Where do I bond neutral/ground for new inverter.

Re: Where do I bond neutral/ground for new inverter.

69MyWay wrote: »

Hello - I'm new here - and new to this technology. I'm getting ready to power up an off grid system that will be running in my house to supply power to the well pump, lights, sump pump, and other basic items.

In general, I highly recommend that you measure your present household loads, and look changes you would make (insulation, LED lighting, new energy star appliances, etc.) and practice extreme conservation.

In general, it is almost always better to conserve energy than to build out a larger solar power system. For sake of discussion, assume that off grid power costs about 10x utility power--Or around $1 to $2 per kWH vs the $0.10-$0.25 or so per kWH from your utility. Off grid power is expensive, and requires a fare amount of money and work to keep running (new batteries every 7+ years, new electronics every ~10+ years, new array every 20+ years, etc.).

I'm doing an AIMS 120/240 12 volt 4,000 watt inverter/charger, four 6 volt T16 Trojans 230 ah. This is being fed by ten 12 volt 100 watt cynergy panels. The Aims plugs to a 240 grid source to maintain the batteries. I've wired this into it's own fuse box...

Not a big fan of AIMs inverters... And getting 4,000 Watts from a 12 volt battery bank is very difficult (to both pull the current and recharge). For example:

• 4,000 Watts * 1/0.85 inverter eff * 1/10.5 volts inverter cutoff * 1.25 NEC derating (wiring/breakers) = 560 Amp minimum branch circuit @ 12 VDC current design

A 4+ kWatt AC inverter would really be much happier on a 48+ VDC battery bank.

In any case, before you start with the hardware, what are your expectations for loads/peak power/average power/kWH per day/full time off grid or emergency power/plans for backup genset/etc.

and I now am becoming confused about the neutral ground bond. The answer may already be in this thread...but I'm missing it. Can't get my brain around it.

Grounding is one of the more complex questions about a large power system...

The AIMS inverter does NOT take ground/neutral in from the grid - but it outputs 240 through two legs and a neutral. It has a CASE ground. The instructions say to run a #8 wire to ground. I'm waiting to do this now - weather dropped to -13 today...so that's not happening real soon. Right now all the loads for this system would be dedicated to the system. I'm assuming the inverter acts as a "main" so my fuse panel would need to be set up as a non-bonded with neutral and ground separate. The question comes in if I should attach the #8 ground from the grounding pole to the case to the grounds in the panel (and remain unbonded) keeping the neutral and grounds on different bars.

Regarding AC ground bonding of Neutral--First is this a MSW or TSW (modified or true sine wave) AC inverter? Can this inverter support a ground bonded neutral or not (in general, MSW cannot and TSW inverters can have a ground bonded neutral).

And with AC wiring, the "standard" is to have the AC Neutral bonded to earth/safety wire in one location. Prevents ground loops in the wiring.

For the Case Ground of the inverter--In general, it is assumed that if there is a short circuit from AC to case or DC to case, the ground bond will carry enough current to pop the AC or DC breakers/fuses... Note that you have a minimum of 560 Amps of DC circuit. Probably need something like a 2 AWG cable or larger for grounding to the common safety ground/DC battery bank grounding. The wire has to be heavy enough to trip a 560 amp breaker and not fuse. An 8 AWG cable will fuse around 473 amps or so... Too small of wire to protect 560+ amps of DC current. NEC code book may have something to say on grounding cable size.

By the way, I'd never considered grounding the batteries to the earth ground too - but it makes sense. I haven't yet mounted the panels, but I do need to bring this thing on-line soon to make sure the SOC on the batteries is all good.

Right...or wrong?

In general, for many reasons, it is good to DC Ground (negative terminal typically) the battery bank. But, as with all grounding, the details matter.

And second question - my house main has the neutral/ground on the same bus bar. If I were to run a back up transfer switch where I could put the inverter power directly to my main house grid (with all the heavy load breakers off) - am I headed for trouble since my main panel is "grounded" and bonded - and I assume the inverter is too. Is this okay????

In general, the main single ground/neutral grounding is done in the main AC panel of the home (note: there is also a neutral/earth at the utility power transformer). In general, the AC inverter would have floating neutral. And the backup genset may or may not have a floating neutral (larger gensets than ~3.5 kWatt (seem to) generally have the AC neutral bonded to frame ground).

It is a complex subject and need the exact AIMs inverter model number, etc. You need to confirm that the AIMs will support a grounded AC Neutral configuration--I do not know if it can or not... And if it cannot, you can short out the AC inverter as soon as it is turned on with a ground bonded AC neutral and DC negative battery bank.

-Bill

Re: Where do I bond neutral/ground for new inverter.

69MyWay wrote: »

Let's see if I can answer.

The inverter is PICOGLF40W12V230VS

http://www.aimscorp.net/4000-Watt-12-Volt-Pure-Sine-Inverter-Charger-120/240.html

TSW

AIMS tells me to NOT hooked up (to the AC input) any ground or neutral...Then it runs a L1 and L2 out with neutral. They explained setting the panel with a "floating ground" - but I honeslty have gotten so consfused on this now that I don't know what to do - and they didn't say anything about grounding the battery bank to earth...so this is a brand new consideration.

