Hard-Wired Inverter

[peterh]

Friday I decided it was time to hard-wired a 120-watt inverter setup into my trailer that would provide electric power to my trailer outlets for charging cell phone, laptop, and camera batteries and running a small LCD TV (if I ever get one) when we're away from a shore power connection and depend on our batteries and solar panel for power.

I have a couple inverters around the house, so I tested each of them for how high their "parasite" power draw is when hooked to the trailer's 12v system but aren't busy charging any batteries or running any appliances. I found my Wagan-brand "Smart AC 120 USB" inverter drew the least power, 0.26 amps, so I installed it over the other inverters.

120 watts is enough to run most small TVs and laptops, but not enough to run power drills, wood burners, refrigerators, coffee makers, microwaves, toasters, water heaters or most anything else, so I wanted to be able to run my trailer outlets on shore power when I have hookups and on the inverter when I'm dry camping. I also wanted some way to switch the inverter on and off so I don't draw my battery down when I don't need the inverter power. (My trailer has a separate 110V circuit for appliances like the microwave, gas/electric refrigerator, water heater element, and air conditioner.)

Here's what the system looks like from the front with the covers off:





The supplies to do this job were fairly simple: a two-gang "old work" switch box, a three-way switch, a standard on/off switch, a switch-plate cover (more on that later), 3' each black and white stranded 12-gauge wire, a couple feet of 14 or 12 gauge Romex wire, a grounded appliance cord (all of these from a hardware store) the inverter, a 15-Amp fuse and a cigarette lighter with an "L" shaped mounting bracket (these from an automotive store) and a bit of corrugated cardboard.

The wiring diagram looks like this:





First I outlined the cutout shape for the box on the front of my kitchen cabinet next to my trailer's converter panel and cut a hole just large enough to accommodate the 2-gang old work box. (If you haven't cut a hole for one of these before, try cutting a hole for the box in an old piece of plywood or even cardboard, then install the box in your test hole. You wouldn't want to practice on your trailer's fiberglass and cut the hole too big!)





Here's a picture of what an "old work" box looks like. If you look carefully you'll see there's a little plastic "wing" at the upper-right (and hidden lower-left) corners that flips out and tightens against the edges of the hole you cut for the box when you tighten the screw, clamping the box into place.





The inside wiring is pretty simple: 110V AC wiring on one side of the box, 12V wiring on the other.





Just be sure to:
<blockquote>
1) Disconnect the 12V battery and your shore power when you're working on this project! 110V AC is dangerous!
2) Make sure your on/off switch has all the wire connectors on the same side (not cross-wise kitty-corner), and put that switch so these screws face away from the 3-way switch with the 110V AC power so there is no chance the two sets of wires will ever come in contact. Also insert a non-conductive "isolator" between the two switches; I used a rectangle of cardboard.
3) This is not a beginner project. I'm not posting step-by-step instructions that'll help a novice find his or her way, just enough detail to show an experienced handyman or handywoman how it could be done. If you haven't worked with household current electricity before, find someone who has.
4) The 12-volt wiring for the inverter should have its own 15 amp fuse and be connected directly to your trailer's converter fuse panel or to your battery. Use 12-gauge or (even better) 10-gauge stranded wire for all 12 volt wiring. Use 14-gauge Romex wiring for all 110-volt wiring.
</blockquote>

I'll post a final picture of my project once I've installed a switch box cover plate. To fit the switch box in between the converter and the wall for the closet space on the other side I had to install the box right up against my converter, which means I have to cut the cover plate to match. I bought a plate with the rest of my supplies, but it shattered while I was cutting it to size, so I need a new one.

[peterh]

And with the switch plate cover installed.





For those who are wondering what the little green LED to the left of the convderter panel is, it's the solar panel controller "system charging" light.

Done!

[Dean & Mary]

Peter,

This post is most informative like many of your others. Thanks

I was just working on an inverter install in our trailer this weekend. I also used the simpler approach to a transfer switch. However, I was also going to transfer the neutral as well, like it is done in the auto transfer switches. I have read that the neutral should also be transferred - thoughts?

In addition, I inserted a fuse on the AC side of the inverter. I did this by drilling a ½ inch hole in the side of my double gang box and inserting a fuse holder with a 6 amp fuse. This will protect my 300 watt inverter that handles 600 watt peaks. I have easy access to the back of the box.

