Taking electricity outside

Under Construction This page is still being written, and has not had any form of review or comment. Do not rely on any information presented here. --John Rumm 07:41, 3 June 2007 (BST)

Taking electricity outside

This article discusses various ways power can be delivered to outbuildings like sheds and garages on a permanent basis. While in concept this does not sound much more difficult than extending power circuit to another room in your house, there are a number of quite subtle details and safety issues that come into play as soon as you venture outside.

Note also that most of the work described here would be classed as a "notifiable work" under part P of the building regulations.

Decide what you want to do

This is probably the most important phase of the project, since if you don't think this bit through well enough you could end up spending lots of time and money on a solution that will not do what you need.

The primary questions to answer are:

• What do you plan to do with it?
• How much power is required?

What you plan to do in the outbuilding will dictate the power required. If all that is needed is a socket or two and some lighting, then the power requirements are fairly modest, a 16A supply would probably be more than adequate. To power a full workshop including equipment, lighting and heating however may require a far more substantial power supply. For the purposes of this article we are going to limit our focus to supplies that can reasonably be taken from a normal domestic household supply. If you need more power than that, then you may need to approach your electricity supplier about providing a dedicated supply to your outbuilding.

To estimate the total power requirement, think of the likely activities, and what equipment will be involved. What devices could you conceivable need to use at the same time. For example in a workshop you may need to use 1kW of dust extraction, a couple of kW of power tool, a kW of heating, and perhaps allow another 400W for lighting. If you add on some contingency for future expansion you get say: 5kW total, so perhaps a 20A or maybe even a 30A supply would be needed.

Design

The design process is a process of working out how you are going to achieve each part of the installation, and what type of equipment and its required current rating is going to be appropriate for each part. We can focus on three aspects of the design separately:

• Head end: This is how you will take power from the house electrical system
• Sub main: How you will wire the connection between the house and the outbuilding
• Outbuilding equipment: How you will select and install appropriate equipment for the outbuilding.

Each of these areas have to be addressed to ensure that the final result meets the required regulations and standards, does what you need, and is above all safe.

Note to complete this design exercise correctly you will need access to copies of BS7671 (the wiring regs) and the IEE On Site Guide.

What you need to know

Distance

One of the primary things you need to know is how far do you need to take power. This is not only the straight line distance between the house and the outbuilding, but also the distance from the consumer unit to the start of the electrical system in the outbuilding "as the cable runs", which may be significantly further.

The primary problem that distance introduces is that of voltage drop. The longer the cable, the bigger its resistance, and the more voltage drop introduced for any given load.

Earthing

There are three commonly encountered earthing systems in use: TN-S, TN-C-S, and TT (see [Earthing Systems] for more detail). It is important that you establish what type of earthing your property uses and this will influence the rest of your design.

Usage

The pattern of usage you expect to make of the power in the outbuilding will also influence your design. A design that is simply intended to provide power to a storage shed will one light, and a socket for running the mower and other garden tools from, probably does not need to worry about details like ensuring that the light does not fail should some other fault cause the power to "trip". Whereas in a workshop where you may be working at night with power tools or machinery this is a far more serious concern. A garage supply that will be used to run a chest freezer will need to make sure there is very little likelihood of a nuisance trip (see RCD) causing the contents to defrost.

Head end design

From an existing circuit

Generally this form of supply should only be used for the simplest and smallest power feeds (i.e. no more than a 16A supply), since general purpose power circuits designed to supply sockets in the house, are not intended to provide power for heavy fixed equipment, or to supply substantial currents as a single "point load". You must also take into account the existing loading on the circuit before deciding to place additional demands on it. For example, many kitchen ring circuits often have existing loads that approach their full capacity.

This type of supply may be ideally suited to the provision of a single waterproof socket mounted on the outside wall of the building.

You will also need to take note of if the chosen supply circuit already has RCD protection. If it does not, then this must be provided for any socket outlets (with a trip threshold of no more than 30mA to offer adequate Direct contact shock protection).

If it already has RCD protection, then this may make it difficult to meet your usage requirements since you it might then be difficult to provide a lighting system that is not vulnerable to interruption in the even of another fault.

From a spare way in the CU

This is the more practical solution for most installations. It will allow you to take more power and in a way that will not affect other existing circuits. If you have a TN-S or TN-C-S system then you would ideally want to take power from a non RCD protected way on your consumer unit, this is to guard against the outside power feed increasing the likelihood of nuisance trips being caused on the household RCD, and also to prevent the RCD from making discrimination difficult for the outbuilding supply.

