MNBCM far from batteries

hemmjo

I posted this in the NEC wiring section last week but got no response;

I am doing another installation for the mission I work with in the Dominican Republic. The panels, inverter, controller, etc, will be in one building, the kitchen. There is little need for electric here (two 5 watt LED bulbs) as all cooking is done over wood, but lots of space for the components.

130 feet away, there is a small house for the property caretaker. We are running 3 strands of #10 thhn wire to provide about 7 amps max to run a refrigerator and a few led bulbs.

We installed 1" pvc pipe underground between the buildings when we ran a water line between the two builldings.

I would really prefer mounting the MNBCM in the house rather than the kitchen. This would allow the caretaker to keep an eye on the system without opening up the kitchen which is only used when we have large groups there, 6-7 months out of the year. More important, it would be good for his wife to see how her energy use is affecting the system.

Would it be terrible to run a pair of #14 thhn wires in the conduit along side the #10's that will be carrying the 120vac? Will I get too much induced energy into the low voltage wires? I have read where it is OK to run high and low voltage together in the same conduit as long as all of the conductors are rated for the highest voltage. The installation will have no inspection but I have no desire for it to be unsafe.

It is not practical to hand dig another 130 feet of trench if I can run the wire in the same conduit.

Thanks,

John

BB.

OK... the unit takes ~12.5 mAmps @ 12 volts (does not give higher voltage power usage). Looking at the battery state of charge table, the meter uses 0.12 for every 10% state of charge change. And say y0uo have 150 of one way cable run.

Using a generic voltage drop calculator, lets see what gauge wire of wire would give us 0.12 volt drop (maximum "acceptable" error voltage.

http://www.calculator.net/voltage-drop-calculator.html

24 AWG wire:
Voltage drop: 0.092
Voltage drop percentage: 0.77%
Voltage at the end: 11.908

So, using 24 AWG would give us 0.09 volt drop.

If you wanted to get 0.0012 volt drop (1% error at meter), then:

14 AWG gives:
Voltage drop: 0.0091
Voltage drop percentage: 0.076%
Voltage at the end: 11.9909

So 14 AWG would seem to give very good accuracy.

Then there are the noise issues... Normally, placing next to AC cable is a good way to get coupled noise into the cable. Shielding would be nice, but probably not practicable.

The two things I would try--Lay out your 14 AWG run (lay the two cables at ~150 side by side on the ground). Hold/take the pair at one end and chuck the other end of the pair in an electric drill. Give the wires a twist rate of ~1-2 feet per twist. That will help prevent coupling AC noise between the AC and the DC cables (you could do the same thing with the AC cables too--But may be too heavy of wire to do that and pull in your 1in conduit. Remember that twisting the cable will shorten the run a bit--So either add some extra cable so the run is "long enough" or take 10 feet and give it 5 or so twists and measure how much it shortens. Also, twisting too "tight" may make the cables harder to pull. You might have to dig up the middle of the conduit run and pull 1/2 the conduit first, then the second 1/2. Also, they do make stuff to put on the cable to make it easier to pull (do not carry on plane--checked luggage only--my guess with TSA).

Perhaps somebody here with more experience can tell you about pulling 10 AWG + 14 AWG pair and how hard it would be in 1" conduit (next time, if you can, bury much larger diameter or even a pair of conduit to give you more options later).

Other things to do--Get some small capacitors (0.01 uF, 0.1 uF, 1 uF) ceramic/polyester capacitors. Solder/screw these to the ends of the wires at the meter... Exact values do not matter--You just want good/small capacitors (high frequency type) with a wide (10:1 to 100:1) range of values so that they "cover" a wide range of frequencies to filter. These help filter out high frequency noise.

https://en.wikipedia.org/wiki/Filter_capacitor

If you can find a ferrite bead (look for ones that have lower frequency ratings)--But anything you can find locally (sometimes a local electronics "junk store" will have these on electrical wiring in power supply/cabinets):

http://products.mercola.com/ferrite-...eads_howto.htm (this is a split bead--There are also solid rings).

Again, reduces the electrical interference.

When routing the cable (near battery bank, near meter). Keep the twists right up to the connections as best you can. When you split the ends to make the final connections, the untwisted portions now act like antenna. Keep the untwisted ends "away" from other wiring (again as best you can). I.e., do not run an untwisted single wire, or make a "wire loop" next to your AC mains or DC inverter cabling for 1 to several feet in parallel, Cross at right angles if you can.

That is what I would try.

-Bill

hemmjo

Thanks BB,

The conduit is plenty big enough, it could handle 15 - #10 thhn conductors at 40% fill. My concern was from the possible noise. I will try the twisted pair and use the ferrite on the end. I have some ferrite "donuts" I could put on the end of the #14's would that have the same effect at the "beads"

John

BB.

Sounds good. Have fun down there.

You can take several turns of wire though the donuts--If room/needed.

-Bill