# Off Grid (RV) Design Check &amp; Critique - Especially wire sizes &amp; circuit protection

Registered Users Posts: 11
I have the makings of what I hope will be a good solar / inverter install for our small (24 foot) RV laying out in my garage.

The major components are already purchased and here, some of the 'balance of system' components are still to be picked up.

All comments are appreciated, especially safety & efficiency related; including wire sizes and circuit protection.

Array size is limited to 4 panels.

• Super Moderators, Administrators Posts: 32,027 admin
Re: Off Grid (RV) Design Check & Critique - Especially wire sizes & circuit protection

A 2,000 Watt AC inverter on 12 VDC and a 300 amp fuse... You are talking about some very large currents and lots of copper wire. Depending on the wiring standard you choose to follow, 2 AWG is barely large enough for ~300 amp fuse:

http://lugsdirect.com/WireCurrentAmpacitiesNEC-Table-301-16.htm
http://www.acbsphl.org/Tips_and_hints/ABYC_Wiring.htm

And look at the voltage drop of each of your high current 12 VDC power runs.

For Charge Controller to battery bank, you want something along the lines of 0.05 to 0.10 volt drop maximum (as we discussed before, those long wire runs from charger to battery bank are not going to serve you well).

For an AC inverter on 12 volts, the voltage drop can be as high as 0.5 to 1.0 volts--But I would try for 0.5 volt maximum if you plan on really running it near 2kWatts.

Remember that your inverter+charger is also a charger--So, large voltage drops during charging will tend to slow it charging.... If you run grid power mostly and not too much generator--It is not a huge problem.

2,000 watts * 1/0.85 inverter eff * 1/10.5 volt battery cutoff = 224 amps max continuous
224 amps * 1.25 NEC wiring/breaker/fuse derating = 280 amp minimum branch circuit+fuse

Use of a disconnect+fuse -- If you can find a large enough, properly rated, circuit breaker--That may be a good solution. Many battery disconnect switches are not really good at running near rated current. And are not good at breaking active current flow without damage.

Regarding inverters... If you stay with 12 volts and still need the large inverter at times... I would look at two inverters. The large one you can use for large loads, and a small MorningStar 300 Watt TSW 12 VDC inverter for small loads (electronics, laptop, battery chargers, cell chargers, a few 120 VAC LED/CFL lights, etc.). The small inverter will have much smaller idling losses and the loads will be much happier on TSW (true sine wave) power. The MS inverter has remote on/off switch (no large disconnect needed) and a low power "search mode" (only "turns on" when there is >6 Watt 120 VAC load. Can save you a fair amount of power usage.

Are you planning on adding solar panels, or is the 4 panels the most you will install? The Midnite 30 amp controller is maxed out on your 12 volt battery bank.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Banned Posts: 17,615 ✭✭
Re: Off Grid (RV) Design Check & Critique - Especially wire sizes & circuit protection

One thing I see is the 600 Watt array will produce more current than the Kid can handle by about 8 Amps. That in itself is not a big issue. However the addition of a second set of batteries will reduce the effective solar charging considerably. If these are GC2's you'd go from (30 Amps into 220 Amp hours) 13.6% peak current to 6.8% peak current. This may still work if you don't mind a bit of gen run/shore power time.

Oh and 2/0 will not handle 300 Amps: that fuse needs to be down-sized to 200 Amps or else the wire size increased to 4/0. Most 12 Volt systems would run 4/0 for the DC side.
• Solar Expert Posts: 10,300 ✭✭✭✭
Re: Off Grid (RV) Design Check & Critique - Especially wire sizes & circuit protection

adding to what was already said, you cannot tie 240vac to this inverter as it is meant only for 120vac. to cover both sides of the breaker box you will need 2 separate inverters or an inverter capable of supplying the 2 hot legs. their ms-pae series is capable of 240vac
• Registered Users Posts: 11
Re: Off Grid (RV) Design Check & Critique - Especially wire sizes & circuit protection
BB wrote: »
2,000 watts * 1/0.85 inverter eff * 1/10.5 volt battery cutoff = 224 amps max continuous
224 amps * 1.25 NEC wiring/breaker/fuse derating = 280 amp minimum branch circuit+fuse
...
MorningStar 300 Watt TSW 12 VDC

Thanks BB for the math and the MorningStar 300 Watt TSW 12 VDC suggestion, that looks excellent. The MS2012 manual states: Max Continuous Input Current: 267 ADC (Maximum continuous current is based on the inverter's continuous power rating at the lowest input voltage with an inverter inefficiency factored in)

The MS2012 sets the minimum wire gauge at 2/0 and maximum fuse size at 300 amps with time delay.

