Help with circuit design

[CarryWise]

Hi,  I need some help in designing a circuit for my motorhome.  I think I know how to achieve what I want with an Arduino, but it seems like it would be faster and more reliable to design a simple circuit instead.  Here's what I want to accomplish:

I have an 2000 watt inverter that I want to wire into my motorhome's 120 AC power system that will run off the 12v DC system.
There is already a 120v AC to 13.6v DC converter which is used to charge the batteries and provide 12v power when AC power is available. (I can plug the motorhome in at a campground, and I also have a generator I can run.)

The problem is that when the inverter is providing power 120v power, the converter will try to turn that back into 12v (to power the inverter), and since perpetual motion isn't likely I imagine the best outcome will be to have the inverter turn off with a low voltage warning.

So here's my rough idea.

• Add a relay to the Converter 120v input.
• Wrap an 30amp AC Current Sensor around the 120v output from the inverter.
• Build a circuit powered by the 12v power from the batteries which powers the relay when there is no voltage coming from the AC Current Sensor, and shuts it off when voltage is detected.
• If the relay requires more than a few milliamps of power, I would like to add another AC Current sensor around the main line coming into the circuit breaker panel, so that I could leave the relay unpowered when there was no AC current at all. (Power relay when Main AC, but no Inverter AC. If no AC sensed, or AC sensed on both, leave the relay unpowered.)

Thanks in advance for your constructive criticism.
-Chuck

[retrolefty]

I would  think you could accomplish that with  just a couple of DPDT relays wired such that the relays only pull in when external AC power is available acting as an interlock. But to be sure I would have to see at least a block diagram of your setup.

[CarryWise]

Here's a simple diagram of what I'd like to do. I didn't mention the Automatic Transfer Switch - it's another box I have which will automatically cut out the inverter when the generator is started or the rig is plugged into shore power (the 30amp power outlet at a campground.)

Can I get away with just a SPST relay for the hot wire to the converter as it exits the circuit breaker?

[MarkF]

Take a look at the circuit breaker panels and power switching used on yachts for some ideas.  They do exactly what you're trying to do for shore power and generator switching.

[CarryWise]

MarkF, generally the Inverter and Converter (aka Charger) are built into a single unit, so they don't have the problem I have.  For space and cost reasons I wanted to keep using the existing converter, so I lack the circuit to cut out the converter when the inverter is running.

[MarkF]

MarkF, generally the Inverter and Converter (aka Charger) are built into a single unit, so they don't have the problem I have.  For space and cost reasons I wanted to keep using the existing converter, so I lack the circuit to cut out the converter when the inverter is running.

I was referring to the circuit breaker panels.  Both AC and DC panels to do the switching using your current equipment.  Then it's just a matter of a little re-wiring.  Something like this photo.  I believe there are source selection switches combined with circuit breakers for distribution.

[CarryWise]

Mark, we're back to the "space" issue.  Here's my circuit breaker panel, in the area below the refrigerator. The second picture shows the AC breakers, DC fuses, and the converter - for perspective the converter is 5.4 inches high.

That's why I opted for the automatic transfer switch - I can throw that in an exterior bay where the main power line is coming in, and pull the AC line of the inverter all the way over to it. Then while nice and dry inside I can start the generator or turn on the inverter and have AC power.

[CarryWise]

Just to clarify - in my post above with the RV diagram, what I need help with is the circuit defined as a box with the question mark in it.

It has two inputs, from something like http://www.ebay.com/itm/SCT-013-030-Non-invasive-AC-Current-Sensor-Clamp-Sensor-30A-NEW/201415007484  and 1 output, to something like http://www.ebay.com/itm/2-ZETTLER-MINI-POWER-RELAYS-AZ2280-1A-12DF-30A-277VAC-28VDC-NEW-/201144478486.  12v DC power would also be available to the circuit.

[MarkF]

Personally, I would not recommend doing the switching automatically for safety.  I would try to setup circuit breakers with lockout selection to prevent  simultaneous connections.