If your CU does not have a spare RCD unprotected way (or has a "Whole house RCD"), then consider using the independent supply method described below.

Using an independent supply

Where there is no available spare and appropriate way in the consumer unit, one can add a separate feed to a new consumer unit or switchfuse (this needs to be nothing more elaborate than a small DIN rail enclosure large enough to hold a switch and suitable protective device).

To feed this new CU, a big junction box called a "service connector block" or "Henley block" is used to split the exiting tails from the meter. The service connector block typically has room for five separate pairs of tails, so one pair of connections is used as the input and two pairs take power to the old and new consumer units. Probably the most elegant way to use this it to take the existing tails from the meter into a master switch, and insert the Henley block after this. That way you retain the ability to kill the power to the whole installation with a single main switch.

<picture of henley>

The size of the tails used needs to match or exceed that of those used for the existing supply. This will typically be 16mm² for a 60A supply, or 25mm² for a 100A supply. If in doubt use the larger size since this will allow for a future upgrade to the supply without needing to replace all the tails. The new CU should have its earth terminal connected to the exiting main earth terminal using 16mm² earth single (6mm² on TT installations).

Choosing protective devices

It is necessary to provide protection for the submain since this will be vulnerable to damage, either en-route to the outbuilding, or in the outbuilding itself (especially where the building is readily combustible)

Provision of fault and overcurrent protection

Appropriate protection needs to be provided to protect the submain cable against fault and possibly overcurrent (overcurrent protection may alternatively be provided at the outbuilding end in some circumstances). The most appropriate devices for this being either a MCB or a HRC cartridge fuse (fuse carriers are available in a MCB shaped enclosure for many brands of consumer unit). The fuse often has a slight advantage in that it will usually offer better discrimination with downstream MCBs. When using a MCB one often has to size it at least two ratings higher than the highest rating downstream device to ensure discrimination. This can be impractical in many designs.

The rating of the protective device will need to be selected so as to adequately protect the cable used. Note however that further calculations are needed to prove this devices is adequate (see submain design section)

Earth fault protection

For TN-S and TN-C-S installs one can usually rely on the fuse or MCB to also protect against faults to earth (i.e. spade through cable errors!). With a TT install, additional protection will be required in the form of a RCD to protect against faults to earth, and maintain adequate shock protection from indirect contact faults. With the simplest outbuilding setups, it may be appropriate to use a 30 mA trip threshold device at the head end of the cable and forgo any further RCD protection in the outbuilding. Whilst this is a cheap solution, it is non optimal in many cases since it prevents the discrimination in the event of a fault to preserve lighting in the outbuilding, and also means that one has to return to the head end to reset the RCD should it be tripped. A more practical solution is to use a 100 mA trip time delayed device.

Sub main design

This section deals with choosing an appropriate cable type, and checking the design parameters to make sure it is adequately protected.

Cable choice

The most commonly used cable types are flat T&E, HiTuff, or Steel Wire Armoured (SWA) cable. For more details including current ratings for the different cable types please see the main Cables article.

Cable types

Cable type	Useage
Flat T&E	Twin and Earth cable is not suitable for use outside unless protected inside conduit or trunking, since the insulation is neither robust enough for direct burial, and the PVC is attacked by the UV in sunlight causing it to harden and crack. T&E would usually only be used outside for short runs in protective conduit, often clipped to the outside of the main building. Typical applications being connections to outside lights. T&E is however often used for the first part of a submain that makes the journey from the house CU to the point of exit from the building. One weakness of T&E that needs to be considered, is that its CPC is usually smaller in cross section than its main conductors. This will have a negative impact on the overall earth fault loop impedance of the submain.
HiTuff	HiTuff is multicore flexible cable. While it is not suitable for direct burial it is robust enough for most other applications such as being clipped direct or suspended from a support wire. It will remain flexible under a wide temperature range, resists abrasion, and it is not affected by UV exposure.
SWA	SWA is usually the cable of choice for many installations since it is available in a good range of core sizes, it is very robust, and can be buried directly into the ground with no need for further protection. Note that SWA is available with two alternative types of outer insulating material: PVC, and XLPE. The XLPE variety has a higher maximum operational temperature (90° C) compared to PVC (70° C), and hence a higher maximum current carrying capacity. SWA is frequently not brought right into the building at the head end since it is relatively inflexible and difficult to work with.

