Voltage Drop - Electrician Talk - Professional Electrical Contractors Forum

brian john · 09-03-2007, 11:09 AM

I recently was hired to determine why a specific site had excessive voltage drop (VD). I did some basic load, no load measurements and took some distance measurements. There was no excessive VD, it was pure math,

I cheated and used an on line calculator, I do have an excel file in my office I devised that uses true k in lieu of estimated k for variuos wire sizes. The difference can be approximatly .3 plus (for small gauge wire)

http://www.csgnetwork.com/voltagedropcalc.html

What surprised some including me is the maximum distance you can run #12AWG and #10 AWG at variuos loads.

Look at the distance for #12 AWG at 16 amps.

**MDShunk** · 09-03-2007, 11:21 AM

Landscape lighting is one time when it's handy to know the capacity of 10 and 12 at various lengths. The lengths on the chart seem pretty long, but when you start wiring lights in a warehouse, it suddenly seems short again.

I'm curious to know who at that site thought they had excessive VD, and by what means they thought they made that determination? Sounds like someone made an expensive incorrect determination. Maybe it's worth it to some people to pay you so that you can say they don't have a problem?

brian john · 09-03-2007, 11:45 AM

Years ago a customer freaked out when we gave a cost for LV landscape lighting, his other contractor was 1/2 our price, I said let him have it.

Months later we were hired to install the system properly, removed all that #12 UF and relocated the transformers, installed #8 to the furthest lights and the customer apologized.

**MDShunk** · 09-03-2007, 12:02 PM

I just re-read your post, and it sounds like the customer might not have had excessive voltage drop. It sounds likey they had all the voltage drop they were supposed to have. In other words, that's what the design called for, so that's what they're stuck with. Excessive is a subjective term. Excessive might imply that there was something other than long wires at play, causing the VD. If it's just long wires, you're just getting all the VD you paid for.

Joe Momma · 09-03-2007, 04:06 PM

I know the obvious answer to this question would be to check voltage at the source and then check voltage after the last device and then figure the percentage.

But would ever be okay to see 120 at the source and assume that 108 would be acceptable at the end, being that 110V is also an acceptable voltage? Or would you assume 115 for all your calculations.

We did a service change not long ago where the utility had us at just over 500 volts at no load. It seemed pretty hot, but I guess it was within tolerences, but that would only you to be at 475 before you reached a 5% VD

brian john · 09-03-2007, 10:04 PM

What we did was take no load and load readings at the receptacle, then calculate VD actual voltage was around 117.2 at no load, we also verified load and no load readings at the panels and transformers.

The job specs for this particular facility required no more that 2% VD or 2.34 volts AC at 117 VAC.

PC's actually have an operating voltage in the range of 87-132 VAC or close to this. Motors are more susceptible to low voltage.

We also do power monitoring and have seen variances from 502 VAC at night no load to 460 at full load peak operating hour.

Had one facility that was seeing 427 VAC last summer, crashed the UPS from depleted batteries several times, we installed a one to one transformer with the taps set to 480 in lieu of the 465 that was standard for this area. The UPS was designed for 480 VAC.

Joe Momma · 09-03-2007, 10:49 PM

I always thought motors were most forgiving with a 10% tolerance, and electronics were most sensitive to voltage variations.

brian john · 09-04-2007, 06:26 AM

More on voltage drop with #12 AWG at various distances.