Length of extension cord

JRR

 Hi, I am in the planning stages of a solar setup.  I have 500 watts of solar panels I want to install on the second story of a house.  The cable will be run down to the front porch to a 360 amp-hour battery bank.  From there, I'll run cables a few feet through the wall to the charge controller and inverter.  I have 2 inverters, a 600 watt inverter for most stuff, and a 2000 watt inverter for the microwave and anything else that might need more power.  It will be used infrequently.  I will need to get power into another room about 30 feet away with a total wire run of about 50 feet.  I'll be powering a desktop computer with modem, router, etc, and a small 3.1 cubic inch refrigerator.  I also have a 700 watt microwave I might use for 5 minutes a day.  I will also have a few dc lights and an Engel refrigerator/freezer on the 12 volt side.  Total wattage of the ac stuff is about 330, not counting the microwave.  DC wattage is minimal, less than 40 watts.  My question is will 50 feet be too far to run an extension cord from the inverter, and what gauge coed should I use.

Thanks for any advice.

cow_rancher

JRR said:

 Hi, I am in the planning stages of a solar setup.  I'll be powering a desktop computer with modem, router, etc, and a small 3.1 cubic inch refrigerator.  Total wattage of the ac stuff is about 330, not counting the microwave.  My question is will 50 feet be too far to run an extension cord from the inverter, and what gauge should I use.

A 3.1 cubic inch refrigerator is real small, we assume that is a cubic foot refrigerator.

A medium duty 50' extension cord is 14 AWG and it will handle 14-15 Amps, you are showing less than 3 Amps.

Rancher

Photowhit

There won't be a huge drop over a 50 foot extension cord carrying 300 watts, a 14 or even 16 gauge should be less than 2% (without looking at a calculator)

I think you might not understand the wattage rating, an item, say your small fridge, that is rated at 80 watts, is 80 watts per hour. A desktop computer rated at 160 watts is 160 watts per hour.

Say a 30 watt DC load for 24 hours would be 720 watt hours, or half of an average days output from a 500 watt array. Solar panels put out about 75% of their panel rating, and average locations in the US have about 4 hours of sun per day...

Also inverters will use some power, if you wire both to the same home wiring be sure to put them on a transfer switch so both can't be sending power at the same time. Even a 3 way battery isolator switch might work.

...and the cables, I suspect will come to the charge controller first then the battery bank...

JRR

Photowhit said:

I think you might not understand the wattage rating, an item, say your small fridge, that is rated at 80 watts, is 80 watts per hour. A desktop computer rated at 160 watts is 160 watts per hour.

...and the cables, I suspect will come to the charge controller first then the battery bank...

I understand, I just used the wrong terminology.
As for the cables, I know how to wire it, I just got ahead of myself. 

Thanks.

cow_rancher said:

A 3.1 cubic inch refrigerator is real small, we assume that is a cubic foot refrigerator.


Rancher

It's bigger on the inside!

Thanks.

Estragon

14ga is good for a 15a household circuit. Assuming the 2000w inverter has 15a outlets, 14ga is okay. If it's a single (hardwire?) output, it should be 12ga.

The DC stuff should be bigger to avoid voltage drop if the loads are also ~50' away - maybe 6 or 8ga. If loads are close to batteries, wire could be smaller. You could plug number into an online voltage drop calculator. Breaker/fusing needed based on wire size chosen.

BB.

You can use a generic voltage drop calculator like this (just plug in different numbers to get the final "close enough" answer):

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

Generically, we use 3% maximum voltage drop and 1% (or less) minimum (you start wasting money on copper going with less drop).

However, you do need to look at the absolute voltage needs too. For example, a 120 VAC device will probably run nice on 110 VAC (9% voltage drop). However, a 12 volt device will need at least 10.5 volts, and 2x surge current on a 11.5 volt (~50% discharge lead acid battery under load)... That works out to 0.5 volt drop at rated load (and allows for 2x surge current) or ~4% maximum drop.

Obviously, you have the maximum current for a particular diameter wire (temperature, insulation, in conduit, etc.) so you need the wiring tables to look that up. Also, NEC tends to be more conservative than typical "boat" standards:

https://lugsdirect.com/WireCurrentAmpacitiesNEC-Table-301-16.htm
http://www.boatus.com/boattech/articles/electrical-wiring-on-boats.asp (table #3 1/2 way down page)

There is usually a good reason to go towards 1% voltage drop--Less voltage drop at appliance, better operation, and less power loss. I.e., even though you could live with a 10% power loss, it means 10% more solar panels, 10% more battery bank, 10% larger inverter, etc.

However, in some cases higher voltage drop is your friend. For example, starting AC compressor motor on the refrigerator, a higher voltage drop will reduce surge current for the motor--Sometimes enough to let a smaller inverter/battery bank start the fridge or circular saw.

For example, 40 Watts and 12 AWG wiring, how far? Assume that a maximum of 0.5 volt drop and 40w/12v=3.3 amps:

http://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=5.211&voltage=12&phase=dc&noofconductor=1&distance=50&distanceunit=feet&amperes=3.3&x=66&y=6

12 AWG and 50 feet:
Voltage drop: 0.52
Voltage drop percentage: 4.33%
Voltage at the end: 11.48

12 AWG is good for at least 20 amps (NEC), and in ABYC 38 to 45 amps--But the realistic limit is 3.3 amps for 50 foot run.

And, for solar power, I always suggest that you use 1.25 NEC derating for designing your system breakers/wiring. For example, a 3.3 amps circuit should have a minimum wire+breaker rating of:

• 3.3 amps * 1.25 NEC derating = 4.125 amps -- Rounding to ~4-5 amps minimum

This will reduce the chances of false trips on fusing/circuit breakers and keep wiring cooler (and wire terminations cooler too). NEC designed breakers and fuses (typical UL/NRTL) devices are designed to "eventually" blow at 100% rated current (can take hours)... Battery charging (solar, genset, utility power) can take many hours to fill a 1/2 discharged (or deeper) battery bank and eventually trip breakers or the excess heat cause failures of breakers/fuses/terminal ends.

-Bill