Dean

[peterh]

. . . I was also going to transfer the neutral as well, like it is done in the auto transfer switches. I have read that the neutral should also be transferred - thoughts?

Hmmm. I'll have to think about that, see if there is power leakage backward from the inverter to the neutral lead. I suspect, however, that the trailer neutral, ground connection, and 12v system ground are likely all tied together and at the same potential, meaning that wouldn't create a problem when we're hooked up.

I'm more concerned that some yahoo might open the door and stick their finger in the trailer's 110 shore power plug, but if that happened I'd suspect the worst that could happen is they'd take a 60 volt hit at one amp or so. Enough to sting, but not enough to injure someone.

Next time I have a chance to spend some quality time tinkering in the trailer I'll get out the multimeter and do some testing.

In addition, I inserted a fuse on the AC side of the inverter. I did this by drilling a ½ inch hole in the side of my double gang box and inserting a fuse holder with a 6 amp fuse. This will protect my 300 watt inverter that handles 600 watt peaks. I have easy access to the back of the box.

Our inverter -- most inverters -- have overload protection, a thermal breaker that will shut the inverter down if the circuit it powers draws too much juice. To re-set our inverter after the breaker blows I have to power it down, let it sit, then power it back up again.

I also installed a 15 amp fuse on the DC side, which will blow the circuit at 180-200 watts, a number well under the 250 watt "surge" rating of the inverter.

[Capt Ron]

Just FWIW,

We found that we could not run our dorm size GE fridge off an inverter smaller than about 700 watts. While the specs for the fridge say it runs on 105 watts, the start up current for the compressor trips out smaller inverters. It may also be that the wave form of the inverter impacts the wattage requirement. Most small inverters put out modified sine wave not pure sine wave like 'shore' power. It may be that a pure sine wave inverter could be smaller and still start the fridge?

Also, there are inverters on the market that have automatic transfer switches such that the machine knows when it's plugged in to the grid and when it's on battery power. No human intervention required except to turn the inverter on and off. I have one installed on my boat and it works fine. If the inverter is off when you plug the boat into shore power, the panel lights on the inverter flash until you turn on the unit. The inverter is a Xantrex Pro 1800.

All is not perfect with this system however. If for example, we're plugged into shore power and I have the battery charger turned on (a separate Xantrex Trucharge 40) and then turn off the shore power (as I would do to disconnect the power cord) the automatic cut-over switch switches to battery power and continues to run the battery charger creating a circular loop. It's not dangerous except to the batteries which will now deplete rather quickly. I just have to be careful to turn off the charger before we leave shore power.

This is likely not an issue if the inverter and charger are combined into the same unit as some models are.

Good detailed post of your installation though.

Note to moderator: Maybe this original post and others like it should be 'booked' into the document center for easy reference to others planning similar mods?

[peterh]

We found that we could not run our dorm size GE fridge off an inverter smaller than about 700 watts. While the specs for the fridge say it runs on 105 watts, the start up current for the compressor trips out smaller inverters.

Different strokes for different folks. When we dry-camp we power our trailer using just one deep-cycle battery and recharge that battery using a single, roof-mounted solar panel that puts out about 10 amp-hours a day. At 10 amp-hours per day we don't have enough power to run an electric 'fridge, cube heater, microwave, or coffee maker. To keep our electric needs low we switch our 'fridge to run on propane gas, stow the electric cube heater and turn on the gas furnace, just plain don't use the microwave, and use our gas stove to manually boil our water that we pour by hand into the electric coffee maker's filter basket.

Inverters take power, too. Whenever they're turned on they suck down energy, even when no 110v appliances are plugged in, and the bigger the power capabilities of an inverter, the more "parasite" power it sucks in. So, with that in mind, we selected an inverter that can supply enough power for the few 110v accessories we don't quite want to part with and no more. That way we can have our electronics "cake" without drawing our battery so low we can't eat it and use our goodies when we want to.

Also, there are inverters on the market that have automatic transfer switches such that the machine knows when it's plugged in to the grid and when it's on battery power. No human intervention required except to turn the inverter on and off.

I'd like that . . . Alas, all the inverters I've seen that have auto-transfer switches are big inverters that pump out lots of watts and draw lots of parasite power. Not a good choice for a trailer that needs to run on just 10 amp-hours a day. So auto-transfer inverters are not for us, but might be hunky-dory for a trailer with two batteries and a gasoline-powered generator that can be used recharge the batteries when they get low.