I don't know what to do with neutral bond... It appears that the AIMS may ground bond the neutral internally--But I don't know.

AIMS Power

www.aimscorp.net

Wiring Option 3

230V split phase

Input: Hot line+ Hot line +Ground
Output: Hot line +Neutral
Remark: In such case, each output
hotline can only carry a max of
half the rated capacity.

WARNING
For split phase models

, AC input neutral is not required in wiring. Never Connect Input Neutral to

Output Neutral. Damage will result which is not covered under warranty.

Always switch on the inverter before plugging in any appliance.

I know 24 or 48 volt would be better. I know that. However, for a back up I have 12 volt stuck in mind as it is more common to everything else I have for any other tie in for any othe device as well as altnerative ways to charge the bank in a pinch. I've toiled over this decision as it is a danged if you do...danged if you don't

Trying to get ~500 watts moved around your DC wiring with 1/2 volt drop--It can be difficult. Don't get stuck in the 12 VDC trap for much more than 1,200 to 2,000 watts expected load. You will be fighting "everything" when you try to get 5kWatt from a 12 volt battery bank (and charging too).

Unless you have a 1kWatt 12 VDC HAM ratio transmitter or something (not very common either). If most/all of your loads are 120/240 VAC, then the battery bank voltage is not "of concern" with respect to your loads. Just wiring the battery bank to 48 volts, the inverter, and the solar charge controller (and some sort of genset/AC charger setup).

Total house load? I've run the kilowatt and measured everything multiple times. I am in the process now of changine over to LED. This is a "back up" system not intending to run the whole house. I've had trouble measuring the 240 watt (well pump) - for total use. My numbers have changed with use. So Plan A...B...or T (not sure which I'm on) turns out to set up on a dedicated breaker panel off the inverter to use on specific targeted loads disconnected from the house main all together).

Do you have any water storage (cistern, pressure tank, etc.)? Having storage and a small water pump to provide house pressure, and use a genset running once per day/couple times a week to get a few hundred gallons back into storage--That can be a good trade off for shorter outages (days/weeks). Longer term, looking at a replacement or alternate well pump (low flow, low power) to reduce the large starting surge that many "traditional" well pumps have.

In this case, I believe from everything I'm seeing that the key is to treat the fuse panel as a "sub" to the inverter - so it would be unbonded at the inverter - but the grounds would all be to earth ground - neutrals not tied in - does this sound right?

I believe you are correct... The inverter is wired to a dedicated sub panel for all your loads. The Inverter has an internal transfer switch (AC input power, or inverter power). But I am still not sure if the sub panel should have neutral bonded or not--Although, guessing you need a floating neutral bus.

HOWEVER...it would be sooo nice to put a transfer on the house breaker - kill all the other loads at the panel (HVAC, water heater, stove...etc) and plug in there...so we have temporary whole house lights...etc. verse running chords or changing connections over to the inverter dedicated panel.

You can get manual transfer switch panels to do something like that--But most do not transfer the neutral--Which according to AIMS is probably a no-no.

Attachment not found.

I've got to buy some DC cables now - the vendor I purchased from sold me too small of cable anyway - so I've got to move to 4/0 out of the battery bank. Cables are under 5'

What size (Amp*Hour) 12 volt battery bank are you using? What size/voltage of battery?

-Bill

Re: Where do I bond neutral/ground for new inverter.

A starting point for discussion (assuming Battery AH rating is C_{20[.sub] rate):}

• C/20 -- Long term average discharge rate (i.e,. 5 hours per night, 2 nights, to 50% max discharge)
• C/8 -- Maximum "cost effective" discharge rate
• C/5 -- Maximum continuous discharge rate (minutes, hour or so)
• C/2.5 -- Maximum surge discharge rate (seconds to minute)

For your 2x370 AH @ 12 volt battery bank:

• 740 AH * 12 volts * 0.85 inverter eff * 1/20 discharge rate = 377 Watt long term average power usage
• 740 AH * 12 volts * 0.85 inverter eff * 1/8 discharge rate = 942 Watt maximum recommended continuous
• 740 AH * 12 volts * 0.85 inverter eff * 1/5 discharge rate = 1,510 Watt maximum continuous
• 740 AH * 12 volts * 0.85 inverter eff * 1/2.5 discharge rate = 3,019 Watt surge (starting well pump, etc.)

Note that charging runs from ~5% to 13% rate of charge for solar, and for a genset around 10% to 20% to 25% is the usual range.

For solar, 5% is good for weekend use. 10%+ for full time off grid living:

• 740 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 697 Watt array minimum
• 740 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 1,394 Watt array nominal
• 740 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 1,812 Watt array "cost effective" maximum

For a genset+AC charger:

• 10% * 740 AH = 74 Amps @ 14.5 volts charging
• 20% * 740 AH = 148 amps @ 14.5 volts charging

Some pretty significant charging current too.

Not saying right or wrong--But are these numbers that "make sense" and will meet your needs?

-Bill