Wiring Regulations Literary Question

Even if it's mentioned in a Wiring Regulations Commentary, it would be better (in your writing) to quote a source whose authors specifically know about lighting.  So Mike's suggestion (a lighting book) or Dave's (a manufacturer specification) sound preferable even if the quoted text can be found in the Commentary.


If you're able to access this book (Handbook of advanced lighting technology) you'll find somewhere halfway down the introduction chapter the text  "The life of incandescent lamps varies inversely as the 13th power of the applied voltage.  Voltage 10 % below rated increases incandescent lamp life by a factor 3.5 over rated; 10 % above, a corresponding factor 3.5 reduction in life."


Whether it's the "13th power" - or a bit more or less - depends on the particular lamp or the person you ask. This old article (The life-voltage exponent for tungsten lamps, Covington 1973) should be a very good source, but I can't access more than the first page. The exponent can also vary when moving away from rated voltage.

The 55% of lifetime that you mention suggests instead an exponent more like 14, because:  1 / (240/230)^14 =~ 0.55. But it doesn't make much difference: with an exponent of 13 it's about 58% ...  it's anyway a big change in lifetime or light output, for not many percent in voltage.

Even if it's mentioned in a Wiring Regulations Commentary, it would be better (in your writing) to quote a source whose authors specifically know about lighting.  So Mike's suggestion (a lighting book) or Dave's (a manufacturer specification) sound preferable even if the quoted text can be found in the Commentary.


If you're able to access this book (Handbook of advanced lighting technology) you'll find somewhere halfway down the introduction chapter the text  "The life of incandescent lamps varies inversely as the 13th power of the applied voltage.  Voltage 10 % below rated increases incandescent lamp life by a factor 3.5 over rated; 10 % above, a corresponding factor 3.5 reduction in life."


Whether it's the "13th power" - or a bit more or less - depends on the particular lamp or the person you ask. This old article (The life-voltage exponent for tungsten lamps, Covington 1973) should be a very good source, but I can't access more than the first page. The exponent can also vary when moving away from rated voltage.

The 55% of lifetime that you mention suggests instead an exponent more like 14, because:  1 / (240/230)^14 =~ 0.55. But it doesn't make much difference: with an exponent of 13 it's about 58% ...  it's anyway a big change in lifetime or light output, for not many percent in voltage.