[Dean & Mary]

Our inverter -- most inverters -- have overload protection, a thermal breaker that will shut the inverter down if the circuit it powers draws too much juice.

I agree that most of the modified sine wave inverters do have overload protection. In which case, it would be redundant to install a fuse on the output (AC) side. My Morningstar Sure Sine-300 does not have thermal protection and an inline fuse is required on the output side. I really like this inverter and after the install is complete I will put a load and an oscilloscope on the output and take a look. It is a compact 10 pound inverter with nice features and at this point I would recommend it.

[b]Note: Like your schematic shows, one must fuse the input side coming off the battery for all inverters.

[b]Capt Ron, Your right about the modified sine wave inverters not working with motor loads. One should use a pure sine wave inverter for motor loads.

[b]Peter, I could not find where I read that the neutral is switched in an auto transfer switch. I looked at my inverter schematic closer and realize that it is not necessary to switch neutral. The output is fully isolated and the neutral is not defined until one lead is connected to ground. This would happen if it is not switched, since it would be hard wired to the camper’s neutral bus bar.

Dean

[Chris Z]

Peter
I've been considering doing something along these lines. You do good work!

A couple of questions
1)Do you check polarity before you plug-in? Wouldn't reversed be a problem?
2)Can you plug-in and forget to throw the switch to 120 ? Just keeps running off the battery?
3)How about a double relay instead of a double pole switch? When you plug-in, the coil would energize and throw the normally closed to inverter over to line volts.

Your thoughts?

Chris

[peterh]

1)Do you check polarity before you plug-in? Wouldn't reversed be a problem?

No. The idea of what defines "neutral" is somewhat artificial when you're talking about an isolated electrical system. A house's 110v electrical system's "neutral" wire has the same (or very nearly the same) electric potential as the "ground" wire, which is quite literally wired to a metal rod pounded into the dirt outside the house.

When a travel trailers is plugged in to shore power the "neutral" and "ground" wires again have the same electrical potential as the ground around the trailer, but when the trailer's running on 12v power, "ground" is the potential of the negative pole on the trailer battery.

So as long as my wiring respects this convention that defines what "neutral" is, the electric in the trailer should work just fine.

2)Can you plug-in and forget to throw the switch to 120 ? Just keeps running off the battery?

Yup, that would be what would happen -- as long as the "inverter" switch is turned on.

The "Inverter" switch is the weak link in this system. Bump it, switch it on, and the inverter will suck down 1/4 amp-hour every hour until the battery runs dry or someone notices and turns it off. I do have a solution: I plan on ordering a 2-hour mechanical spring-wound timer switch that will limit the run time for the inverter up to 120 minutes. That'll give me a way to "set it and forget it" for charging a laptop, camera battery, or watch a DVD without risking my precious battery power.

3)How about a double relay instead of a double pole switch?

Wouldn't really buy me anything except another electrical system I'd have to engineer and maintain. Manual switches have the
advantage of being cheap, simple to operate, trouble-free and easy to troubleshoot:
"Hey, the coffee maker's not heating up!"
"Oh, the "Power" switch is set to the Inverter."
"Click."
"<sound of coffee maker doing its thing.>"

Another advantage of this system design is I can easily replace the inverter if it decides to die on us or I decide I need another unit. I just pull the pots-and-pans out of the cupboard, pull the bottom panel out, un-plug the current 120-watt inverter from the 110v power cord and the cigarette-lighter outlet, plug another inverter in, and put the cabinet floor and pots and pans back in. A ten minute job, tops.

Simple and low maintenance . . . counts.

[Chris Z]

No. The idea of what defines "neutral" is somewhat artificial when you're talking about an isolated electrical system. A house's 110v electrical system's "neutral" wire has the same (or very nearly the same) electric potential as the "ground" wire, which is quite literally wired to a metal rod pounded into the dirt outside the house.

When a travel trailers is plugged in to shore power the "neutral" and "ground" wires again have the same electrical potential as the ground around the trailer, but when the trailer's running on 12v power, "ground" is the potential of the negative pole on the trailer battery.

So as long as my wiring respects this convention that defines what "neutral" is, the electric in the trailer should work just fine.
Thanks you Peter. You seem to be a very talented individual.
I guess I'm still confused and before I do this I need to clear this up in my head. IF the neutral and hot were reversed at the plug, wouldn't this same hot wire be directly in contact with the neutral wire from the inverter if your on/off switch was on? I know you intend to never have the on/off switch on when plugged in but my concern would be that it could happen.

