Dimmers & Switchbanks

Dimmers & Switchbanks are both ways to control lighting levels, improving both utility and comfort. This article sets out the advantages of controlled lighting levels, and looks at the relative merits of different ways of achieving them. Some installation notes are also included.

Advantages of Lighting Level Control

Controllable lighting brightness permits

• Bright lighting when its needed for a task
• Relaxed lighting in the evening
• Improved safety by allowing low light levels to be used at night; particularly advantageous if small children are about.
• Give better comfort and better visibility.
• Can reduce energy usage

How it affects energy use and running costs will depend on which method of control is used, and the pattern of use.

Methods of Lighting Level Control

Switchbank

A Switchbank is a bank of 2 or more switches, where each switch controls a subset of the total number of lights in a room. Lighting levels can be controlled in a number of discrete steps by selecting which lights are turned on.

To operate well, switchbanks require lights to have overlapping lighting areas; so a long room with only two lights - one lighting each end, may not be an ideal candidate for a switchbank, since turning one off would leave half the room dark and the other half bright. A room with 10 smaller downlights however could have control of the lights interleaved such that lights 1 3 5 7 & 9 are one one switch and 2 4 6 8 & 10 are on a second. This would be a far more satisfactory arrangement for a switchbank.

The more switches used (and hence separately controllable lamps), the finer the control possible.

Dimmer

A Dimmer is an electronic switch that reduces the power flowing through the lamps it controls, which in turn causes a continuously variable change in brightness to be achieved. Note however that the reduction in brightness is not matched in proportion with a reduction in energy consumption since a good proportion of the energy requirement for a filament lamp is just getting it hot enough to begin to glow. As a rough estimate you can say that a 90% reduction in light output may only represent a 50% reduction in power.

Unlike switchbanks dimmers don't require overlapping lighting areas to archive their effect.

Combination

It is also possible for both techniques to be combined. Here each or some of the multiple switches of a switch bank are implemented with dimmer switches. This gives some the advantages of a switchbank, while retaining the continuously variable nature of the dimmer.

With this arrangement the cost savings of switchbanks are only realised if you use the switches to control the lighting.

Patterns of use

Different people use controllable lighting in different ways. Some will tend to use their lighting at or near to its maximum level most of the time, and then wish to use lower levels occasionally. Many people retrofitting dimmer switches to existing installations will fall into this category.

Some will want to do the reverse, using lower lighting levels everyday, but having the option of having more (or less) brightness on occasion.

Comparison of Energy Efficiency

The pros and cons of switchbank lighting versus dimmer control will vary depending on use patterns. For users who nearly always use maximum brightness, neither technique will save much energy.

Dimmers at full brightness have a slight energy efficiency disadvantage since the triac voltage drop will run the lamps at very slightly lower rms voltage and efficiency, but the amount of the effect is trivial. This does also extend lamp life by a trivial amount, with a tiny consequent saving on bulb manufacturing energy, but this is much smaller in size than the energy efficiency reduction due to operating at slightly lower rms voltage. In all these effects are trivial.

For users who routinely use less than full brightness illumination, the energy saving potential of a switchbank is far greater.

This section compares the energy usage requirements for a hypothetical room that is equipped with 300W of filament lighting or the approximate equivalent of 100W of CFL lighting. We will assume the usage pattern in a typical day would be:

1 hour a day of 40w filament equivalent output
5 hours a day of 150w filament equivalent output
1 hour a day of 300w filament equivalent output (i.e. full brightness)

Prices are given for 10p/unit electricity cost.

No Control

Firstly the no control option, for which we will like most typical householders, have to select the wattage best suited to all round use, in this case 150w since that is the most common lighting level required. This is a compromise since we cant have the levels we really want some of the time, so comfort and utility are not optimal.

7 hours at 150W = 1.05kWh / day = £38 pa = £960 per 25 yr installation life.

If we really must have the full brightness option, then obviously the costs above will double. A more realistic option might be to provide additional table lights etc and keep the main lighting at a more conservative level.

If we opted for 50w of CFL instead:

7 hours at 50W = 0.35kWh / day = £13 pa = £319 per 25 yr installation life.

Switchbank

A total of 300W of filament lamps on switchbank will use: 1 hour at 40w = 0.04kwh + 5 hours at 150w = 0.75kwh + 1 hour at 300w = 0.2kwh

Total usage = 0.99kwh/day = £36 / year or £903 per 25 yr installation life.

CFL Lamps on switchbank will use: 0.015 + 5 x 0.8 + 0.1 kWh = 0.195kWh / day = £7 / year = £177 per 25 yr installation life.

Dimmer

For 300w of halogen on a dimmer: we will assume to get 40w equivalent brightness we run at 50% full current, and for 150w brightness we use 80% of full power

0.5 x 300 + 5 x 0.8 x 300 + 300 = 1.65kwh / day = £60 / year = £1,505 over 25 years

Summary

Lighting and control	Total cost / year	Total cost / 25 years
CFL, Full Power No Control	£26	£638
CFL, Optimal Power, No Control	£13	£319
CFL, Switch Bank	£7	£177
CFLs on dimmer	n/a	n/a
Filament, Full Power, No Control	£76	£1,920
Filament, Full Power, Dimmer	£60	£1,505
Filament, Optimal Power, No Control	£38	£960
Filament, Switch Bank	£36	£903

From this comparison table several things can be seen:

1. The most expensive option is the use of full power lighting with no control. This also does not achieve the goal of controllable lighting levels.
2. Full power lighting with a dimmer is a little cheaper, but not as cheap as using optimal bulb sizing in the first place. It does however give the greatest flexibility of control over the lighting.
3. Switchbanks generally give a lower run cost than control using a dimmer, or opting for no control at all. Sometimes the cost reduction will be significant and switchbanks will pay back their extra installation cost many times over if fitted at rewire time.

