Project Notes

Sep 16, 2010

Guidance on a variety of marine electrical issues...

We hope that 'Project Notes' will eventually build into a valuable resource. At first it will be a small collection including basic advice about Marine Cable, Fuses and Circuit Breakers, Calculating Current and Marine Inverters. This should appear soon and is just a modest start but we will be adding more topics over the coming weeks and months.

If you have any ideas or suggestions for other Project Notes which you think would be helpful to you and our other website users, please email your ideas to ask@boatelectricals.co.uk

Marine Cable

Sep 15, 2010

Why use marine cable? Voltage drop. Selecting the right cable thickness.

Marine (tinned) Cable 

Automotive cable is cheaper than marine cable and is easier to obtain, so it’s tempting to use automotive cable on your boat – this is a mistake. Un-tinned copper cable corrodes rapidly in the marine environment, turning black so that some connections become unreliable. The current capacity of the cable is also reduced. With marine cable each strand of copper is coated with a thin layer of tin which prevents the copper from corroding in damp conditions.

Voltage Drop

All cables have an electrical resistance; this causes a voltage drop to occur as some of the current is converted into heat. In an extreme situation the temperature increase could result in a fire hazard. The thicker a cable is, the better it conducts electrical power and the higher the current (amps) it can safely carry. By the same token, thicker cable, given the same current (amps) will result in less voltage drop than thinner cable. Furthermore any loss of voltage increases in proportion to the distance travelled by the current; this must be measured along the entire circuit, i.e. double the cable length.

Any rise in cable temperature caused by bundling of cables or by external factors such as ambient air temperature will also result in an increase in resistance, e.g. additional allowances should be made for cables in hot engine rooms.

The significance of any voltage losses is greater with low voltage (12v and 24v) systems. So, more care must be taken when working with 12 volt or 24 volt systems to ensure that voltage losses are minimised, a 3% loss is generally considered a necessary maximum for items such as navigation lights, electric fridges, electronics, inverters, macerators, bilge pumps, bilge blowers and high current devices. At 12v a 3% loss equates to 0.36v or less while at 24v this would be loss of 0.72v or less. A 5% voltage drop would be acceptable for less critical items such as cabin lights, bait pumps etc.

Cable thickness is graded either by the cross sectional area of the conductor in mm², or an AWG number (American Wire Gauge).

Choosing the correct cable thickness

Each cable that you install into your boat’s electrical system needs to be of sufficient thickness to avoid an unacceptable voltage drop and ensure that no fire hazards occur. To do this, consult the appropriate 12 volt or 24 volt Project Notes Cable Thickness Guide

Follow this procedure:

• Work out the total current loading for each cable in Amps. Amps can be calculated by dividing watts by the voltage.

• Measure (or estimate) how long the cable run will be and double this number to arrive at the total circuit length.

• Once you have these figures you can use either the 12v or 24v table (separate note) to select the correct cable.

Inverters

Sep 16, 2010

An inverter will provide a 230v AC supply from your boat’s 12v DC or 24v DC system enabling you to operate mains appliances or equipment, e.g. hair dryer, microwave oven, mobile phone charger, power tools, etc. Obviously the rated output of the inverter will need to exceed the total that you intend to draw at any one time.

Start by making a list of all the 230v electrical equipment that might be running simultaneously. For each item on the list allocate its consumption requirement in Watts. Watts = Volts x Amps. Be aware that appliances with motors (fridges, hairdryers, micro-waves etc) may require a larger start-up current so, check their specifications carefully and read about ‘surge capacity’ below. Add all the items on the list and you will have the total Watts required.

Surge Capacity

Some appliances momentarily require a higher current (surge) when they are switched on. Fridges are notoriously hungry starters (check specifications carefully), hair dryers, micro-waves, vacuum cleaners and basically anything with a motor will usually need a high start-up current.  A good inverter will usually cope with at least double its max output for a few seconds. Even if your inverter has adequate capacity to cope with such start-ups, if your batteries are inadequate or not fully charged, this can cause the inverter to cut out.

Battery Capacity

Many 230v AC appliances draw a lot of power and you need to be sure that you have sufficient battery capacity to cope with the demand. As a rough example, a fully charged 100Ah 12v battery will run a 1Kw 230v AC hairdryer for about 30 minutes, while a similar 24v battery would last for twice as long. Of course if the engine is running, this will extend the available time.

Bear in mind that inevitably there are system inefficiencies; a top specification inverter (including Victron and Sterling) will lose about 6-8% (at 20°C). Make sure that the inverter is installed in a well ventilated area as its efficiency will decrease as the temperature rises. You also need to be aware that all inverters draw power when in standby (quiescent) mode – this is low with both Victron and Sterling inverters, normally less than 5%. Even so, an inverter left in standby for a couple of weeks might flatten your batteries.

