NEC Wiring Questions

Re: NEC Wiring Questions

In general, you should be using the rated solar wiring for the panels/roof runs. UV is very hard on insulation and so can animals (squirrels and other other animals with less good press). So conduit would be a requirement too--And I believe the NEC would require rated conduit into the home until the DC disconnect (and fusing/breakers, if required).

Using your welding cable for the battery bank may be OK with your inspector. As always, lead acid batteries can output a crazy amount of current (probably over 1,000 amps into a dead short). So insuring that the cables are properly supported, no short metal to cut insulation, and a cover over the battery bank to protect against dropped tools and kids too.

Now, before we get into the hardware--Let us look back at your needs/existing hardware. Two ways to go--First start with your loads, or second to start with your existing hardware.

Solar power systems are very difficult to expand in a cost effective manner. And, in your case, starting with a 24 volt battery bank will make it a bit easier to design/build out your system with a good amount of available energy.

I would suggest that you get a Kill-a-Watt type meter to measure your energy usage (peak watts, average watts, how much energy per day you use in Watt*Hours/kWH).

To run an energy star rated fridge/freezer and a few other random/small loads, you probably are looking at a 1.5 kWH per day minimum. And I would suggest around 3.3 kWH per day for a serviceable off grid system that could keep you going for months (or longer).

Since the battery bank is the "heart" of your power system, I like to design a balanced charging system around it. Lets pick 1,000 WH (or 1 kWH) per day as your loads (I cheated, that is what your present battery bank with "ideally" support--but to show the math...). Assume 1-3 days (pick 2 days for "balanced" system) and 50% maximum discharge for long battery life... A battery bank would look like:

• 1,000 WH per day * 1/0.85 inverter efficiency * 1/24 volt battery bank * 2 days storage * 1/0.50 max discharge = 196 AH @ 24 volt battery bank

So, close to the battery bank you already have. Next, we need to look at charging that battery bank. Our rule of thumb is ~5% to 13% rate of charge for a happy battery bank. The size of solar array needed would be:

• 210 AH battery bank * 29 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 395 Watt array minimum
• 210 AH battery bank * 29 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 791 watt array nominal
• 210 AH battery bank * 29 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 1,028 watt array "cost effective" maximum

So, your present single 310 Watt panel is a bit on the small size... Two to three of your 310 Watt panels would be a much better match for your existing battery bank.

Now, how much power will an array supply... Using PV Watts and guessing you are around Milwaukee Wisc., with an array tilted to your latitude (tilted from horizontal, fixed array, and you might think of tilting to near vertical in winter if you have a lot of snow for a "self clearing" array), we get:

Month    Solar Radiation (kWh/m 2/day)
1      3.32     
2      3.96     
3      4.28     
4      5.11     
5      5.58     
6      5.88     
7      5.66     
8      5.60     
9      5.19     
10      4.34     
11      2.96     
12      2.49     
Year      4.53

Toss out the bottom three months (assume utility power/backup genset use during poor weather), we get February at 3.96 hours of sun (noon time equivalent) per day as the "break even point". A 791 Watt "nominal array" would output a minimum (long term average) of:

• 791 Watt array * 0.52 off grid system efficiency * 3.96 hours of sun per day = 1,629 120 VAC Watt*Hours per day

Note that running a freezer on an AC inverter may need the inverter to run 24x7 -- And inverters have significant "tare losses" (power used just because the inverter is "on")--Which can easily be in the 6 to 20+ watt range... For example a 20 watt tare loss (larger inverters have larger losses), that is 480 Watt*Hours per day--Or over 1/4 of your available power in February.

So, before we go further--What would your plans be for the system (larger array now, or expansion plans later, etc.)?

-Bill