If I use the continuous power ratings and 1.25 derate, that actually takes me a bit over the 300 Amps. -- Then again, the current batteries would last about 15 minutes...
One thing I see is the 600 Watt array will produce more current than the Kid can handle by about 8 Amps. That in itself is not a big issue. However the addition of a second set of batteries will reduce the effective solar charging considerably. If these are GC2's you'd go from (30 Amps into 220 Amp hours) 13.6% peak current to 6.8% peak current. This may still work if you don't mind a bit of gen run/shore power time.

Still very much learning on how to calculate these values:
If the kid can handle 30 amps, and the Isc is 5.85 Amps per panel, wouldn't that make Isc (4 panels) to be 21.84 Amps? I understand that the output side of the Kid (at between 13.6 - 15.6 volts depending on charge state) would peak at the Kid max of 30 Amps. -- Our longer term plan is to add another Kid, likely in 'follow me' mode, if we use additional PV off the roof.
Oh and 2/0 will not handle 300 Amps: that fuse needs to be down-sized to 200 Amps or else the wire size increased to 4/0. Most 12 Volt systems would run 4/0 for the DC side.

Thanks! -- What is the best practice to calculate these (or look them up)?

I've been using values found on the BlueSea website, and treating our RV more like a boat than a house (reasonable?):
http://www.bluesea.com/resources/529/Allowable_Amperage_in_Conductors_-_Wire_Sizing_Chart
http://circuitwizard.bluesea.com/
http://www.calculator.net/voltage-drop-calculator.html

Attachments:
Voltage drop calculations using above calculator: Attachment not found.
Updated system drawing (wire increased to 4/0): Attachment not found.
• Banned Posts: 17,615 ✭✭
Re: Off Grid (RV) Design Check & Critique - Especially wire sizes & circuit protection
ScoCan wrote: »
Still very much learning on how to calculate these values:
If the kid can handle 30 amps, and the Isc is 5.85 Amps per panel, wouldn't that make Isc (4 panels) to be 21.84 Amps? I understand that the output side of the Kid (at between 13.6 - 15.6 volts depending on charge state) would peak at the Kid max of 30 Amps. -- Our longer term plan is to add another Kid, likely in 'follow me' mode, if we use additional PV off the roof.

Charge controller maximum output is 30 Amps @ any Voltage. For an MPPT controller the input current is largely irrelevant; only the Watts matter. 600 Watts @ 77% typical efficiency / minimum system Voltage (usually nominal) 12 = 38.5 Amps potentially. The MPPT controllers can down-convert available Voltage to charge current, which gives them a slight increase in current potential as opposed to the PWM type.

There are a huge number of wire sizing charts out there with an amazing tendency to disagree with one another. Much of a wire's ability to handle current comes from how well heat is dissipated, and that can be affected both by the actual wire type and the way it is installed. The one I use is from Powerstream http://www.powerstream.com/Wire_Size.htm and indicates 2/0 would cook at 300 Amps. Now if you're not really going to put that much current through it you can downsize the breaker/fuse to suit what the expected maximum current is. As long as the wire can handle that and the over-current protection is smaller than the wire's maximum it will be safe.
• Registered Users Posts: 11
Re: Off Grid (RV) Design Check & Critique - Especially wire sizes & circuit protection
niel wrote: »
adding to what was already said, you cannot tie 240vac to this inverter as it is meant only for 120vac. to cover both sides of the breaker box you will need 2 separate inverters or an inverter capable of supplying the 2 hot legs. their ms-pae series is capable of 240vac

Hi Niel,

Our main bus is 30 amps, 120VAC. Both leads are from the same phase (we only have 1). The MS2012 has 2 transfer relays, so I am running in 2 circuits.

That said, you can run in 2 120VAC leads which are 180degrees out of phase, and get 240VAC across the pair. See p 24 through 33 of the MS2012 Manual Here. - This only provides 240 VAC when both in standby mode & shore power is connected (pass through).
• Registered Users Posts: 11
Re: Off Grid (RV) Design Check & Critique - Especially wire sizes & circuit protection

Yikes,

Checking my wire sizes from the PV to the Controller and Controller to the Batteries, and my current sizing looks way off (at least to get a drop of only 0.1 volt)

The maximum one way run lengths (which will be shortened where possible) are
PV Combiner to Controller: ~12 feet
Controller to Batteries: ~8 feet

Here are the Panel/Array specs: Attachment not found.