Sine Waves

AC power is characterised by a curved waveform called a sine wave. Now, without getting too technical, it is difficult (and expensive) to produce AC current with a true sine wave from a low voltage DC power supply. This has resulted in the availability of two kinds of inverter; pure (or true) sine wave inverters and modified (or quasi) sine wave inverters. Pure sine wave inverters are much more costly to produce than modified sine wave inverters and therefore cost a lot more.

Top specification pure sine wave inverters are capable of running virtually any type of 230v AC equipment or appliance with virtually no adverse effects. Some sophisticated telecoms equipment might be the rare exception.

Top specification modified sine wave inverters (Sterling) will run all but the most sophisticated appliances. Mobile phones, laptops, TVs, drill chargers etc. all tend to work fine as do most appliances. However there are rare (and getting rarer) occasions when the odd micro-wave, drill or vacuum cleaner will not work. Remember, it will be cheaper to change a £50 micro-wave to a different model than to buy a pure sine wave inverter instead. The firm exceptions to all this are washing machines and bread makers which will not work with modified sine wave inverters (or anything with a thyristor control). Sophisticated telecoms equipment may also have problems.

This advice is based on the Victron and Sterling Power inverters which we supply, much cheaper brands are available but expect more compatibility issues and do not expect the same performance.

Inputs & Outputs

Inverters are supplied with heavy input cables, normally 1m or 1.5m long for connecting the inverter to your batteries. As a rule, site the inverter as close to your batteries as possible and keep the input cables as short and fat as possible.

Victron inverters are normally supplied with a choice of 230v Ac output connections, including IEC plug sockets and/or terminals. Sterling inverters usually offer euro and British 230v AC output sockets.

© Boat Electricals Ltd August 2010

Fuses & Circuit Breakers

Sep 16, 2010

If the positive (live) side of a low voltage DC circuit connects directly to the negative side, a large current will flow. This is known as a short circuit because the current will take the shortest path back to the battery and bypass the rest of the circuit. If the current exceeds the maximum capacity of the cable this will cause the cable to heat up and can result in an electrical fire or damage the wiring. Short circuits can be caused because insulation is damaged by chafing or due to prolonged exposure to sunlight, oil or corrosive chemicals; by a damaged or faulty electrical device; or because a poorly secured connection comes loose. It is also worth noting that although it is becoming less common nowadays, some equipment may have its case or body wired to negative.

In order to protect against the danger of electrical fire all circuits must be protected by introducing either a fuse or a circuit breaker into the circuit. This includes battery cables which are often overlooked.

A fuse or circuit breaker will carry current up to a specified load rating. If the current exceeds the load rating a fuse will ‘blow’ while a circuit breaker will ‘trip’. A fuse includes a short section of wire which melts rapidly when a specific current is reached, thereby creating a physical break in the circuit and preventing any current from flowing. Alternatively, a circuit breaker senses a rise in current either thermally or magnetically and will ‘trip’ if its specified current rating is exceeded, also creating a break in the circuit.

The fuse or circuit breaker fitted must have a lower rating than the maximum current capacity of the wire or cable which it is intended to protect. Where multiple circuits are protected by a common fuse or circuit breaker, this must be rated for the lowest rated wire. Individual devices and components are not protected by fuses and circuit breakers installed within wiring circuits. Some equipment may be fitted with an internal fuse.

Fuses

Fuses have the benefit of being cheap as are most fuse boxes and fuse holders, some of which can be inserted ‘in-line’ if an extra device is added at some later stage. However ‘blown’ fuses need to be replaced, so spares for each type and rating must always be kept readily at hand. Fuses can corrode, shake loose due to vibration or simply deteriorate with age and eventually fail without warning.

Circuit Breakers

Circuit breakers are generally more expensive than fuses but can be infinitely reset without the need for spares. Circuit breaker switches are available for many situations, thereby halving the number of components needed.

If a fuse blows or a circuit breaker trips, you should allow only one replacement or reset before investigating the cause. Small current surges can occur on circuits without any short circuit; these can be a very occasional nuisance.  However, if a blow or trip recurs then the cause must be found. 

Blade Fuses

These handy automotive fuses are economical and readily available in ratings from 1 amp to 40 amps. Blade fuse boxes are also available for marine installations, which can make a rewiring job much easier.

Midi and Mega Fuses

A high current fuse should be installed on both the negative and positive side of battery cables and can also be fitted to protect high current devices. Midi fuses cover a range from 30 amps to 150 amps while Mega fuses cover 100 amps to 300 amps. Midi and Mega fuses must be fitted into an appropriate holder which encloses the fuse and will prevent melted metal from escaping in the event of a short circuit. All these items are listed in our Fuses & Breakers section.

