Dimmers & Switchbanks
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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
- Relaxed lighting in the evening
- Bright lighting when its needed for a task
- Improved safety by allowing low light levels to be used at night; particularly advantageous if small children are about.
- Better comfort and better visibility.
- Can often reduce energy use, but doesn't always
Methods of Lighting Level Control
Switchbank
A Switchbank is a bank of 2 or more switches, where each switch controls some of the lights in a room. More than one lighting level is thus achieved depending on which lights are on.
To operate well, switchbanks require lights to have overlapping lighting areas; so a long room with only two lights, one at 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. This can be an issue when fitting switchbanks to an installation with a minimum number of lights, as was common practice in the 1970s and before.
When using a switchbank with multiple downlighters, there are various ways to distribute the lights among the switches. Groups of lights on each switch and alternate lights on each switch are both popular options. What works best depends on the room and lighting layout.
2 or 3 switches is enough for most people. More switches make for more possible lighting patterns or levels if desired.
Dimmer
A Dimmer is an electronic control that reduces the power flowing through the lamps it controls, which in turn causes a continuously variable change in brightness to be achieved.
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. Measurement of a halogen lamp showed 60% rated power consumption at apx 8% light output.
Dimmers don't require any overlap in lighting areas to achieve their effect.
Combination
A less popular option is to combine both techniques. There are a few possible ways to do this:
- Dimmer followed by switchbank
- Multiple dimmers
- A switch for some lights, and a dimmer for the other lights
This gives some the advantages of a switchbank while retaining the continuously variable nature of the dimmer.
Compatibility
Lighting Compatibility
Switchbank
Switchbanks are compatible with all types of lighting.
Dimmer
Standard dimmers have restrictions on the types of lights they will run.
- Fully compatible with mains filament bulbs
- Fully compatible with mains halogens.
- Compatible with most electronic 12V halogen lighting transformers, but not all.
- Not compatible with toroidal halogen lighting transformers.
- Not compatible with CFL
- Not compatible with linear fluorescent
- Not compatible with discarge lighting (HID, sodium, mercury)
Dimmable fluorescent lighting ballasts are available. When used the dimmer & ballast chosen must be compatible with each other.
Special dimmable CFLs are available, but are relatively high priced and little used.
Wiring Compatibility
Dimmer
Dimmers can easily replace a single lightswitch in an existing installation, with no need for any extra wiring to be installed. This makes them an easy option for existing installations.
Switchbank
Switchbanks can sometimes be retrofitted with no change in wiring, but in the majority of cases this is not possible, and extra cable would need to be run to enable them to be retrofitted. This heavily limits their retrofit use.
The Lightbulb Ban
There have been reports of the government seriously intending to ban filament lightbulbs. If this plan goes ahead, there may be compatibility issues with non-filament types of lighting and dimmers. For DIYers this may be a minor matter, most can replace dimmers with switches, but for the less DIY able, hiring an electrician may prove an unwanted expense.
Some are considering stockpiling filament bulbs as a way to circumvent this issue.
Reliability
Switchbanks
Switchbanks are as reliable as any other light switch arrangement, ie very reliable.
If more lights are fitted at the same time, that translates to more lightbulb replacements. This is not generally a cost disadvantage due to greater running energy efficiency than dimmed lamps, but can be if installation costs are high.
A switchbank also provides component redundancy, meaning that if any one element fails, the system as a whole still continues to work.
Dimmers
Dimmers have lower and more variable reliability compared to switches, and failures do occur occasionally. 250w dimmers are rather less robust on the whole than 500w dimmers. A dimmer is also a single failure point, meaning a failure can cause whole room lighting failure.
Filament bulbs occasionally consume a heavy current surge on failure, due to arc-over. Such a current surge has a high chance of destroying a dimmer. These surges are more likely to occur and be of higher currents with
- small form factor mains filament bulbs (especially mains halogen)
- short cable runs.
12v bulbs and non-filament type lamps don't suffer from this.
Specifying a 500w dimmer with well below 500w of lighting load can improve reliability, although it does not solve the above minor issues.
Dimmers should not be loaded past their max rating, as this will elevate failure rate greatly.
Bulb Life
Switchbanks have no effect on lamp life.
Dimmers usually soft start lamps. This extends mains halogen lamp life to some extent, but makes insignificant difference with all other types of lamp.
Installation Costs
Switchbanks
Fitting a switch bank requires more materials (a multi-gang switch & cable), and a little extra labour than a dimmer. The extra cost of DIY fitting is usually paid back many times over in the life of the installation by reduced electricity use.
This is not the case with retrofits where channelling and plastering is required to make good.
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 filament, and their partner CFL, or they might use a mixture of lighting types. One example of this would be concealed fluorescent uplighting with halogen spots for task and feature lighting.
Dimmers
Fitting a dimmer is normally a simple low cost job.
Installation Tips
Switching for Switchbanks
With standard faceplates, a single sized light switch can come with up to 3 switches. A 3 gang switch is plenty for most rooms.
More Switches
Grand buildings, public halls and specialist lighting installations may require more switches. Standard double faceplates come with upto 6 switches. To get even 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.
- A 2 gang switch gives 3 lighting levels
- A 3 gang switch gives 7 lighting patterns/levels
- A 4 gang switch gives 15 lighting combinations & levels!
Dimmers can also be incorporated into gridswitches.
Wiring Arrangements
Dimmers
A dimmer usually replaces an existing switch, with no added complication.
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.
Switchbanks
Loop-in
The majority of 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, but is not compatible as it stands with switchbanks. To retrofit switchbanks into such a system would require additional wiring, complicating the wiring layout.
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The other method in use for domestic lighting is to run the power feed to the switch first rather than the lighting (so power feed to the next room is taken from the switch position and not from a light fitting). From the switch, a cable with neutral, earth & switched live is run to the lighting. A minority of house lighting is wired this way.
When more than one cable is run from the light switch to more than one light fitting, a switchbank can be retrofitted without adding any further wiring. However in most retrofit cases such wiring is not already present. Hence switchbanks are more often fitted at new installation and rewire times rather than retrofitted.
Energy Efficiency & Running Cost
Dimmers
While it is often assumed that fitting a dimmer will reduce energy use, the reality is more complex. In some cases energy is saved, and in some cases energy use increases. See this discussion for explanation and detail.
For users who frequently use less than maximum brightness illumination, the energy saving potential of a switchbank is much greater.
For users who rarely use less than maximum brightness illumination, neither option could save significant energy or cost.
Switchbanks
All lighting on a switchbank runs at full energy efficiency, hence switchbanks are an inherently more energy efficient option than dimmers.
Switchbanks are also compatible with all lighting types, whereas dimmers only work with the higher energy consumption types of lighting (tungsten filament & halogen). Energy efficiency and compatibility are the main advantages of switchbank over dimmer.
Examples
Run cost and energy use depend on what amount and type of lighting is used at what setting for how long, so is a very variable figure.
This section will compare lighting energy use for a hypothetical room that is equipped with 300W of filament lighting or the approximate equivalent of 100W of CFL lighting. For this example we will use a daily use pattern of:
- 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
Energy Efficiency 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:
- 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.
- 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.
- 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.
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.