Friday 5 May 2023

Power Outlet Sockets

Recently I responded to a forum where members tried to find the cause of cigarette lighter socket and plugs failing. While the replies were varied I don't believe any really addressed the root cause of the problem. In the past I have worked through a variety of problems with these very popular power connectors and have a few ideas of my own I would like to share.

Cigarette Lighter Plug and Socket, the better quality units are good to 20 Amps if wired correctly.

I hope to give you some thing to think about before you quickly write off the cigarette lighter sockets and plugs and advocate to install Anderson style plug in connectors. I don't believe it’s sensible moving away from the cigarette lighter sockets totally considering a wide variety of consumer electronics are made specifically to plug into this type of connection for power. In reality the best option is to have both, but keep in mind both have to be installed properly, if done incorrectly your back to where you started, with ongoing problems.

                                Common cigarette lighter accessories found on most cruising boats

Common USB power outlets for a cigarette lighter socket

Laptop Charger and USB outlet for a lighter socket.

A small inverter, various models are powered from the lighter socket.

An Anderson connector in an easy to install panel mount.

Sockets and plugs

Not all cigarette lighter sockets and plugs are created equal. There are a lot of plugs and sockets for sale in the market place that are just plain junk. Unfortunately these are the cheapest and due to this fact are usually the first to catch our eye. I have noted a lot that are really stretching the truth as far as the ability to carry the rated current (amps) advertised on the box. I have even seen install kits and extension cables with the wire size so small there is no way they could possibly supply the advertised rated current (amps) at a usable voltage. If these cables do eventually supply the advertised rated current, the voltage will be so low it would surely do damage to the equipment connected.

Cigarette Lighter Plugs

A good quality marine grade plug, this one has side contacts that will lock into slots in the matching socket. There is also a sealing ring to help keep moisture from entering the socket and plug connection in use. 

The good quality marine lock in plugs are listed as being able to carry 10 amps at 12 volts (120 watts) there are also some on the market listed as being able to handle up to 20 amps at 12 volts (240 watts). Not all plugs carry the same ratings and there are some that do not have a current rating on the unit or the packaging, of course that is if they are in packaging. In this case the best course of action is to undo the end cap and check inside the plug for a fuse, then use this fuse size as the rating of the plug. You may find the cheaper brands of plug are only good for as little as two (2) amps. A far cry from the ten to twenty amps that the better quality plugs are capable of. Don’t be tempted to up the size of the fuse, I have on a couple of occasions, and found that the plugs fail while in use, some times spectacularly in a puff of smoke.

Some equipment I have seen capable of drawing 15 amps has been fitted with cigarette lighter plugs, and as long as the socket is wired correctly to carry 15 Amps continuous there should not be a problem. Take a little time and do the detective work to find out if the plugs you have are safe for the purpose you intend them for. Equipment that comes with a cigarette lighter plug fitted should be OK to use as supplied, if a fuse is fitted don't replace it with a larger fuse. Prior to use make sure both the socket and plug are clean and the spring loaded tip has plenty of resistance when pushed back into the case. I have regularly seen the side contacts of the plug can get a rust build up on the contacts and will need to be cleaned before use.

This plug is cheap @ $2.95 however you get what you pay for and it did give problems before being replaced.

While its hard to see the side contacts of this plug are starting to rust and will require cleaning, better still would be to replace the plug with a plug made from better more conductive material. 

The better quality plugs and sockets made for marine use have a lock in place mechanism, simply line up the contacts on the side of the plug with the arrows on the socket and the plug will stay put until it is removed. Some plugs can take a bit of force to remove which is very good sign as the tension is kept on the contacts while in use.

Cigarette Lighter Sockets

Marine grade cigarette lighter socket, lock points shown on the body of the case. 

The good quality marine grade sockets are rated for 15 amps at 12 volts (180 watts) to 20 amps at 12 volts (240 watts), and usually come with a cap to keep out the moisture when not in use. These sockets are usually made from better spec materials and the metal components resist corrosion.

Old style cigarette lighter socket with a metal case and ceramic centre pin, this socket has been in use for over 14 years and works fine. 

Also on the market are sockets with an all metal body and a ceramic disk holding the centre contact, these were the norm when used to heat the cigarette lighter coil. I will say the body will not melt, and I have had one in use regularly on my vessel for over 14 years and haven't had a problem. These units do not have a lock in function so care has to be taken when selecting plugs to fit. Check the Amp (current) rating before purchasing as now days they vary any where from 5 – 10 Amps at 12 Volts.

Then there are the standard plastic sockets rated for 10-15 Amps at 12 volts. I think for safety you should make sure they are from a reputable supplier/manufacturer. I have seen unbranded sockets that in my opinion wont have a hope of being able to run fully loaded, the new contacts while looking nice and shiny brass start to bubble rust once in the humid marine environment.

The other variable here is that we can all be sucked in by the numbers printed on the packaging or sales brochure. What’s irritating is the losses caused by either bad advice or product advertising can be significant.

Power Accessories

What else to look for when using plug in power cables. Most, if not all off the shelf splitters (one plug connected to 2-4 sockets) or extension cables are really made to a price and the size of the wires used are not usually big enough for the advertised voltage/current.

There are similar looking extension cables for sale, most are advertised with specifications way beyond what they can actually achieve. Before purchase investigation into the suitability will need to be done before use. We got stung a couple of times before, and now make our own. The above unit is good to 3 amps at 12 Volts, I have seen similar units advertised at 10 Amps at 12 Volts, so buyers need to be aware.  

Watch out for the voltage drop when in use, I have had experience of these when running at the advertised rated 10 amps, the voltage drop is excessive. For example a spot light will be dim however a motor (bilge/water/air/fuel pump) is very likely to overheat all the while running slower, fridges will not run or become damaged, some electronics will stop working temporally or suffer permanent damage.

