A very simple lighting setup. For the sake of discussion, let’s say I have a 12/2, 125' main run to a VOLT Pro Junction hub, and that hub has three (3) 16/2, 25' lead-wire, 4W fixtures and four (4) 18/2, 25' lead-wire, 5W fixtures attached to it. How do I calculate various cable constants on a single run? Do I combine all the similar-gauge wire with their respective wattages and come up with two different voltage-loss figures from the single hub, and then factor in the main run back to the transformer? Four 18-Gauge/5W Fixtures (20W): 100' x 20W x 2 = 4000 / 1380 = 2.9 Drop Three 16-Gauge/4W Fixtures (12W): 75' x 12W x 2 = 1800 / 2200 = 0.82 Drop One 12-Gauge/0W: 125' x 1W x 2 = 250 / 7500 = .033 Drop TOTAL DROP: 3.75V OR Add up each individual fixture/lead wire to the hub and multiply by the number of similar fixtures: One 18-Gauge/5W Fixture: 25' x 5W x 2 = 250 / 1380 = 0.18 x 4 (fixtures) = 0.72 Drop One 16-Gauge/4W Fixtures: 25' x 4W x 2 = 200 / 2200 = 0.09 x 3 (fixtures) = 0.27 Drop One 12-Gauge/0W: 125' x 1W x 2 = 250 / 7500 = .033 Drop TOTAL DROP: 1.02V Can someone please confirm which method is more accurate or if there’s another method? Thanks.
Hi LCD, Thanks for your question. Years ago, I spent hundreds of hours working with load balancing experts to come up with calculators that would predict voltage loss. That was back in the time when we were still using halogen lamps - sometimes mixing them with LEDs. Now, those calculators are gathering digital dust as we become firmly embedded in the age of low wattage LEDs and wide acceptable voltage ranges. If you call our customer support line, they will suggest a transformer size and wire gauge based on a simplified formula - that you can find in our transformer manual. I haven't heard any complaints about those recommendations. Still, as the world's biggest voltage-loss geek, I don't want to ignore your specific questions. Your first set of calculations is off because you are using each lead wire gauge as though that is the gauge for the entire run. Your second set of calculations would be fairly accurate if you used the total fixture wattage (32W) for your main 125' 12/2 line. Using the above correction, I get 2.05V loss. If we ignore the lead wire, then we get 1.07V loss. If you're starting at 15V at the transformer, you're ending up with either 13V or 14V - both well with the 10V to 15V acceptable range. That's why we don't obsess over lead wire losses. By the way, keep in mind that these are all rough predictions. Regardless of how many years I spent with the calculations, I was still often surprised by voltage loss measured in the field. That's why we keep a voltage tester on-the-hip to confirm that the numbers are good.
Thank's Steve. So, just to be clear, a proper calculation for the above hub/fixtures would look like this: All fixtures to Hub without including the Main Run One 18-Gauge/5W Fixture: 25' x 5W x 2 = 250 / 1380 = 0.18 x 4 (fixtures) = 0.72 Drop One 16-Gauge/4W Fixtures: 25' x 4W x 2 = 200 / 2200 = 0.09 x 3 (fixtures) = 0.27 Drop Fixtures-to-Hub DROP: 0.99V Hub-to-transformer main run (combining all fixtures/watts) One 12-Gauge/32W: 125' x 32W x 2 = 8000 / 7500 = 1.07 Drop Hub-to-Transformer Drop: 1.07V TOTAL Drop: 2.06V OR, using 10/2 wire... Hub-to-transformer main run (combining all fixtures/watts) One 10-Gauge/32W: 125' x 32W x 2 = 8000 / 11920 = 0.67 Drop Hub-to-Transformer Drop: 0.67V TOTAL Drop: 1.66V I already own a VOLT 300 watt multi-tap transformer. The formula in the manual assumes using a single gauge in its calculations, hence the multi-gauge question. I suppose that adding more hubs to this single main-run, via daisy chaining or a T-connection, is similar in that you add each extra hub's voltage drop and then combine them all for your final main-run calculation. I agree with testing final connections, but these calculations are valuable in the planning and cost-estimating stages, especially with long main-runs and wire gauge. Thanks again.
