The IEE regulations for current ratings Table 4E1A single core 90Â°C thermosetting insulated cables non armoured. foot print and local representation allows us to meet the needs of unique CURRENT RATINGS/GENERAL INFORMATION. 2 x 1 Core PVC The Nexans Olex Cable Range. 94 current carrying capacity smallest conductor sizes. Nexans. and control cables to power cables with rated voltages up to 35 kV. technical resources mean that a cable can be designed to meet the exact needs of a customer. .. Table Bunched Circuits of Single Core or Multicore Cables in Air . The factors are to be applied to “spaced from surface in air” current ratings. Notes: 1.
Your meter box is supplied through a cable, sockets are supplied by cables and your ceiling and wall lights are fed through cables. A cable can carry many wires depending on the job it needs to do.
Most domestic cables carry a blue wire which is usually for the neutral current, a brown wire for a live current and a bare wire to take residual current to earth. This cable is called 2 core and earth, or twin and earth. The previous colours for live and neutral wires was red and black respectively.
It is OK, when working on a building with the old colours, to joint the new to the old as long as the joint itself is done correctly. For details of these changes please see the link to the Institute of Electrical Engineers. In essence, for domestic use, the cable wire colours will change to those of the flex colours. Lighting circuits are usually fed by 1. Another cable used a lot in domestic lighting is called 3 core and earth. The extra core wire is in an insulating sheath and is used as an extra conductor to carry power between 2 or more switches operating lights.
The brown wire, in a 3-core and earth cable should always be used as permanent live. The other two are interchangeable as long as they are sheathed in order to make it obvious which cable is neutral blue sheathing and which is the other live brown sheathing.
Special lighting switch cable can be bought. This is called "Twin red core" and is used as switch cable for your lights. Often this is replaced, by electricians, who use an ordinary 2 core and earth cable as a switch cable and place a little red tape around the black wire in the cable. See our project on lights and switches.
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Cable with 2 lives a neutral and an earth Appliance Flex and why it is Used Flex is short for flexible and the reality is that a flex is simply a flexible cable. Flexible cables are used for appliances because appliances usually get moved around a lot and the inflexibility of a fixed cable, either 1. Appliance flex with live, nertral and earth Why are Different Cables and Flexes Used Different cables and flexes are used for different jobs because they are thicker and can carry more current and have more, or less resistance.
Resistance can be thought of as electrical friction and the wires in the cable or flex will absorb some of the energy in the current, allowing a little less to reach the target than was sent.
High energy users such as electric showers, cookers and immersion heaters are supplied by thicker wires than for example radios, as the current that the appliance needs is considerably greater. Cables supplying circuits etc should be sized as per the following cable ratings table. The methods mentioned in the table are methods for how the cable runs through the home to carry the power from the consumer unit to the outlet. Cookers require high amperage as they use lots of power Please note again that installation of cables depends on the location they are to be installed in.
Also taken into consideration is the temperature of the area or void, the length of the run, the grouping of the points they serve and the type of device Fuse, RCD etc by which they are protected. The rest, and the full table follows. The second table is for cables installed by Method C, "clipped direct". As you can see there is quite a difference in rating so be absolutely sure you are doing the right thing. Also remember that while it is not an offence to make electrical installations yourself, it is an offence to use them without them having been checked by a qualified electrician.
Even a simple socket addition now has to have a minor works certificate. The mixture of these two things is called a Method and there are 7 methods. Method A — Enclosed in conduit in an insulated wall Method B — Enclosed in conduit or trunking on a wall Method C — Clipped direct Method — In contact with plasterboard, ceiling or joists, covered by thermal insulation not exceeding mm Method — In contact with plasterboard, ceiling or joists, covered by thermal insulation exceeding mm Method — In a stud wall with thermal insulation with cable touching the wall Method — Surrounded by thermal insulation including in a stud wall with thermal insulation with cable not touching the wall IET On Site Guide to wiring regulations 17th Edition incorporating Amendment 3 — 17th Edition Regulations available to purchase here Your house insurance may not be valid if you do not comply with the regulations as you have a duty of care to protect anyone who enters your home.