3)How about a double relay instead of a double pole switch?
Wouldn't really buy me anything A simple double pole relay with 120v coil would eliminate the switch and wouldn't have to worry about changing it. Make sense?

[peterh]

I guess I'm still confused and before I do this I need to clear this up in my head. IF the neutral and hot were reversed at the plug, wouldn't this same hot wire be directly in contact with the neutral wire from the inverter if your on/off switch was on? I know you intend to never have the on/off switch on when plugged in but my concern would be that it could happen.

When I wired my short little 3-prong power cord to the switch box I wired all the black wires from the trailer's 110v system and the power cord to the 3-way switch and used a "wire nut" to connect the trailer's 110V system white wires, and wire-nut the green wire of the extension cord to the bare copper wires for the trailer's 110v system. In other words, hot to hot, neutral to neutral, and ground to ground. Because 3-prong plugs will only insert into the outlet one way and assuming the inverter I use is wired correctly it would be impossible to connect the inverter incorrectly.

When we get down to the things we plug into the trailer's outlets, I'm not worried there, either. Any appliance I'm likely to use in the trailer (UL/CSA approved) would be "double insulated," with both the hot and neutral sides of the electrical connection wired up as if they are both "hot," so they don't care what pole of the plug is hot or neutral. To them it's just a stream of electrical energy they can use.

Fuses and overload breakers are there to protect us when these safeguards somehow fail.

A simple double pole relay with 120v coil would eliminate the switch and wouldn't have to worry about changing it. Make sense?

It would certainly work, and if that's what you want, that's fine. For me a simple 3-way switch does the job and does it cheaper, lighter, and won't ever vibrate or buzz like the core of an old fluorescent light ballast.

[Dean & Mary]

I have completed the installation and testing of my 300 watt inverter and found the following:

If your inverter is going to be hard wired into your existing electrical system, the neutral has to be switched as part of the transfer.

Why:

My trailer, like most modern trailers, has the neutral and ground isolated from each other. The ground and neutral connection is done at the shore panel. Connecting them together before the panel would create a ground loop.

So, when another AC source, like an inverter, is used the neutral and ground are still isolated in the camper and a ground fault will exist at the outlets. To prevent this, the neutral and ground should no longer be isolated from each other. The only way to connect them together temporarily, is to transfer the load neutral to the inverter neutral that shares a connection with ground.

This simple transfer can be accomplished via the use of a 120 volt 20 amp DTDP (double pole double throw) switch.

Testing results when neutral was not transferred:

Checking the voltage at the outlet, I measured 100v ac between hot and neutral and 10v ac between neutral and ground. This is not good! My plug in tester of course indicated a ground fault.

Testing results with neutral transferred so connection to ground could be made:

There was no ground fault and a nice clean AC voltage measuring 110.7 volts under load was measured at the outlets (see photo). I would highly recommend the Morningstar Sure Sine-300 inverter for features, output and ease of installation.

The output is now verified safe and laptop can be connected.

[Ed Harris]

Peter

Just a small note,you can now get a double box with an isolated low voltage side at most home depot now.
It has only taken Carlon until now to come up with them but they are great and pass code too.

I will also echo your advice when trying to cut in one of these boxes......they do not look like what they are,measure 5 times so you can get away with cutting just once!

I install these every day and have made templates to aid the processand I still have trouble some times with irregular surfaces and finishes.

Ed

[peterh]

Peter

Just a small note,you can now get a double box with an isolated low voltage side at most home depot now.
It has only taken Carlon until now to come up with them but they are great and pass code too.

I've seen the new boxes, but all of them seem to be new-work boxes, which doesn't work for a fiberglass install. Have you seen otherwise?

I will also echo your advice when trying to cut in one of these boxes......they do not look like what they are,measure 5 times so you can get away with cutting just once!

Yup. Done the template thing, too. Made the template just undersized so I could screw it to the wallboard, check level, sink a second screw to anchor the thing, then RotoZip around the profile.

[peterh]

I have completed the installation and testing of my 300 watt inverter and found the following:

If your inverter is going to be hard wired into your existing electrical system, the neutral has to be switched as part of the transfer.

[b]Why:

My trailer, like most modern trailers, has the neutral and ground isolated from each other. The ground and neutral connection is done at the shore panel. Connecting them together before the panel would create a ground loop.