I'm unable to read the writing in your pictures.  So, I don't see how the interconnections to the AC, battery and convertor are made.
Here is an idea for your circuit if you insist on doing it automatically.  The relay has 120VAC coil with 120VAC normally closed contacts.  If the invertor is on, the convertor is disabled.

You do realize that your invertor will only provide 16 Amps and will NOT power your entire RV.  You will need a 15A breaker on it's output if it doesn't have something built-in.

Here are two possible AC source selector switches:
Blue Sea 1482
Blue Sea 8366

[CarryWise]

Hi Mark.

First, thank you for taking the time to respond and help me.  I'm sorry for the chicken scratchings.  I like your diagram - what did you use to create it?

Yes, I know that I won't be able to power the whole RV from the inverter. It does have output overload protection built in although I expect that I will probably hit a low DC input voltage shutdown first until I someday switch to Lithium Iron Phosphate batteries.

The automatic transfer switch is this ats301-transfer-switch so it's designed for that purpose.  I will have the generator/shore power be dominant - the inverter will be disconnected whenever power from the generator/shore is available.  The generator is wired to an "outlet" which I plug the input line into when I don't have shore power. In a campground I pull the plug from the generator outlet and plug into the campground outlet, providing a kind of manual switch.

So it's starting to sound like a whole circuit is unnecessary. Here's my refined idea:

• Use a normally open relay, based on sensing power from the generator/shore power. (So if I forget to turn off the inverter, the relay doesn't stay powered and prevent battery charging.)
• Use a parallel circuit to wire the relay from the generator/shore power.
• Use a single pole normally open relay on the hot line to the converter.

So my new questions:

• I think I can get away with relatively light wire gauge for the circuit from the relay to the generator/shore power, since it should be drawing in the milliamps. How small a gauge can I use?
• The circuit to the relay is parallel, connected to the Hot and Neutral from the generator/shore power line. Right?
• How do I protect the relay wiring? It would be nice to have a simple inline fuse or small circuit breaker. Is there something like this for AC? 30-Amp-In-Line-Circuit-Breaker-Stereo-Audio (I've used these to protect the circuit from the solar panel and charge controller and cut the circuit easily so I can work on things.)
• Would something like this work for the relay? 2-New-NAIS-HE1aN-AC120-V-30-amp-N-O-Relay

[MarkF]

First, thank you for taking the time to respond and help me.  I'm sorry for the chicken scratchings.  I like your diagram - what did you use to create it?

I used Photoshop to make the drawing.

Yes, I know that I won't be able to power the whole RV from the inverter. It does have output overload protection built in although I expect that I will probably hit a low DC input voltage shutdown first until I someday switch to Lithium Iron Phosphate batteries.

Not at all.  Even with a full battery, the output will over current if multiple high current items turn on at the same time.  For instance, the A/C, Microwave and especially something with a motor (high start-up in-rush current).

The automatic transfer switch is this ats301-transfer-switch so it's designed for that purpose.  I will have the generator/shore power be dominant - the inverter will be disconnected whenever power from the generator/shore is available.  The generator is wired to an "outlet" which I plug the input line into when I don't have shore power. In a campground I pull the plug from the generator outlet and plug into the campground outlet, providing a kind of manual switch.

I assume then that you have a portable generator that is NOT permanently mounted and wired into the RV.  I choose an AC Source switch with 3 inputs that you could wire it in or get the 2 input version (one to external power connector and one to the inverter).  Or leave the 3rd free for expansion.

So my new questions:

• I think I can get away with relatively light wire gauge for the circuit from the relay to the generator/shore power, since it should be drawing in the milliamps. How small a gauge can I use?
• The circuit to the relay is parallel, connected to the Hot and Neutral from the generator/shore power line. Right?
• How do I protect the relay wiring? It would be nice to have a simple inline fuse or small circuit breaker. Is there something like this for AC? 30-Amp-In-Line-Circuit-Breaker-Stereo-Audio (I've used these to protect the circuit from the solar panel and charge controller and cut the circuit easily so I can work on things.)
• Would something like this work for the relay? 2-New-NAIS-HE1aN-AC120-V-30-amp-N-O-Relay

  1.  I would just use #12 AWG stranded wire with crimped wire lugs to wire the relay, inverter AC to breaker to AC source selection switch (20A max). 
  4.  No.  You need a relay with NORMALLY CLOSED contacts.  That why when there's NO power on the inverter, the converter will be connected to the AC breaker panel and work like it currently is wired.