I have also noticed plugs advertised as capable of carrying 20 Amps which is all well and good, as long as the socket is also rated and wired to carry the current properly. However if it was me I would not use a cigarette lighter plug & socket at this high current level, I recommend and would use Anderson plugs.

A cigarette lighter and anderson plug mounted into an easily installed housing, great for dash mounts and the Anderson plug can handle up to 50 Amps if wired correctly. 
What we have experienced.

Excessive voltage drop in an extension cable. We inherited a 3 metre extension cable with the boat and it never did work at the amp and voltage specs advertised on the box. We unsuccessfully tried using it for running a fuel transfer pump, during trouble shooting we found low voltage of 7 volts at the socket end of the cable with the pump on. After a little detective work we found the wire size in the extension was 18 AWG (0.83mm2) or close to it. There is no way we could ever get the advertised rated 8 amps at 12 volts from this cable, it would be impossible. To be able get close to the specs on the box the cable size would need to be at least 12 AWG (2.8mm2) which is a significantly larger wire size. So how do we get around this, we make our own cables now, or have installed sockets closer to where we work. We cannot trust the manufactured extension cables to perform to the numbers written on the box.

We have seen so many advertisements of cables that can not possibly work at the listed current and voltage. After a little more detective work we have found that while the advert states 8, 10 or 15 amps, they get around that by stating the wire "amp rating", and the "amp rating" explanation is far beyond the scope of this post. Its all in the wording, obviously the marketing department had their thinking caps on and come up with this one. Why you ask? copper costs money and if they used a wire size capable of performing to the advertised specs they would either need to put up the price or lower the profit margin. Sadly we the unsuspecting consumer get sold a piece of equipment that will not perform.

Melting cigarette lighter plugs. We have had our share of problems with these until we worked out why. I was under the assumption these plugs were a universal 10 amps, or heavy duty model at 15 amps and now there are 20 amp units on sale, truth is stranger than fiction and as it turns out these plugs are rated at any thing from 1 up to 20 amps. Yep 1-20 amps and while they look the same outside its the quality of the components inside where the additional cost is. Prior to purchase always look for a fuse in the plug, this fuse size is a give way on the plug amp rating. This is where I caused myself grief, I would change the fuse to a larger one to run heaver equipment and caused the plug to overheat and sometimes distort badly. How & Why? Resistance in an electrical circuit causes heat, and the lower grade material in the cheaper units caused resistance and the flow on effect is heat. The more current (amps) drawn through the resistance the hotter it it will get, and most times the hotter things get the more resistance is created so its a spiral into failure, some times in a puff of smoke. For trouble free use of equipment get top branded plugs and sockets, yes I know the plugs for $2.95 look attractive when compared to the $15.00 plugs. The real question you need to ask your self is what is the failure of either the plug or the connected equipment going to cost you in the long run.

While these fuse ends look ok the fuse on the left has solder on the ends holding on the fuse wire, for whatever reason this causes enough resistance creating high temperatures at higher current.  

I mentioned fuses in the plugs, and this is also important, the fuse used has to be good quality. We had a batch of fuses with soldered ends and these caused all manner of problems due to poor contact inside the plugs. The same can happen with fuse holders either in the line of an extension cable or power supplied to the socket, the slightest resistance in the fuse holders contact with the fuse, can at higher currents cause the fuse/fuse holder to melt with out the fuse actually blowing. On these installs where the socket will be subjected to current over 10 amps we use a bolt in fuse, the holders don't cost much more than a standard style and have far better method of securing the wires or fuse.

Fuses hot enough to melt the housing and fuse holder, all with out blowing the fuses, most likely cause was a resistance and at high load created enough heat to melt the fuse and holder..

Bolt in fuses are certainly a better option to ensure better contact and less chance of failure due to bad connections. 
Failures of sockets

With out doubt one of the biggest problems is sockets wired in with undersized wire for the length of the run, this will cause large voltage drop for the connected equipment. I have also seen poor joins or improperly crimped connectors used on either end of the cable run causing poor contact. Also when doing an install buy good quality tinned/silver coated copper terminals or push on connectors, there are some on the market made from mixed metal and do not conduct as well as copper. The other flow on effect of these mixed alloy terminals is they can corrode quickly when in the marine environment.

As explained earlier regarding resistance in an electrical circuit causing heat, if the plugs used in the socket cannot maintain a good connection then there will be resistance and in effect heating, if enough current is drawn eventually the connection will heat up and if enough heat is generated the socket, plug or both can melt and become distorted. Again better quality sockets are built from better more electrically conductive material the flow on effect is a socket less prone to failure from contact problems. The better quality marine sockets also come with a built in locking mechanism and when used with the matching plug help maintain good contact with the plug.

Keep the sockets clean, we have found the green scourers can do a good job of removing any surface corrosion on either the socket or the plug contacts. We don't have to scourer for very long and the contacts come up like new. A quick blow out with canned air or use the vacuum cleaner to clean up after the job.


Be aware of what is for sale if your out looking to buy new equipment, it is truly a mine field. The sockets have to be wired correctly in your vessel. If your doing the job yourself get an idea of the wire length required before you do the size calculation to get the right current carrying capability. Remember while the socket may only be a metre from the power distribution bus bar you may require double or triple that length to do the job correctly by keeping the wire in the cable run or trays.   

While the better quality plugs and sockets are more expensive, the convenience of reliable connections is well worth the initial out lay. 

Here is a simple wire size calculator, and it can be compared as a secondary check with a chart I have used for several years. Keep in mind the calculator uses one way distance the chart uses wire length in the circuit which is double the one way distance.

DC_wire_selection_chartlg.jpg (3189×1574) (

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