So, when another AC source, like an inverter, is used the neutral and ground are still isolated in the camper and a ground fault will exist at the outlets. To prevent this, the neutral and ground should no longer be isolated from each other. The only way to connect them together temporarily, is to transfer the load neutral to the inverter neutral that shares a connection with ground.

It'll be a couple weeks before I can correlate with you on this, but hopefully I'll get back to this topic. I can say, however, that I'm currently hooked up to a 110v GFCI outlet, and with y inverter switched "Off" and my outlet circuit switched to "Shore" power, I'm not tripping any breakers (GFCI or overload breakers) or smoking any circuits.

I suspect, however, that if I went over and flipped my 12v power supply to my inverter to "On" I might well create a difference in voltage between inverter and shore neutral. That would, as you point out, be bad. I'll have to avoid doing that.

[Pete Dumbleton]

Remember that the ground and neutral are only tied together at the first load center in the trailer, which is the 120VAC circuit breaker box -- Downstream from there, just as in your home, they are separated. The neutral and ground should NOT be connected at the shore panel.

OTOH, the shore power cable is analogous to the power company entrance into your home, and YOUR trailer load center SHOULD have the neutral and ground connected, just as your main load center at home. The only difference is that your home will (should) have physical ground rods.

What you seem to be doing is inserting a second load center (aka sub-main), but the neutral and ground should still remain separated all the way down to the load.

Likewise, the neutral and ground at the RV pedestal in the park should be separated, but these are often wired by the owner or a friend who may not know the code -- I have found all sorts of wiring problems in RV parks and never plug in without using the cheap test plug first. Another reason for being off the grid when I can help it.

We are chasing problems on a friend's house because the pro electrician combined the neutral and grounds in at least two sub-main boxes (well and dock) as if they were the primary load center. I'm afraid to open the sub-mains he has in his basement...

Also, keep in mind that, despite the title, GFCI devices do NOT detect ground faults, they detect imbalance between hot and neutral. You can lift the ground and nothing happens (although they will react to an imbalance even with the ground lifted).

[Dean & Mary]

Remember that the ground and neutral are only tied together at the first load center in the trailer, which is the 120VAC circuit breaker box -- Downstream from there, just as in your home, they are separated. The neutral and ground should NOT be connected at the shore panel.

OTOH, the shore power cable is analogous to the power company entrance into your home, and YOUR trailer load center SHOULD have the neutral and ground connected, just as your main load center at home. The only difference is that your home will (should) have physical ground rods.

Thanks for the response,

I made an assumption that the neutral and ground were connected at the shore panel. A more correct statement would be: A real ground is provided after connecting to shore power.

[b]In hind sight, I should only state what I know for sure, and that is:

[b]1) My Bigfoot trailer, for sure, does not connect ground and neutral at any point in the trailer. I have metered and double verified this. Only when I'm connected to shore power do I have continuity between neutral and ground.

[b]2) In order for my inverter to work correctly, the neutral and ground must have a common connection?

I did not want to change the electrical engineering of my trailer by connecting them permanently. So, thought it best to transfer the neutral to make the connection. When the load is transferred to my inverter, the neutral / ground connection is made only in that branch. This way, it does not matter if both shore and inverter were active at the same time.

I have only worked on one other trailer and it was the same. So, I made the assumption that all modern trailers were this way.

[Pete Dumbleton]

The camp ground's ground and neutral should only be connected way back where the power company connects to the main service entrance box -- After that, they are split all the way to the shore power box (where there should be at least one ground rod).

Likewise, your trailer should be wired just like you house or the CG, with the ground and neutrals connected on in the sevice entrance box and then separated.

Be advised that there are several ways manfs installed converter boxes; some with the 120VAC and 12VDC in the same box, some with 120VAC separate and some with both separate.

[Dean & Mary]

Camping this week at one of Colorado's State parks gave me a chance to continue my testing.

Choosing a vacant site with electric hookups for my testing; I found the neutral and ground were connected together at the panel.

[Pete Dumbleton]

If the panel was being fed from something like a meter then it is a SERVICE panel and that would be correct, but if it's just being fed by a breaker in another box (service or sub), then it's a SUB panel and doesn't meet my understanding of the code if the ground and neutral are bonded.

http://members.tripod.com/~masterslic/FAQ-2/18.html