Where do I bond neutral/ground for new inverter.

mike95490 wrote: »

not quite. Inverters have internal losses. Figure your max starting load x 1.3 and that will be pretty close. Then you oversize the wire so you don't have much voltage drop.


But now I gotta say, if you plan on pulling 1800watts, you need to change to a 24V system. That's 150a flowing in your wires. Way too much - wait till you price that out wire & fuse.

BB. wrote: »

The continuous power calcuation (from an engineering/code point of view):

• 1,800 Watts * 1/0.85 inverter eff * 1/10.5 minimum inverter voltage * 1.25 NEC fuse/breaker/wire derating = 252 Amp minimum branch wire+fuse rating

If you think you will only use 1,200 Watts (not a bad guess):

• 1,200 Watts * 1/0.85 inverter eff * 1/10.5 minimum inverter voltage * 1.25 NEC fuse/breaker/wire derating = 168 Amp minimum branch wire+fuse rating

There is what is practical from the battery bank... I did not see your battery / bank specifications, but lets say you have flooded cell, golf cart 6 volt @ 220 AH, and you have 4 of them (series/parallel) for a 12 volt 440 AH battery bank.

Roughly, I would not draw more than C/5 continuous current from the battery bank (reliably over time, state of charge, temperature, aging--And C/2.5 for maximum surge current):

• 12 volts * 440 AH * 1/5 max current * 0.85 inverter eff = 898 Watts approximate maximum power

So, somewhere around 0.9 to 1.2 kWatt would be a nice continuous maximum draw limit to design for.

AGM batteries can supply much more surge current than flooded cell... But by size/weight, they supply about the same amount of AH of power at C/20 as the same size/weight of flooded cell batteries. So--I recommend sticking with the conservative numbers for flooded cell unless you have specific high current/short duration loads.

-Bill

BB. wrote: »

The typical Energy Star refrigerator typically needs around a 1,200 to 1,500 Watt AC inverter with good (heavy/short) DC cabling properly bolted to a good size battery bank.

The typical inverter is rated for ~2x rated power for surge current--But the typical numbers would suggest a 5x running current for surge current (120 Watt running compressor would have ~600 Watt surge or ~5 amp @ 120 VAC surge---Or a bit over 50 amps @ 12 VDC surge).

You probably have some other loads (lights, radio, TV, etc.) and not all inverters support the same amount of surge current the same way--Why the relatively high wattage rating for what should be a "smaller" load...

We try to be conservative and want the system to always start/run without any major headaches.

Wayne from NS Canada was able to get his fridge/freezer to run on a 300 Watt MorningStar 12 VDC TSW inverter (rated for 600 Watt surge for ~10 minutes). However, he did a bunch of shade tree engineering to make them work (experimenting with motor capacitor values, using auto transformers to drop voltage/raise starting current with timed relays, allow only one compressor to run at a time, etc.).

So, it can be done with less, and Wayne will tell you how... But it is really not where most folks want to spend their time/energy.

Typically, you would run a heavy gauge "green wire" ground from the inverter case ground to the battery negative bus. Remember, you could get that 200 Amp DC line shorted to the inverter's metal case.

If you are going to earth ground, drive a ground rod near the outside wall foundation (you want the lightning to go outside the building). If you have any other grounds (AC outlet green wires, etc.), you would bond those to the same ground rod (+cold water pipe+metal gas pipe+etc. in most codes).

Electrically, all of the grounds are common at the master ground rod (trip fuses/breakers/GFI outlets, etc.). But you do not see the 200 Amp short circuit going through the 14 AWG green wire ground to the common ground rod back to the battery bank.

Just as and FYI, a 6 awg copper wire will fuse around 600 amps of current.

-Bill

BB. wrote: »

OK--Try again... Words are difficult, and I don't have time for drawing at the moment.

The "cook book" would be:

Say you are using 2 AWG cable from battery bank to AC inverter DC input. Run a 2 AWG cables from Inverter Case ground back to DC Ground Bus (you can use ~6 AWG minimum if your DC breaker/fuse is ~200 amps or less).

Run a 6 AWG from the battery ground bus common to the main ground rod (installed withing a foot or so of the outside wall.

Your AC power panel grounding should have a 6 AWG green wire from the AC panel's ground bus/green screw (not Neutral Bus, this is grounded in the Inverter as I recall--Correct?). Run that green wire to the same outside ground rod.

Run 6 AWG wire from (metal) cold water pipe to same ground rod.

Run 6 AWG from cold water to hot water pipes at water heater (new code in my area).

Run 6 AWG from gas line to main ground rod (new code in my area).

The ground rod is really for lightning suppression. That all green wires connect in one place (at ground rod, but can be a different location for common connection) so that if there is a short circuit anywhere in the AC or DC power system, there is a "return path" to trip protective fuses/breakers.

Single point grounding is to make sure each ground wire is large enough for short circuit loads.

AC systems typically have 1/10th the current vs a 12 volt DC system (i.e., 10 amps at 120 VAC is 100 amps at 12 VDC). And your AC inverter can output a few 10's of amps maximum. Your battery bank can output 1,000's of amps into a dead short. You don't want any of that current flowing through a 14 AWG green wire--Bad things will happen (why there is only one common AC to DC ground connection at the common ground rod).

Note there should be no ground connection from the AC Inverter Chassis to the AC Main panel--To prevent a DC short inside the Inverter from flowing through your AC green wire ground system and frying those AC green wires.

I hope this is clear--Please ask again if not. We are after safety here.

And very happy to hear your system is working well--It makes it worth the work helping here.

-Bill