fuse is required for these, also many units come supplied with an in line fuse which should be retained. 

On Sancerre I needed more than one bus bar to support the number of services I have, so I dedicated one bus bar for services only requiring low power and connected that to the high power bus with a suitable fuse, that might be considered over the top but by isolating clusters of equipment like this if there is a fault in one section only the single item or a small group will be lost. Fault finding is also simplified by checking which services still work and which don't and you work through the hierarchy until you find the fault which 9 times out of 10 will be a loose connection, unfortunately that does not work for the negative side.


Do not skimp on cables, and keep run lengths to a minimum! Undersized cables are a serious fire risk and "Adequate" cables, especially on longish runs result in a bigger voltage drop than "oversized" cables, if in doubt use a size or two bigger. 

35mm2 Hi-flex cable (my preference, more expensive but much more resistant to vibration and generally easier to handle although the radius of a turn will be a bit bigger than a more ridged cable)  is rated at 240 Amps and 25mm2 at 170 Amps, but that is their maximum capacity and at that the resistance and voltage drop will be high and it will get hot. Within reason this may not be a problem for short duration loads such as engine start (35 mm2 should be considered a minimum for smallish engines with starters rated at 1KW), more if the cable run is particularly long) but for charging the cable has to be capable of handling the maximum alternator output for extended periods with minimal voltage drop.

Sterling Power recommend that for charging with a run length of 0 - 1.5m, and a charging rate of 45 - 85 Amps, 25mm2 cable should be used and for a run of 1.5 - 4m, 35mm2, which is one reason the Yanmar engine uses the heavy starter cable to return the alternator output to the battery. If a single run of 35mm is not practical then a parallel run of two or more cables of approximately the same length can be used, so:
  • 1 x 25mm2 cable + 1 x 10mm2 cable = 35mm2 can used instead of a 35mm2 or
  • 2 x 25mm2 cables = 50mm2 = 2 x 170 Amp rated capacity and is OK for charging between 85 & 180 Amps over 1.5 - 4 metres and gives more headroom at lower currents.
And remember this applies to any negative cables as well as the positive.

Mains Power

This is where you may need to get professional help, I have not researched the legal aspect of working with mains power on a boat (the safety issues should be obvious!) but on land some electrical work MUST be carried out by a "competent person" as I am qualified to HNC level in electrical and electronic engineering I consider myself a competent person so did not research this. I suspect that because the shore power system is all behind a plug and socket this does not apply but I have not confirmed that.

I also suspect that the initial fit of Sancerre's shore power was not done professionally because there was no consumer unit, CB's or fuses! The CB's at least are essential and the consumer unit is where you put them. CB's should be 2 pole and you should have both MCB and RCCB/RCD types, a consumer unit with a reverse polarity indicator is probably worth having but in UK a marina is very unlikely to suffer from this, things could be different elsewhere. Chandlers to not generally stock these but they are easily obtained from suppliers to caravaners who have the same issues, I got mine on eBay.

Sancerre's shore power installation.

I strongly recommend fitting a galvanic isolator / zinc saver, for more on this see my post How to turn a 3 hour job into a 3 day one - upgrading the Battery compartment wiring. (opens in new window).

Continue to Types of Battery.

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