It should still be read with the knowledge that electricity is dangerous and you should check to ensure that you are actually allowed to do the work you would like. Understand lighting and wiring in the home — Available to purchase here form Amazon What Does 1. This is not the combined cross sectional area.
Each one of the live and neutral wires has a cross sectional area of 1. Obviously this is a circle. The area, or cross sectional area of this circle is 1. It is purely the area of the copper wire. MCBs are available in various types 1. Each has different characteristics, and is appropriate for a particular application. In a domestic system, we will normally use a type '1' or a type 'B' device, as these are general-purpose units.
If you have a distribution board with re-wirable fuses, and don't want to replace it yetyou can get adapters that will let you plug in an MCB in place of the fuse. If you are replacing wiring with a system that is rated on the basis that you will eventually be using MCBs and not fuses, this is a very sensible thing to do. The amp rating given on the fuse or MCB body is the amount of current it will pass continuously. This is normally called the rated current or nominal current.
We normally assume that if the current in the circuit is lower than the nominal current, the device will not trip, however long the current is maintained. This isn't quite true, but it's a reasonable design assumption.
Many people think that if the current exceeds the nominal current, the device will trip, instantly. So if the rating is 15 amps, for example, a current of This is not true.
There isn't any reason why it should be true: So when will it trip? This is where things start to get interesting. It turns out that there's a rather complex relationship between the tripping current and the time for which an over-current is maintained. The vertical axis shows the duration for which the device can stand this current before it trips.
There are a few things to note about this graph. The fuse and the MCB, even though their nominal currents are similar, have very different properties. For example, to be sure of tripping in 0. The fuse clearly requires more current to blow it in that time, but notice how much bigger both these currents are than the '30 amps' marked current rating.
Neither device will trip at 30 amps, in any length of time shown on the graph, but the lines get closer and closer to 30 amps as the time increases. There is a small likelihood that in the course of, say, a month, a amp fuse will trip when carrying 30 amps.
If the fuse has had a couple of overloads before which may not even have been noticed this is much more likely. This explains why fuses can sometimes 'blow' for no obvious reason. Both fuse and MCB will stand currents of over 40 amps for an hour or so. If the fuse is marked '30 amps', but it will actually stand 40 amps for over an hour, how can we justify calling it a '30 amp' fuse?
The answer is that the overload characteristics of fuses are designed to match the properties of modern cables. In fact it would be very impractical to use a fuse or MCB that tripped at a current very close to the nominal value. This is because many electrical devices take higher currents for the first fraction of a second after they are switched on, compared to normal running.
Take an ordinary lightbulb, for example. The resistance of all metals increases as they heat up. When the lightbulb is first switched on, its filament is cold, and it has a very low resistance.
As it heats up, the resistance increases, so the current decreases. For the first tenth of a second or so, the current flowing in a lightbulb may be times higher than its normal running current 1.
So we have to allow some margin for start-up currents, or the fuse or MCB will tend to trip by accident, which is inconvenient. Because the MCB trips very quickly once a particular threshold is reached, the concept of an 'instantaneous trip current' is appropriate for MCBs. This is the current that will trip the device in 0.
For type 1 MCBs the instantaneous trip current is guaranteed to be between 2. When selecting the correct MCB or fuse to use, we have to consider its role in both over-current protection, and short-circuit protection.
The basic principles are as follows. For example, a amp MCB is suitable for a current of 30 amps in a amp cable circuit. Tripping rule A current of 1.
In practice you haven't normally got to worry about this, it's the job of the MCB designer. All modern devices meet this requirement except re-wirable fuses. This is why re-wirable fuses are discouraged. These fuses normally require about twice the nominal current to blow them in one hour. The 'disconnection time' rule is the most awkward to ensure compliance with in a domestic installation; it will be discussed later. In practice it doesn't affect what rating of fuse or MCB to use, but it often affects whether to use a fuse or an MCB, and may impose the use of additional protective devices.
Assume we are installing a lighting circuit, which will nominally have 8 light fittings of watts each. So the MCB must have a nominal current that is, the current marked on its body of at least 3. Furthermore, its tripping current must be less than 1.