Reliability

Switchbanks are typically as reliable as any other light switching arrangement (i.e. very), however the need for a larger number of independently controlled lamps will reduce the overall reliability figure a little, since there will typically be more lamps to fail and replace.

From a light availability point of view however a switchbank provides redundancy, meaning that if one element fails, the system as a whole still continues to work.

A dimmer is a single failure point, and dimmers have variable reliability. The cheaper 250w dimmers seem on the whole to be less robust in this respect. It is generally better to specify a dimmer with greater power handling than you actually require by a reasonable margin (say 50%). A dimmer can operate with a single lamp or cluster of lamps and will hence tend to improve lamp reliability. Many also include a soft start capability that is beneficial in extending halogen lamp life in particular.

Dimmers are rated for their maximum power ratings (and in the case of ones used with LV lamps often a minimum load as well). If dimmers are used over their ratings, early failure is likely.

Compatibility

Standard dimmers are compatible with mains filament bulbs & mains halogens. They are also generally compatible with 12V lighting systems, but a little care mut be taken to pair the correct type of dimmer and transformer or power supply.

Dimmable fluorescent lighting ballasts are available. Dimming these does reduce the energy efficiency of fluorescent lamps to some extent, but nowhere near as much as with filament lamps.

CFLs and dimmers are generally not compatible (special dimmable CFLs are available in some countries, but generally not the UK).

Dimmers have a significant compatibility advantage, which is that they can easily replace a single lightswitch in an existing installation, with no need for any extra wires to be put in. However, depending on how much power and time dimmed lighting is used, retrofitting a switchbank and adding any extra wire needed can work out cheaper, as well as being more energy efficient.

Run and installation Costs

Run cost depends entirely on what lighting is used at what setting for how long, so is a very variable figure. One must also allow for the fact that the heat produced from the lighting will contribute to the heat released into the building. In the winter this may be desirable to an extent and will partially offset heating costs (although using more expensive electric heating in place of cheaper gas or oil). In summer this is counter productive, especially if you have to pay for air-conditioning to vent the unwanted heat.

Fitting a switch bank obviously requires more materials (switches, cable, & luminaries), and may also require significant extra installation time and cost. However this extra install cost can be paid back many times over in the life of the installation.

A switchbank can also be used to mix or choose between filament lighting and CFL. For example, someone that is not keen on CFL might use CFL just for background light, with filament as the main lighting, or use linear fluorescent uplighting with halogen spots for task and feature lighting.

Fitting a Dimmer is usually a simple and low cost option, however the energy saving payback is far less dramatic.

Installation Tips

Switching for Switchbanks

With standard faceplates, a single light switch can come with up to 3 switches. Standard double faceplates come with upto 6 switches. A 3 gang switch is enough for most rooms.

More Switches

More switches are not normally used outside of grand buildings and public halls. To get more switches requires either a gridswitch, additional faceplates, or a custom switch assembly made from a blanking plate. The gridswitch is the usual choice.

Each additional switch and set of controlled lights will double the number of available lighting patterns. Hence 3 and 4 gang switchbanks are normally plenty.

If you ignore "off" as a brightness level, we are left with:

Number of switches	Patterns available
2	3
3	7
4	15
5	31
6	63

It is also possible to incorporate dimmers into gridswitches.

Wiring for switchbanks

Most domestic lighting circuits are wired using the "loop in" method. Here power is daisy-chained from each lighting fitting to the next, and a separate switch wire connects a switch to each fitting. This is a simple and easy to implement system for general lighting wiring, however this approach can become a bit cumbersome for switchbanks with many controllable ways.

A simpler way is to take the power feed (complete with its associated neutral) to the switch position rather than lighting position - so power out to the next room is hence taken from the switch and not from a light fitting. From here a single cable with neutral and switched live can be run to each lighting position (or group of positions). This does away will all of the daisy chain wiring between each lighting position, and just requires a single wire out to each lighting position.

Wiring for Dimmers

A dimmer will usually simply replace and existing switch, and requires no special wiring consideration.

The exception to this is when two way operation is required. If you wish to control brightness from both switch positions then a special master / slave dimmer switch will be required. Conventional dimmers will allow two way switching, but not two way brightness control.

Summary

• Switchbanks are a good idea for new installs, and if used in the intended way, may save many times their cost.
• Dimmers won't save as much money as a switch bank, and if lamp powers are not carefully chosen may end up costing more than a non controlled lighting system.
• Dimmers are the neat option for multi-bulb chandeliers.
• Dimmers are significantly easier and thus cheaper to retrofit than switchbanks.
• Dimmers give the greatest amount of lighting level control
• Dimmers prevent use of CFL Lamps.

See Also

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