Parts List:
  OMRON G2R-1-T-AC120 Relay
  Blue Sea 7210 Single Pole 15 Amp Circuit Breaker
  Blue Sea 9010 AC Selector Switch
  Blue Sea 8080 DC Panel

The DC Panel will allow you to connect 2 batteries and parallel them if you want.  The little toggle switch will turn on your inverter assuming it requires less than 12VDC @ 100Amps.  I saw a 2000W unit online that required 12V @ 80A as a guide.  Your battery is NOT going to last very long at that current level.  You will probably need #4AWG stranded copper wire to connect to the battery and the inverter.

You should be able to drill a couple holes to mount the AC breaker from the inverter and place it inside a metal box.

All AC wiring should be physically separated from the DC wiring and in-closed in their own metal boxes.

See if any of this makes sense to you.

[CarryWise]

Mark,

The generator is permanently mounted, but as I explained, it it wired to a NEMA TT-30P female oulet. So I already have a "very manual" generator/shore power switch, where I unplug the cord from the generator outlet and plug it into the campground outlet or vice versa.  Since I have to coil up the cord and put it away when I unplug from the campground, it's no effort to plug into the generator outlet in case I decide to turn it on later.

Can you explain why you don't like the idea of running a NO relay off the generator/shore power line instead of the NC relay from the inverter? With the NC/Inverter approach, if I forget to shut off the inverter the circuit to the relay will still be live and the converter will continue to be disabled.

-Chuck

[CarryWise]

Mark, here's a picture of what I'm envisioning. Included is a picture of the electrical cabinet on the driver's rear side.

Everything in this diagram is already in place except for the Inverter, Automatic Transfer Switch, and Converter relay and the wiring between them.

I need to pull a AWG #12 3 conductor wire from the inverter to the electrical cabinet underneath the rig. The inverter needs all 3 conductors, because when it's running, it's responsible for the neutral/ground bond.

I also need to pull a two conductor wire back from the electrical cabinet for the converter relay. Could I put a GFCi outlet in the electrical cabinet and then just put the relay at the end of #18 lamp cord with a 2 prong plug?

[MarkF]

Can you explain why you don't like the idea of running a NO relay off the generator/shore power line instead of the NC relay from the inverter? With the NC/Inverter approach, if I forget to shut off the inverter the circuit to the relay will still be live and the converter will continue to be disabled.

If you wire the relay coil directly to the shore/generator power before it goes through the inverter output switch (See diagram).  Let's take a look at what you're proposing: 

   Case 1.  When shore power or the generator is ON, the normally open relay will close the contacts.  At this point the converter is powered up and will charge the battery.

   Case 2.  Shore power is OFF.  The relay contacts are open and the converter is disabled.  Turning ON the inverter will provide AC power to the RV.

   Case 3.  If you FORGET to unplug shore power, the relay is closed and the converter is charging the battery.  If you switch ON the inverter and select source switch for it to feed AC power, now the inverter is feeding AC power. The converter is still ON charging the battery and the battery is powering the inverter.  JUST THE CASE YOU WANT TO AVOID.

With the normally closed relay solution I'm proposing, there is NO WAY for the converter to be charging the battery while the inverter is ON.  There is NO way to complete the loop, even if you FORGET something.

[CarryWise]

Hi Mark,

I must not have explained the concept behind an automatic transfer switch well.  With the shore/generator line wired into the switch as the alternate source (if both are powered, the alternate is connected to the output), then the Inverter can never be powering the converter.

There's nothing to forget - it's hard wired behavior.  You might even say it's.... Automatic.

