On Schedule !

Planning the Electrics

The first task to tackle when we arrive at our mooring will be laying in the electric cable runs so that we can get the basic wiring installed in order to start work on the lining out.

I'm no stranger to wiring things up. In my younger days I've wired up a large number of model railway layouts - all 12 volt. I've also carried out numerous bits of domestic wiring - before the recent change in the law which makes it more or less illegal now. Fortunately, no such restrictions appear to apply to narrowboats - as far as I know, but we will need to get what we do cleared by the BSS surveyor.

The boat plan sat for several days, untouched. We knew that we needed to get down to drawing out the wiring diagram so that we could order the wire in good time. However, we'd been hampered by a number of technical difficulties. The first was deciding what cable sizes to use. Voltage drop is not normally a problem on model railways or in routine house wiring, but on a 57ft narrowboat it certainly can be. So we needed to ascertain what size cables to use. Here was the first hurdle. The part of Graham Booth's book that deals with electrics was written by an electrician and he has quoted the area for wire sizes. However, everywhere we'd looked we'd found wire sizes that gave the size expressed differently.

The type of wire that is allowed to be used for electrical circuitry within a narrowboat has to be made up of a number of strands - single strand wire, similar to that used for some domestic house wiring and (domestic) telephone cables, is not permitted on narrowboats. This is because the movement of, and within the vessel may cause the wires to fracture. This, in turn may result in an intermittent connection that can lead to sparks that can cause a fire.

For 12 volt circuits the strands of the wire are generally either 0.3mm or 0.4mm in diameter. The number of strands within a wire (and their diameter) determines the maximum current that can be safely carried by that wire. As indicated above, wire sizes can be expressed either by the overall area of the collection of strands within the wire or by a measurement that indicates the number of strands and the diameter of those strands. To make matters more complicated the cable statistics appears to depend on where you shop. For example, we've seen different suppliers selling 0.5mm², 1mm², 1.5mm² 2mm² and 2.5mm² cable, but not all of them and, in addition, we've found what appear to be different maximum safe current ratings expressed by different retailers for the same size cable!

Another factor to be considered is the voltage drop over the proposed length of wire. The thinner the wire and the higher the current, the greater the voltage drop. For lights and similar things, a significant voltage drop may simply mean that the lights won't shine so brightly - although for things like fluorescent lights, a significant voltage drop may make the difference between the light working and the light not working. The greater the voltage drop, the more current an appliance will attempt to draw. The more current flowing in a wire, the greater the heat generated within that wire. The more heat, the greater the risk of a fire.

We were surprised by the wire needed for a fridge/freezer. We'd been advised that we should allow 1mm² area of wire for every metre there is between the battery and the fridge. Our calculation is that we'll have a 12 metre run and had initially thought of using 80/0.40mm (10mm²) wire. However, on reflection we'll probably end up using a pair of 7mm² wires just to be on the safe side.

Fortunately, within a couple of days of getting this far, we made a trip to see progress on the boat at the builders and were able to call in at Midland Chandlers at Braunston on the way. Here we were able to tap the brains and experience of the staff and learned that they recommended using 97/0.30mm (7mm²) cable for the television, shower and water pumps. The boiler pump, which we believe uses less than an amp and the oven ignition circuits can be in 44/0.30mm (3mm²) cable. The bilge pump, which will only be a couple of feet from the battery will probably use the same size cable for no other reason than we'll have some!

We'd anticipated - because it had been suggested to us by others - that 44/0.30mm (3mm²) cable would be sufficient for all the lighting circuits. We were gobsmacked when it was suggested that we'd need 120/0.30mm (8.5mm²) cable for the front of the boat interior circuit and for the outside lights circuit. We could probably getaway with 84/0.30mm (6mm²) cable for the centre lighting circuit and 65/0.30mm (4.5mm²) cable for the rear lighting circuit. There was a slight respite! We'd only need these thicker cables from the battery to the light switch, we could reduce these sizes to 44/0.30mm (3mm²) cable for the short runs from the switches to the lights.

Why such large cables? We were planning to have nine 20 watt halogen bulbs on each of three interior lighting circuits. This equates to around 180 watts per circuit, if all are on together, which could be possible at the front of the boat. Unfortunately, the front of the boat is the furthest from the battery and, at about 15 amps, has a relatively high continuous current load for the length. Alas. before we got thus far, we found problems locating a suitable control panel with the appropriate value circuit breakers and had to revise our plans..

We tried to locate a control panel that we could use. As will be seen from above, we needed a minimum of three circuit breakers - much more user friendly than fuses when things go wrong - that could handle 15amps. We found one control panel that had two 15amp circuit breakers, but nothing bigger. In the end we located a supplier who was prepared to populate their control panels with any combination of circuit breakers that we required. However, by that time we'd worked out that the cable sizes required for such long runs was very large and ended up splitting all the lighting circuits in two. We should add that the other reason why we need such heavy cable is that all but two of the light circuits will be able to be switched at either end of their respective areas. This makes the run a lot longer than would otherwise be the case.

We shouldn't forget the 240v side of things either. We have a Mastervolt combi unit. Apart from battery charging when we're connected to a land line, we also plan to use 240v for the washing machine, microwave and vacuum cleaner. This is in addition to any power tools that we use whilst fitting out the boat. We've decided that we'll have a single ring main around the boat, into which we will plug the relevant appliances. The wiring of this will be via 2.5mm flex, with a consumer unit near the combi. We'll also fit a galvonic isolator to help protect the metalwork from the impact of stray electrical currents from other boats.

Running the cables will be one of the first things that we do before we start lining out. We envisage that we'll start connecting up lights and sockets pretty soon after the lining out is complete as this will enable us to work a little longer during the shorter winter days. As a result, it shouldn't be too long before this web site features some more about the electrical side of a narrowboat.

In the meantime, the following two web sites do have some useful information on electrical circuits. The Vehicle Wiring Products web site has a page that lists cable sizes in both area and number of strands. The same page includes the prices that they sell the cables for. The TB Training web site offers a wealth of information, including numerous pages on electrics and is well worth a visit.

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