Choosing the Right Extension Cord and Wirings

Many a time,  we use an extension cord to extend our power supply for connection to a laptop,  Hifi Audio,  Television, and many other appliances.  They are cheap & easily available.  Sometimes,  we use such extension cords for slightly heavy current appliances such as electric irons & kettles.   

Most extension cords we buy off the shelves are not suitable for very heavy current appliances,  not because the plugs and sockets cannot take the load but because the flexible cable used is too small for carrying such higher electric currents.  A quick test one could do is to turn on the appliance for about a minute,  we should feel that these underrated cables will start to warm up.   The better extension cord will always have the ratings specified on the extension cord but the cheaper ones might not. 

This article will try to give some information on how to choose the extension cords and how to size up the flexible cable if one decides to DIY an extension cord.

Extension Cords

There are generally 3 types.  The common ones are sold as switch socket outlets.   They are either in single,  double, or multiple gangs as shown in the top left-hand picture. The other common ones in cable reels where the cables can be reel in and out for storage purposes as shown in the top-right-hand picture.  For those requiring heavy-duty for industrial applications,  they are shown in the lower pictures. 

What to look for?

When buying an extension cord,  it is important to know what we will be using the extension cord for and how long will be the cable or rather how far the appliance will be located away from the wall power sockets. We will be looking at the load-carrying capacity of the extension cord and the loading distance of the extension cord.

a)  Load-Carrying Capacity

For example,  if we are using the extension cord for television,  HiFi audio set,  laptop, or Christmas lights, most extension cords will be able to do the job.   But if we use the cord for electric irons and kettles which will drain higher current,  we will need to be careful and pay some attention to selecting the correct type of extension cord;  otherwise,   the cable might not be big enough to "carry" the electric current or the load.  As a result,  the cable might be heated up over long hours of operation. The wiring insulation could melt causing electrical short-circuiting,  giving rise to fire incidents.

The better ones and branded ones will usually have a rating specified on the extension cords.  Most of them will have a rating capable to handle a maximum load of 13 A or 3,000 watts.  That would mean it could be used for an electric iron or a kettle which usually consumes about 4 to 5 amperes or 1,000 watts.

b)  Loading Distance

The household extension cords usually come in fixed-length of around 2, 3, or 5 meters;  some, 3 meters long;  extension cords the heavy-duty or the industrial type can be of lengths 20 meters or more.  Users do not usually have to worry about the cable ratings of extension cords so long as one is using the extension cords within the manufacturer's maximum specification.

Extension Cord Ratings & Safety Labels

Various countries have different statutory requirements or safety codes for the use and application of extension cords.   Most regulatory authorities will want the extension cords to comply with the local manufacturing safety standards.  Nonetheless, it is always wise for users to choose a brand that has a safety label sticker over those which neither give any ratings or brands or operating ratings.

What about DIY?

DIY is not any cheaper than those available off the shelves but if one needs a specific length of cable,   the only way is to design and self construct the extension cord.

What to look for?

The things to look at are about the same as the commercial extension cord except one will have to size up the cable.   One would need to size up the cable to handle not only the carrying load but also to account for the voltage drop caused by the cable.  

Take, for example,  a cable of a certain size that might work fine when the cable is short but might not even be enough to run the same appliance when the chosen cable is about 30 meters long. This is because the voltage at the receiving end might have dropped to below the acceptable limit for the proper running of the appliance.

How to size up the cable?

There are 3 common and different sizes of flexible cable one can easily buy off the shelves;  these are specified in the following manner under the imperial system:   23/0076 & 40/0076 & 70/0076 as follows.

The Technical specification of the flexible cable under the imperial system is as shown in the following.  There is also one for the metric system.

  

Column 3 of the table indicates the maximum ampere that the cable can carry and volume 4 and 5 indicates that voltage drops for the cable to carry the maximum ampere. For household application which is usually 1 phase ,  we will be looking at column 4 to size up the cable. 

Let's assume we have an electric iron consuming about 1 kW @ 220 Volts,   that is roughly 1000/220 or 4.5 amperes. The above table says 23/0076 can take up to 6 amperes which might be able to carry the load.  But in turn of voltage drop,  it will have 4.5/6*11 or 8.25 volts over 100 ft or 30 meters,  this is about 8.25/220 or 3.75% which is higher than 2.5% specified under the regulation, Therefore,  this cable is not suitable under such usage.

The next cable size (40/0076) will have 4.5/13*14/220 or 2.2% voltage drop.  It might just make it but the one size larger (70/0076) would be more suitable for prolonged hours of usage.

Testing

It is always good to ensure that the correct size of the cable has been selected for the operation by also measuring the temperature of the cable when it is under operation.   The cable temperature should not exceed 70^oC.   One should also ensure the cable is always installed according to the acceptable color code which is green/yellow for earth, blue for neutral, and brown color for the line or live cable.