Voltage drop

Your design must ensure the maximum voltage drop allowed between source and point of use is not exceeded when at full load. This is usually defined as 4% of the nominal supply voltage (about 9.2V at 230V AC).

You also need to allow some of this voltage drop "budget" for the final circuits in the outbuilding.

SWA Cable

Conductor CSA (mm²)	PVC (max 70° C) Voltage drop mV/A/m	XLPE (max 90° C) Voltage drop mV/A/m
1.5	29	31
2.5	18	19
4.0	11	12
6	7.3	7.9
10	4.4	4.7
16	2.8	2.9

Calculation Examples (PVC SWA):

1) 20m of 4mm², maximum load of 30A would drop 20 x 0.011 x 30 = 6.6V
2) 40m of 6mm², maximum load of 45A would drop 40 x 0.0073 x 45 = 13.14V
3) 10m of 1.5mm², maximum load of 16A would drop 10 x 0.029 x 16 = 4.64V

(1) and (3) are adequately specified with respect to voltage drop. 
However (2) is out of  spec and a larger cable will need to be selected, 
even though the current handling capacity of the 6mm² cable has not been exceeded. 
Upgrading to 10mm², gives a result of 40 x 0.0044 x 45 = 7.92V which is better, 
but still only leaves just over 1V of remaining drop available for the 
outbuilding wiring. 

Disconnection time

In the event of a short circuit fault in the sub main, the circuit protective device is required to disconnect the supply within 5 seconds. Two short circuit faults are possible: phase to neutral, and phase to earth. In the case of a TN-S or TN-C-S installation at the head end, we calculate the disconnect time for a phase to earth fault since this usually represents the worst case and the phase to neutral disconnect time will usually be the same or faster. For TT installations the phase earth disconnect time is dependant on the RCD, and hence can be ignored, and we instead calculate the phase to neutral fault disconnect time.

TN-S & TN-C-S Earthing at the head end

To establish the disconnect time we first need to calculate the earth fault loop impedance. This will comprise the sum of the impedance of the suppliers earth (Ze), plus the round trip impedance of the selected cable. Where a submain has more than one cable type (i.e. a T&E feed through the house, with a SWA section outside) the total impedance for each section should be added. In the absence of a measured value of the suppliers earth impedance one should take this as 0.8 ohms for TN-S and 0.35 ohms for TN-C-S systems (note that these values are pessimestically high, and may cause difficulties when designing sub mains for higher current supplies. In these circumstances is it advisable to actually measure the value and use the measured value in place of these worst case figures).

Example: A submain with 5m of 6mm² T&E and then 15M of 6mm² SWA on a TN-S supply 
protected by a 40A type B MCB.

From our Wire Resistance Table, we know the 5m of 6mm² cable will 
represent 10.49 mOhm/m or 0.052 ohms. 

From table 9A of the OSG, we know the 15m of SWA will give 6.16 mOhm/m or 0.092 ohms

So total earth fault loop impedance Z(s) = 0.8 + 0.052 + 0.092 = 0.994

This gives a maximum prospective fault current of 230 / 0.994 = 231 A

Reference to figure 3.4 in BS7671, shows the 0.1 to 5 second disconnect time for a 
40A type B MCB will be achieved with a fault current of 200A (i.e. less than 231A)

Therefore we can conclude that the design meets the disconnect time requirements. 

If the required disconnect time is not met then the cable CSA will to be increased or the protective device rating reduced.

CPC sizing

The final stage is to check that the CPC of the submain will withstand the duration of a fault condition for long enough to allow the protective device to operate. We know the section of T&E cable will have the poorest performance in this case, so we can check to see if its 2.5mm CPC is ok.

Continuing with the above example:

s = sqrt( V² x t ) / k 

Where k is 115 for PVC insulated cable (table 54C of BS7671)

s = sqrt( 230² x 0.1 ) / 115 = 0.62mm²

Which is smaller that the 2.5mm² of the 6mm² T&E cable, so we can conclude this part 
of the design also meets BS7671

Outbuilding equipment design

Know your environment

Access to a "local" earth Floor material Dampness

Exporting an earth

TT or not to TT

When something goes wrong

Discrimination, Emergency lighting

Installation

Installing the sub main

Ducted

Permanent cabling may not be run on a temporary structure such as wooden fencing.

See Also

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