Help Understanding Voltage Drop To Outbuilding / Possible Need To Upsize Existing Wiring

[greeneggsandcrack]

I tried to make this post as detailed and clear as possible, to get the best advice. It is therefore long.


We bought a house that was built in the 90s. There is an outbuilding with a subpanel in it about 300 feet away. This subpanel is supplied by a 240V / 30A breaker in the main panel. Ordinary residential single phase power. 3 wires, two line and a neutral.

The outbuilding roof is the only suitable location for a solar array, and we want to put about 5KW on it, fed back to the home over the existing line.

Even at max theoretical production (which solar arrays never hit, and our install certainly wouldn't due to not-quite optimal orientation and panel angle), 5000W at 240V is just ~21 A, so this existing circuit appears adequate to carry the array's output back to the home.



The area where I think there might be a problem is that they used 10g wire for the run (buried). This seems rather undersized for the distance.

The drop calculator at Southwire's web site estimates a 10.49% voltage drop for a 20A load at 120V, a 5.25% drop for a 10A load at 120V, and 7.87% for a 30A load at 240V. That seems to be not OK.


I went ahead and measured actual voltage drops from the main panel to the subpanel, with no load and under two different loading conditions. I have an Iotawatt monitor installed in the main panel, so I get accurate realtime readings of the load to the house as a whole and various circuits, including both legs of the 240/30A line to the outbuilding.

The results are a little puzzling and I'd like some help understanding them, and deciding if the wire buried in the ground actually needs to be upsized. (The existing setup has worked Just Fine for running lights and tools in that building for the 3 years we've lived here and presumably for the decades the previous owners did.)


For loading the outbuilding, I had three scenarios:

• nothing on except a network switch on one leg (70W + 3W)
• lighting and space heater on different legs (965W + 1265W)
• lighting and space heater on one leg (1930W + 3W)

It was a little difficult to get all the readings with the same total load on the home, since the HVAC and other loads occasionally came on and off. But for each load condition in the outbuilding, I got main and subpanel readings with similar total household loads:

Measurements were made with a Klein CL390 meter directly at the main panel circuit breaker, and the subpanel input.


Voltage drops with ~2000 watt total load on home, and 70 + 3 watt load to outbuilding:
242.0 to 241.7 (-0.1%)
R 120.3 to 119.8 (-0.4%)
B 121.8 to 121.8 (no drop)

Voltage drops with ~4500 watt total load on home, and 965 + 1265 watt load to outbuilding
240.1 to 233.1 (-2.9%)
R 120.1 to 118.1 (-1.6%)
B 120.3 to 115.1 (-4.3%)

Voltage drops with ~4000 watt total load on home, and 1930 + 3 watt load to outbuilding
239.4 to 234.4 (-2.1%)
R 117.8 to 106.7 (-9.4%)
B 121.8 to 128.2 (+5.3%)


No surprise, no voltage drop with no load. I can wrap my mind around that.

Small surprise, with a balanced load of about 1000W per leg, there was somewhat less voltage drop than predicted by the calculator: 5.25% predicted but 1.6% and 4.3% measured. I guess that's not too far off predicted.

Weird surprise, with the unbalanced load of about 2000W on one leg and nothing on the other, a 9.4% drop on one and a 5.3% INCREASE on the other. I don't understand that


Can someone explain the measurements with the unbalanced load?

Will this arrangement be OK to leave alone? It has apparently worked fine without spontaneously combusting for 30 years.

Any thoughts about using the existing line to feed (at most) 5000W at 240v back to the main panel from a solar inverter? Even under load the total voltage drop was under 3% for the overall circuit.


There are two phases of permitting required for a DIY solar install. One from the county (which is primarily concerned with structural issues for the roof loads and zoning) and one from the power co for the grid tie. Eventually I'll pay for professionals to assess those things. But as I'm in the planning stages now, I'm trying to understand all of the issues and get an idea of what to expect.


Thank you.

 

[James]

Weird surprise, with the unbalanced load of about 2000W on one leg and nothing on the other, a 9.4% drop on one and a 5.3% INCREASE on the other. I don't understand that


Can someone explain the measurements with the unbalanced load?

ok so on a 3 wire connection you have +120v N and -120v

in actual fact, being ac, the lives are actually the same voltage but 180 degrees out of phase but for simplicity we will call them + and - because they are opposite to each other and for simplicity the difference between -120 and + 120 is 240v

the N connection should be at 0v

therefore on one live you get 120v to N and on the other live you get 120v to N

when you put a heavy load on the cable, you will get a voltage drop along the cable.
this is true for both the live cable and the N cable. assuming they are the same size and length then the drop should be equal in both of the cables.

therefore you are seeing aprox 10% drop when measuring between the live that has the load attached and the N.
call it 5% per cable
the 120v has dropped by 6v to 114v
the N (ov) has risen by 6v (in relation to the live cable under load) meaning instead of the original difference between 0V and 120v
you are now looking at a difference between 6v and 114v (108v) that is 10% less than the original voltage.

now, remember the other live cable that has no load on it and is giving -120v the difference between 6v and -120v is 126v or 5% more than 120v

so assuming your new solar kit is generating at 240v and supplying 5kw balanced across the 2 lives, it would work out to be 2.5kw per live wire (not strictly true but should help you understand)
therefore from your calculations i would expect you to be seeing a volt drop of just under 12% when loaded to 5kw

 

[greeneggsandcrack]

Thank you - that makes sense.

I suspect this'll all be a moot point. Code for solar here seems to require a PV disconnect located immediately adjacent to the MAIN circuit breaker panel. I assume this is so any electric co workers coming to mess with the meter will know there's a PV system and easily see how to disconnect it. I've asked the electric co if the disconnect can be adjacent to the subpanel, but haven't heard back yet.

So I'll probably have to run a dedicated line from the solar inverter back to the house to the main panel. And if I'm going to dig a trench for that, I might as well use something more appropriate than 10 g wire.


Related question -

I assume all 3 conductors should be the same size. At present the line to the outbuilding is three 10g conductors - two line and the neutral. No ground. The subpanel has a separate bus for a bare copper ground wire that appears to be bonded to a spike in the ground.


6g wire seems more appropriate for that distance.

I was browsing around for some 6 g direct burial wire, and I see that the 6/3 stuff has a smaller non-insulated ground wire. For example -

https://wirewireelectricsupply.com/product/6-3-uf-b-wg-underground-feeder/

Is that 4th ground wire even necessary? Why is it smaller? Is there a reason to use this vs a product that has no 4th ground wire, and grounding devices directly to the earth at that location?

Thank you for the education.