 

Edit: I'm sorry, at one point I could have sworn I provided a link to the automatic transfer switch: http://www.parallaxpower.com/ats301-transfer-switch-ats301

Edit 2: To clarify the ATS wiring - I will actually be connecting the inverter as the "line" or default source on the ATS, and the shore/generator as the "Generator" or alternate source on the ATS. To quote the manual:

The default power source is the shore line or utility power input “side” of the switch.
The alternate power source or “Generator” input side of the switch will “take command” of the switch when the alternate source is energized.

[MarkF]

I need to pull a AWG #12 3 conductor wire from the inverter to the electrical cabinet underneath the rig. The inverter needs all 3 conductors, because when it's running, it's responsible for the neutral/ground bond.

I also need to pull a two conductor wire back from the electrical cabinet for the converter relay. Could I put a GFCi outlet in the electrical cabinet and then just put the relay at the end of #18 lamp cord with a 2 prong plug?

I did NOT show the earth ground routing in the diagram.  Just the "line" and "neutral" wires.

#18 awg lamp cord IS NOT ACCEPTABLE in any part of this!  Lamp cord is for lamps.  Do not use inside walls!

If you use #18 wire, you will need to provide current protection for it near the source.


Just noticed you have the inverter under your table seat.  I expect that it will require a lot of ventilation.

[CarryWise]

Hi Mark,

Thank you again for taking the time to answer my questions and help.  But while I've been trying very hard to remain receptive and positive in my replies, I feel a bit like I'm being yelled at. I really do want to understand what I'm doing and be safe - honest.

Thankfully, I won't be running anything inside the walls. If you think it's hard to run new wiring in a fully dry-walled house, you have no idea how difficult it is to do in a 7' x 18' box where the furniture and appliances are all bolted down. I'll be drilling through the floor and running wiring along the underside of the vehicle in a plastic cable protector. That way I only have to work around moving parts, exposure to water, road debris and chemicals, and heat sources like the exhaust.

If you look back you'll see that I've been consistently asking for ideas on protecting the wiring to the relay, since it's a separate parallel circuit and I don't want to run #12 wire to power a 20 milliamp relay.

What do you think of this breaker? W28-XQ1A-025  If I understand the data sheet, it looks like it would trip in less than .3 seconds if at 2.5 amps.

Do you have any thoughts for low current wiring on how to determine the required gauge? http://www.engineeringtoolbox.com/wire-gauges-d_419.html seems to indicate that #22 can handle 3 amps, and #24 can handle 2 amps.  If I use Wikipedia American_wire_gauge, the ratio for NEC seems to be 1/11th the Preece ~10s fusing current. That would give #22 gauge a 3.7 amp rating, and #24 a 2.6 amp rating. From either source, with a 250 milliamp breaker, it looks like 22 or 24 gauge wire would be safe even if it's not code.

That would allow me to use Southwire-22-2-Shield-Security-Cable for the relay wiring.  However, that's only 300v. Safe, again because I have a hardwired energy management system (fancy surge protector) which protects the RV against over and under voltage on the shore and generator lines.

If I need to have 600v rated wire, I have to go with something like this: http://www.mouser.com/ProductDetail/TE-Connectivity-Raychem/44A0121-24-0-9-US/?qs=sGAEpiMZZMv1%2f%252b2kKkGMBR432jei1SPkxrIu7AJt0tY%3d

Thoughts?

[MarkF]

Sorry.  I didn't mean to yell at you.  The National Electrical Code does not allow lamp cord to be used in place of a permanent wiring method.  I just wanted to drive the point home.

I'm having trouble dealing with the relay too.  I would only use #22 or #24 inside enclosures in short lengths.  I would not run such small wire in long raceways between locations.  The little breaker is okay, but where would you mount it.  The inverter can only supply 16 amps so the 15A breaker and #14 wire would protect the inverter, inverter wiring and relay wiring.  Don't design to just the minimum requirements.

Why use the Automatic Transfer Switch for just part of the AC switching?  Just add another outlet from the inverter.  Then plug your power cable into the desired source (shore/generator/inverter).  I imagine the only time you'll use the inverter is if you are moving.  So the power plug will be connected to the inverter outlet.  The generator or shore power would only be used will parked at the campground.  (Just a guess here.  A 2nd outlet would be a lot cheaper too).

