20w brighter than 50w

[sitelights]

I recently serviced an outdoor lighting system (not one of mine) with very poor light output. The parameters were: 500w transformer (13v tap), 12/2 gauge cable in a "T" configuration with a total of 160' (why use 12/2 when 10/2 gauge provides so many advantages?), and 10 50w MR-16s. By changing the MR-16s to 20w the light levels came up so sharply that I had to change to the 12v secondary. I quantified the before and after results with my Sylvania footcandle meter by placing a white sheet of paper 12" from the lamp front and reading the reflected light. The light output at 20w per lamp was twice as bright as with the 50w lamps which was why I went down a volt at the transformer which was panting with effort (very hot to the touch). If you hit a 20w (vs. 50w) lamp with 11.5v to 12.0v instead of the 8.0v to 9.0v (to the 50w lamps) lamp output goes up and energy costs go down. As a plus, since 8-9v will not trigger the halogen cycle, there will be no lamp blackening which was obvious on the 50w lamps. As Mies said "less is more".

[steve gass]

Interesting. Could similar results have been obtained by using a larger transformer, or would the 12/2 cable be too limiting?

[sitelights]

The original set-up (500w load on a 500w rated transformer) had the transformer running flat-out and the resistance of the 12 gauge cable added about 100w to the load. Think of towing a boat with a passenger car where the available torque is sufficient on level ground (low load) but too meagre on the hills (high load). When you go from a 600w load (500w lamps, 100w resistance) to a 250w load (200w lamps, 50w resistance... the resistance decreases relative to the lamp load) the engine (tranformer) can rev more freely resulting in better performance. This better performance in a low voltage system results in higher voltage to the lamps which, in turn, creates higher lumen output. This is quantifiable and repeatable, not just anecdotal. Under the original conditions, replacing the 12 gauge cable with 10 gauge would have produced only a marginal gain not really worth the cost of replacing the cable.

[sitelights]

To answer your question directly, the addition of a larger transformer would further exceed the amp rating of the system in question. The original lamp load was 500w + 100 = 600w divided by 12v = 50 amps which exceeded the maximum secondary rating of 30 amps. The modified lamp load plus cable was slightly over 20 amps. The addition of 10 gauge cable might lower that to 20 amps but it would not have lowered the 600w scenario significantly. A more powerful engine puts the drivetrain at risk. unless you balance the system. The most costly fix would have been to add another 500w transformer and split the system into 2 300w loads but the light output of the 10 50w lamps would have been overkill since 200w at full output was adequate. The energy costs were cut in half.

[Pouly]

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