Main Lamp Types

Sep 16, 2010

Incandescent (conventional filament bulbs)

First of all we can say that pretty much whatever you need to use a light for, efficiency-wise conventional incandescent lighting is the worst choice (typically 10-15 lm/W) (lm/W = lumens per watt, dependent on bulb wattage). Most of the energy used is turned into heat. Furthermore, they rely on a filament which by its nature is fragile and has a relatively short life span (typically 750-1,200 hours).

Halogen (Quartz Halogen or Tungsten Halogen)

Halogen lighting is more efficient than incandescent lighting - typically 20-25 lm/W. Bulbs, including halogen tubes and capsule lamps are also more compact, have a significantly longer life (typically 2,000 - 2,500 hours) and give a whiter light. Halogen lamps are best suited to high powered exterior flood and search lights, and to interior spot lights. Their main disadvantage is that they run at a higher temperature and because the gas inside is under higher pressure, they are occasionally prone to bursting. So it is safer if halogen bulbs are used within an enclosed lamp. Also, when replacing any halogen bulb, do not touch the glass envelope. The salts in your skin oils penetrate and weaken the glass. The bulb not only has a shorter life, but when the bulb dies the filament doesn't merely burn out, but rather the bulb envelope shatters.

Xenon (Halogen-Xenon)

Xenon bulbs are more efficient than halogen (typically 24-30 lm/W). Similar to halogen lamps but up to 3 times brighter and have up to 3 times the life span (typically 8,000 - 10,000 hours). Xenon lamps are best suited to high powered exterior flood and search lights, and to interior spot lights. Because xenon gas is filled at a lower pressure they are not prone to bursting and are therefore safer. As with regular halogen bulbs, xenon bulbs should not be handled directly.

CFL (Compact Fluorescent Light)

CFLs use around 20% of the energy required by an incandescent lamp of similar output - typically 46-72 lm/W. Because of their compact shape, brightness and diffuse illumination they are well suited to wide area illumination in both work and living areas. CFLs last about 8-15 times longer than incandescent lamps (6,000 - 15,000 hours). The colour temperature is warmer and 'friendlier' than fluorescent tubes.

Fluorescent Tubes

These really are a very good, energy efficient method of bright area lighting - typically 70-100 lm/W (Lifespan 6,000 - 15,000 hours). They are the most efficient room/space light available. Many people find the colour temperature too cool for living areas but they are ideal for galleys and other work areas. Their very low power consumption and diffuse illumination make them a good choice for deck lighting on fishing boats and workboats, although they can be slightly temperamental to start up when operating at very low temperatures and don't cope well with voltage drops.

LED Lighting

Sep 16, 2010

LED Lighting

LEDs have some remarkable properties when compared to other types of lighting:

• Very low power consumption - typically 26-70 lm/W.
• Very long lifespan. 30,000 hours - 80,000 hours is quite normal. This makes them virtually maintenance free or 'fit & forget'. Ideal for inaccessible lights like masthead navigation and signal lamps.
• Shock and vibration resistant - no filaments to break.
• Low heat output - increases the life of small lamp enclosures.
• Easy to seal - high levels of water resistance can be achieved.

LEDs are best suited to: navigation lighting and replacement navigation bulbs, courtesy lighting and step lights, chart lights, small interior lights and berth lights, lighting subject to high vibration or shock loads. LEDs are generally not as suitable for illuminating large deck or saloon areas.

Important note: LEDs vary greatly in quality and output, we take care to source the best. It is difficult to make an efficiency comparison between LEDs and some other types of lighting. The light from an LED is usually concentrated in a narrow area of about 20-25°. So it is pretty meaningless to compare the output of an LED light directly with, say the output from a fluorescent light. Beware of claims which directly compare lm/W (lumens per watt) output for LEDs with outputs for other types of lighting.

Fluorescent Deck Lights

Sep 16, 2010

Installation, Care & Maintenance Guide

'StormSave' Fluorescent Deck Lights 24v & 12v

MOUNTING

Each unit has two reinforced mounting points located on the back of the case. Mounting point centres are 460mm apart. Bolts, nuts, washers and rubber washers are supplied to assist in mounting the unit.

After removal of the diffuser and tubes, access to the inside of the case can be gained by pinching the 2 plastic catches which protrude through the white gear-tray. Assemble the stainless steel bolts, washers, rubber washers, half-nuts and nylock nuts as shown in the main drawing supplied. The application of silicon sealant or Sikaflex to the mounting holes is recommended.