Looking in the manufacturer's catalogue, I find a 6 amp MCB, that has a trip current of 8. This appears to be just right. Relationship between the fuse nominal and tripping currents, and the current carrying capacity of the cable, for the example given in the text Note that in this example, the MCB trip current is not only below the short-term current capacity of the cable which it must bebut it is even below the nominal current rating of the cable.
This means that the MCB will prevent the cable reaching even it normal working current. Of course that's fine in this case, because we know exactly what the load will be: The tripping current does not have to be below the nominal current of the cable, but it does have to be below 1. Note that we need also to check the disconnection time in the event of a live-earth fault, but doing so requires more information than has been supplied; see below.
Some wiring configurations can lead to overload currents that cannot be detected. Extending a spur with a second spur: Suppose we have a standard ring circuit wired using 2. This is a perfectly standard, reasonable configuration. Now suppose we extend this circuit with a spur 'spur 1' on the diagram. What is the maximum current that can flow in the spur cable? Assume that we have used standard single socket outlets. We can plug an appliance rated up to 13 amps into the outlet, so in the worst case the current load could be 13 amps in the spur cable.
This is well within the current carrying capacity of the spur cable, so no problem. Now suppose we extend the system further by connecting a second spur 'spur 2' to the first spur. Now the maximum current that could be carried by spur 2 is 13 amps, and the maximum current that could be carried by spur 1 is twice 13 amps: The cable used might carry 26 amps, in ideal circumstances, but we shouldn't rely on it.
IEE Current Ratings Regulations Table 4E1A
If we plug several heavy duty appliances into the outlets in the main ring, we may cause an overload that will trip the MCB. But this isn't the problem: In the spur, the current carried could be too high for the cable, but too low to trip the MCB.HOW to choose RIght MCB breaker current rating
This is a potentially dangerous situation. It's worth noting that we could, in principle, avert this dangerous situation by using a heavier cable for spur 1. However, spur 1 was probably put in before the need for another spur was recognized, and it would probably have been constructed from the same size cable as the ring.
The rating of the fuse or MCB should be no higher than 13 amps, which means that the total current taken by the spur can never be higher than the current that would be taken by a single outlet attached to the ring by an unfused spur.
Since we have already established that this current is insufficient to damage the cable, the fused spur allows the ring to be extended in a safe way.
Note also that we can wire the fused spur in lighter cable 1. You could argue that this won't ever happen: The problem with this logic is that you may know this, but not everyone else does. What's to stop someone a visitor to your house, for example from plugging an electric fire into each one? The example described above crops up in two common situations.
Suppose we have ten electronic appliances of this sort, all stacked one on top of the other in a small area. It might be convenient to fit a row of, say, six double socket outlets just behind the system. We might do this by running a spur from the main power ring.
Electric Cable Sizes and Amp Ratings for the UK – Electrical Resistance and Cable Rating Tables
One could reasonably argue that these appliances actually have a small power consumption. A watt amplifier is staggeringly loud in a domestic lounge, but it takes less current on average than a lightbulb.
In the circumstances one might be tempted to use an unfused spur. The problem, as before, is that although you only plug hi-fi equipment in here, who's to say what the next owners of your house will do? A related problem is this one: I am commonly asked whether it is safe to fit an extra mains outlet in a room, by connecting it to an existing one on the other side of the wall.
This is a very handy way to add an extra socket, if there's an existing one in position. In a modern house, wired to comply with the IEE Wiring Regulations, then this almost certainly is safe.
Such houses are wired to a standard scheme, where all the power outlets on each floor perhaps excluding the kitchen are wired into a simple ring system.
If you tap one of the socket outlets then you will be effectively installing an unfused spur. As there will only be one new socket on the spur, this is fine.
However, if there's any possibility that the socket you want to tap is itself a spur, then you should probably not do it. If the existing socket has only one cable entering it, this is probably what it is. Even if the socket has two cables, you can't be certain it's part of a ring.
It might be part of radial system, or even a dodgy unfused spur fitted by someone else. The older your house is, and the more haphazard the wiring is, the more likely this is to be the case. There is no straightforward solution to the problem: This time may, in some circumstances, be as much as an hour.
However, when a short-circuit fault occurs, it may be because a live part has come into contact with a metal casing. In this case lives are at risks unless the supply is cut off very quickly.