2D bulb question

Need to replenish my stocks, but confused to find I can get 2 pin in 16 or 28W, and 4 pin in 16/21/28/38/55 watt.
Am I right in thinking there just two physical sizes: 2 pin about 14 cm across and 4 pin about 20 cm across, or are there maybe more sizes? Don't want to order 38s only to find they won't fit in my light fittings.
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On 19/12/2012 21:19, newshound wrote:

Oops just found data sheet on TLC (how did I miss that first time I looked?). Confusingly Compton (at least) do 2 and 4 pin in both sizes.
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The four pin tubes are just conventional fluorescent tubes bent up. ie from days of yore. I expect they will be phased out at some point.
The two pin ones incorporate the starting gear in them, ie they are equivalent to a CFL
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They're all available in 2-pin and 4-pin versions (and you missed 10W, although it's getting rare now).
2-pin is for series mains ballast and has a starter hidden in the lamp base. 4-pin is for electronic ballasts.
16W and 21W are same overall size, but different tube diameter.
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on 20/12/2012, Andrew Gabriel supposed :

4-pin are also used with standard starters and ballasts.
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They can in theory, but that was the whole point of the 2-pin type, which reduces the luminare and maintenance cost in this simple case.
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You need to order the ones which match the power rating of the ballast, not the physical size. The ballast determines the power the lamps run at. You can't just stick in a bigger one or smaller one (it might work, but it might give significantly reduced tube life and/or overheat the ballast). Obviously, also needs to match the 2-pin or 4-pin fitting too.
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On 21/12/2012 11:10, Andrew Gabriel wrote:

Thanks, will check ballasts before fitting anything. Local DIY shop just stocks one wattage for each size (and is expensive).
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B&Q stock both the 16W and 21W, although they are now energy saving versions (GE Wattmiser) which run slightly underpower.
The original 2D lamps don't meet future EU requirements for energy efficiency of fluorescent lamps, and I presume these have been introduced in order to keep 2D lamps available.
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On 21/12/2012 12:26, Andrew Gabriel wrote:

Normally my shed of last resort, but when I get round to getting an "Old Fart's Wednesday" card I suppose I might use them more.

Not mad on 2D, but I quite like the Screwfix "glass dome" luminaire for kitchens. Shallow, reasonably bright, easy to clean, design which doesn't really date.
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