The Controller is a MidNite Solar Kid (30 Amp)

What are reasonable wire sizes, % voltage drop, and absolute voltage drop for [PV Combiner to Controller] and [Controller to Batteries]?

My calculations are coming up around 4 AWG for a 8' run with 0.1 volt loss.
There are a huge number of wire sizing charts out there with an amazing tendency to disagree with one another.

Agreed, and rather amusing. Thanks for the link.
• Banned Posts: 17,615 ✭✭
Re: Off Grid (RV) Design Check & Critique - Especially wire sizes & circuit protection

You do not need to limit V-drop to 0.1 Volts, you need to limit it to % of V.

Array combiner to controller distance of 12 feet, max current 22 Amps, 8 AWG is more than enough (this is at panel Vmp not system Voltage).
Controller to batteries distance of 8 feet, max current 30 Amps, 8 AWG is still fine.

Remember that drop only occurs at maximum current; as the Amps go down so does the V-drop.
• Super Moderators, Administrators Posts: 32,027 admin
Re: Off Grid (RV) Design Check & Critique - Especially wire sizes & circuit protection

Specifically, you limit the Array to Controller voltage drop and general purpose 120 VAC wiring to 1% to 3% voltage drop. In some cases you can even have more drop--But then it is simply lost as waste heat--Which is something that most people want to avoid with off grid power systems (conservation, conservation, and more conservation).

For specific applications, you may have tighter requirements. For charging a 12 volt battery bank--0.1 volts makes a difference (i.e., charging voltage is around 14.5 volts +0.1/-0.1 volts is a pretty wide voltage band).

For 12 VDC loads, if you assume that the battery can discharge to ~11.5 volts in operation, and the AC inverter can operate down to 10.5 volts before it shuts down, then the maximum voltage drop can be 1.0 volts--If you assume the inverter can supply 2x surge current, then you should design 0.5 volt maximum drop for your wiring (again, based on your actual power needs--Not necessarily the ratings of the inverter... If you have a 2kW inverter and only need 1kW of power--Then you can design smaller wiring/fuses).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 11
Re: Off Grid (RV) Design Check & Critique - Especially wire sizes & circuit protection
You do not need to limit V-drop to 0.1 Volts, you need to limit it to % of V.

Thank you much Coot & BB.

Also, thanks to Niel, BB, S.G. and anyone else who worked on the Voltage Drop Calculator found Here

I calculated using 6 gauge (as I have some handy): Attachment not found.
and 8 gauge, as there is a 8AWG solar pre-wire in our unit: Attachment not found.

As the OE inverter is coming out, and it has 4 AWG homerun wires available, those will be repurposed as charging cables.
• Solar Expert Posts: 3,123 ✭✭✭✭
Re: Off Grid (RV) Design Check & Critique - Especially wire sizes & circuit protection

If the CC includes remote battery voltage sensing (separate wires to the + and - battery terminals which do not carry any current), then you can get away with higher voltage drops and only have to worry about the lost power.
But not all remote voltage sensing actually works as intended.
SMA SB 3000, old BP panels.
• Solar Expert Posts: 135 ✭✭
Re: Off Grid (RV) Design Check & Critique - Especially wire sizes & circuit protection

Just a few notes

1- I wouldn't worry about being a little over the AMP limit of the Kid....it will simply clip the extra amps when they are present.....but will help in lower light conditions.

2- You don't need a Trimetric with the Kid.....buy the WhizBang jr. ....it's cheaper and not only does it do the same basic thing, it will improve the charging capabilities of the Kid.
• Solar Expert Posts: 203 ✭✭✭✭✭
Re: Off Grid (RV) Design Check & Critique - Especially wire sizes & circuit protection

Take my advice with a grain of salt since I'm still a newbie, but I've read all I can on circuit protection here at the NAWS forum and elsewhere. This forum has definitely had the most useful information on the subject.

You don't need a breaker between your combiner box and your charge controller. The individual fuses you have in your diagram are plenty. I'd use Midnite breakers instead of fuses, it's handy to be able to switch off individual panels and they're just as easy or easier to install than fuses.

You should also install circuit protection as close to the batteries as possible for the inverter and other loads and for the charge controller. I'd put the fuse before the switch running from your battery to your inverter.

I'd also increase the size of the 30 amp fuse you show in the charge controller to battery circuit. 30 amp fuse might blow if the controller output is at maximum. 40 amp should be fine.