We recommend that you do not mount the unit close to sensitive electronic equipment, antennae or onto structures where the unit will be subject to continuous vibration.

CABLE CONNECTION

INTERNAL COMPONENTS OPERATE AT INCREASED VOLTAGE.  ENSURE THE UNIT IS DISCONNECTED FROM THE SUPPLY BEFORE OPENING THE CASE.

Connect your supply cable (specified voltage only) to the unused positive/red and negative/black terminals on the connector block. As a general indicator, the maximum cable run for 2.5 mm² cable would be approximately 5m for a 12v twin deck light and 10m for a 24v deck light. Longer cable runs will require heavier cable. See ‘project notes’ on the boatelectricals.co.uk website for further guidance on cable.

A grey nut-gland is provided and should be fitted to the 20mm hole at one end of the GRP case. The nut-gland will allow for water-tight cable entry. Use marine grade (tinned) round multi-core cable and tighten the outer nut to seal.

CARE AND MAINTENANCE - HOW TO GET THE BEST OUT OF YOUR DECK LIGHT

TUBES

This unit is supplied with 2 x T8, 18 watt, 600 mm fluorescent tubes which should be routinely replaced every 2 years. These are normal mains type tubes which are usually available from us ex-stock or at most good electrical suppliers and DIY stores.

The operation of fluorescent tubes in hostile environments (vibration, extreme heat/cold etc.) may result in early deterioration and/or premature tube failure. As tubes deteriorate they become less efficient and draw an excessive current. Continuing to run the unit when it is fitted with a faulty tube will cause the unit’s ballast-inverter to overheat and eventually to burn out.

IT IS VERY IMPORTANT THAT TUBES SHOWING ANY SIGNS OF DETERIORATION ARE REMOVED IMMEDIATELY. CHECK TUBES FREQUENTLY FOR ANY DISCOLOURATION, ESPECIALLY NEAR THE ENDS. TUBES SHOULD BE REPLACED ROUTINELY EVERY 2 YEARS REGARDLESS OF APPEARANCE.  

Where vibration is excessive, you may need to consider ways to protect the unit, i.e. absorbent mountings etc. 

LOW VOLTAGE SYSTEMS AND SUPPLY CABLES

Although power consumption is low with fluorescent units, adequate voltage must be available or the tubes will not run properly or may be difficult to start, especially in cold weather. Voltage loss due to supply cable resistance can be significant with low voltage DC systems and can be a possible cause of problems. The thickness of cable required increases in relation to the distance that the current is carried. Units should be positioned as close to the source of power as possible and cable of sufficient thickness must be used if the unit is to run reliably. As a general indicator, the maximum cable run for 2.5 mm² cable would be approximately 5m for a 12v twin deck light and 10m for a 24v deck light. Longer cable runs will require heavier cable. See our website ‘project notes’ for further guidance on cable.

VOLTAGE TOO LOW

All units are fully tested before leaving our workshop. If the tubes do not start, this might be because insufficient voltage is reaching the unit. If you experience any problems with starting, firstly; try increasing engine RPM and turn off other equipment, secondly; try testing the unit with a short 2.5mm² supply cable connected directly to the boat’s battery terminals. If either test results in the unit operating correctly you may need to increase the supply cable thickness to achieve a sufficient voltage to operate the unit over greater distances, or you may need to consult a marine electrical engineer.

VOLTAGE TOO HIGH
Short bursts of high voltage (spikes) can often occur on low voltage DC electrical systems. These high voltage spikes are usually caused by non-isolated autopilot units, by a faulty alternator or by some other faulty item of equipment on the system. The ballast-inverters fitted to your deck light have anti-spike protection built-in; this should cope with all but the most extreme situations.

Calculating Current

Sep 16, 2010

How much current will you use?

You need to have a clear idea of the total current usage on your boat, mainly so that you can determine the capacity of the batteries you need. Battery capacity is measured in amp/hours (Ah). The results will also be useful when selecting cable.

Start by making a list of all the electrical equipment on your boat. The engine starter motor will normally have its own dedicated battery, if so don’t include it on this list. Firstly, for each item on the list allocate its current requirement in Amps. To find the Amps you can divide the consumption (Watts) by the system voltage, i.e. 12v or 24v.

Amps   =   Watts
                    Volts

Then, secondly you should multiply the current requirements of each item by its total hours of use between one full battery recharge and the next. This will give you the amp/hours (Ah) needed for each item. Add all the Ah figures together for all the items on the list and you will have the total Ah requirement for your system.

Ideally batteries should not be discharged by more than 50%, so you should install sufficient battery capacity to meet at least double your calculated requirement.