View Full Version : Voltage Drop Calculators

Chris J
06-11-2008, 09:12 PM
Speaking of calculators, does anyone use these computer based voltage calculators? Do you find them helpful? I have alway just computed my formulas manually with a hand held calculator. I have gotten to the point were I can do it extremely fast with pretty fair accuracy. Of course I always confirm everything with my meter.
Just wondering if any of you find these calc's useful.

06-12-2008, 02:20 AM
its rare but we do use the cast one when we will be close on voltage just to do a quick check during the planning stage. I got the unique wiring one but never really used it.

Like you we get real close with pen and paper then verify with the meter. Somewhere at lightapalooza someone had a sliding card one... Was that the unique rep ? I thought I grabbed one but who knows.

David Gretzmier
06-12-2008, 02:25 AM
I'm more a paper guy on the jobsite.

Funny thing is, I use the FX graph on the jobsite and what is fairly unpredictable is how the trans will perform. on a similar 100 watt load on 12 guage wire, at aprox the same distance from the trans, It seems nightscaping trans will crash the voltage as it is loaded, FX trans hold voltage no matter how you load them, even seeming to prevent normal drop, and the ryco's are in the middle. go figure.

06-12-2008, 02:33 AM
Funny topic,
I am doing a rewire on a system that was installed with Vista single tap transformers. I had one that read 120.4 volts on the incoming primary and 11.7 volts on the outgoing taps which were (unloaded) at the time. Loaded, they were reading 11.2 at the tranny. This is a problem. Looks like it's time for a new tranny.
I usually just calculate VD with a measuring wheal and wire constants. Once I have the tap outlet performance recorded I can get pretty close and verify with my meter.

Chris J
06-12-2008, 02:45 AM
I had one that read 120.4 volts on the incoming primary and 11.7 volts on the outgoing taps which were (unloaded) at the time. Loaded, they were reading 11.2 at the tranny.

Tim, this is typical of all transformers. Some will seem less than others, but all transformers will drop when loaded. Vista is pretty good about their windings so that you get more up front to compensate for the drop, but they will all drop. Another consideration is the incoming line voltage (which you know already). The lower the incoming, the lower the outgoing low voltage is going to be. Not preaching to you, just giving info for other readers. 120v in = 12v out. 110v in = 11v out. It's a ratio of 10:1.

06-12-2008, 02:51 AM
Yeah, I got that. But I had 11.7 volts with the circuits unloaded. Generally you might read 12 or a little higher because they are going to compensate on the high side for VD. All the other trannies on site were ok but this one. It didn't matter what guage cable I used, I was fighting a losing proposition with a single tap tranny.

David Gretzmier
06-12-2008, 03:10 AM
I'm not so sure of the 1-10 ratio. I've seen single taps all over the place, sometimes 12.4 volts loaded on 120 volts. i've seen several nightscaping trans with 11.5 volts unloaded with the switch on HIGH. I think that maybe trans might lose some volts as they get older? I know I've lost hair...

06-12-2008, 09:41 AM
Transformers will vary to some degree and thats why you need to verify with a meter.

Even MDL transformers from different "manufactures" vary widley. Burt and I have both noticed when we swapped out cast 600 for another brand of 900 that we had to bump up to the next higher tap.

I got some 7 year old vista multi taps at my home that seem to hold the voltage. The Gambinos fall right in line with the rating when loaded... but alot of MDL and definatly nightscaping units we have worked with fall flat on thier faces and lose over 1v when loaded.

I think I have one MDL out there per calculation should be on the 13-14v (14v should give me 12v) tap. Its now on the 15v tap and we are still borderline low @ 10.9 at the fixtures.

06-12-2008, 10:35 AM
I encourage everyone to look at the voltage their units are putting out at the taps before putting a load on it then test it with a load. Most of you will be blown away with the results. Rarely do you have a unit that actually has 12v at the 12v tap even before being loaded.

As for the Voltage Calculators. We have had numerous different charts, slide rulers, and our wiring wizard calculator for many years. They are very useful. We use the wiring wizard primarily for the job doucumentation side. When I am helping a contractor or designer to solve VD or know what the VD will be I always refer to our slide ruler. It is very usefel and accurate. Of course these tools are only as accurate as your measurments! I cant wait for our new one, it will be the best for sure. Right now we have some programs for us to use in house that we developed that allow us to calculate both for 12v and 24v. They are beta versions that we will probably dress up and share with you guys in the near future.

06-12-2008, 11:34 PM
Hey Billy,
What typically will Mike G's trannies read unloaded. I think you said his is bumped up initially to compensate for the load drop.

06-13-2008, 01:42 AM
no load about 6 to 7 tenths high in my findings. If I understood correctly they are engineered to put out 12.8v on the 12v tap at 120v input voltage. Keep in mind there is an 11.5v tap.

I have a 600w unit I just checked on tonight that is loaded to 510w and its reading dead on 12v at the 12v tap and dead on or within 1 tenth on all the others. 119v going in.

I have a 2 yr old Kichler trans at a home I serviced...I did not install it. There is no 14v tap on it but the 15v with 400w load on the 600w trans is reading 13.2 ... nearly a 2v drop off the mark.