I've been struggling with how and where you're going to mount and enclose the relay, breaker and switches.  The 120 AC should be enclosed and physically separated from the 12 VDC wiring and devices.  This will steer a lot of the design choices.

[CarryWise]

Mark, if you want more background on the inverter, it's here: Samlex PST-2000-12

The inverter wiring requirements I've seen have never mentioned a breaker on the output, but they've clearly required a fuse on the input. Probably  because the inverter is designed to support <1s surge of up to 3500 watts, and also has two protections (output overload and input low voltage) to protect the inverter.

That said, I should wire the inverter output with #10 to support the surge amps.

I've been struggling with how and where you're going to mount and enclose the relay, breaker and switches.  The 120 AC should be enclosed and physically separated from the 12 VDC wiring and devices.  This will steer a lot of the design choices.

What do you think of this:

• Mount the relay in the breaker panel.
• Mount the automatic transfer switch in the external wiring cabinet after the energy management system/surge protector. (The ATS is a metal enclosed box.)
• Option 1, cut a hole and mount the relay circuit breaker in the ATS.
• Option 2, mount a separate metal box for the relay circuit breaker, and run the shore/generator line to that, then wire that to the ATS. Not excited about this, not much room in the external wiring cabinet with the coiled wires in there. However, if I have to do this it would make sense to place in there a 24v dc transformer so the wiring only has to meet NEC class 2 circuit standards.
• Use #18 600v wiring for option 1 - seems to be covered in the NEC as a Class 1 remote control circuit - 725.43

[MarkF]

SAMLEX PST-2000-12 INVERTER

Installation requirements:
   DC input wire - AWG #2/0 with 300A fuse
   AC output wire - AWG #12-3 with 20A current protection
   Earth ground - AWG #8 stranded copper (Lug #7) to RV metal chassis

Thoughts:

• The unit is rated for #12-3 wire for the output.  The wire gauge is specified for the continuous load, not <1 second surge currents.  If you want to use #10-3, make sure the terminals will accommodate that size wire.  My reason for using #10-3 wire would be to minimize the voltage drop from running the wire from one end of the RV to the other.
  
• How are you going to control the unit?  Crawl under the dinette seat to the unit power switch?
  
• This unit will generate a lot of heat.  Ventilation should be a major concern.  I suspect that the fan noise will be very high.  Maybe consider another location (within 10 ft of the batteries).
  
• Have the #2/0 DC cables professionally made with crimped connectors.  Don't try to put the ends on yourself.
  
• Are you going to depend on the units internal overload circuitry to protect its output?
  

ATS-301 TRANSFER SWITCH and CONVERTER RELAY

Thoughts:

• The ATS301 enclosure and the AC Distribution portion of the Parallax Power Center are sized to meet the minimum cubic volume needed for wiring and air cooling.  The Omron relay is 1"x2"x2" alone and would not fit in the Power Center and still meet the minimum volume requirement.  I DO NOT recommend added additional components into either unit.
  
• I don't understand this obsession with using the minimum wire gauge for the relay coil.  To the point of adding additional components that you don't have space for.  Really, a 24V AC transformer!  In what world does this make sense?  My recommendation in the attached diagram is to place ALL the new components into the external wiring cabinet in the back corner of the RV.  If I can convince you to use a short piece of #12-2 wire for the relay coil, place the relay in a 4"x4" electric box and run #12-2 to the Power Center to enable the converter.  Remove the converter power wire from it's breaker, put one side of the relay wire on the breaker and tie the other relay wire to the converter power wire with a wire nut.
  
• Duplex Boat Cable 12/2 at $1.18 per foot or Triplex Boat Cable 12/3 at $1.50 per foot (100ft. rolls available) would due for the relay wiring.  I believe a very short piece of #12 wire to the relay coil would be okay without a fuse.  I'm trying to minimize component count because of your limited space.
  

[MarkF]

The #12 gauge wire I mentioned in my last post has very fine strands.  This may be difficult to work with and pretty expensive.  This #12-3C THHN-PVC Tray Cable from Wire & Cable would be a better choice.