RV wiring? Shore, generator, battery inverter. Back feeding question.

[Deezl Smoke]

  Preface: This is not about home backup generators etc. back feeding to the grid in an outage.

 I am aware of automatic switching etc. I am not against such devices in any way. However, my rig is very simple in design and I wish to keep it that way.

 So my question(s) pertains to back feeding power into components when they are OFF. If I were to directly connect hot, neutral and grounds from all 3 sources (each independent to their own circuit), at a junction box, and say plugged into shore service, but the generator and inverter are off, would back feeding the generator (inverter type) and inverter harm them? What if I boon dock, forget to turn off the inverter (non charger type) and start the generator?

 How about a manual switcher for 3 inputs to 1 output? Can you recommend a reliable brand or model?

 Thank you.

[Lou Schneider]

Absolutely backfeeding power into an inverter or generator will destroy it.  The rule is one and only one AC source may be connected at a time.  The only exceptions are inverters with built-in transfer switches that disconnect incoming power when they're switched on and inverter generators that are designed to load share with a second, identical unit.

Manual transfer switches are available, but they're not any cheaper or simpler than a double pole relay wired so it's coil is energized when a generator or inverter produces voltage and switches the load from external power to that source.  

For 3 sources (shore power, generator, inverter) you'll need two relays connected in series.  Relay 1 has it's coil connected to the generator and when it's coil is not energized passes shore power to it's output.  When the generator starts it energizes the coil and the relay disconnects from shore power and connects to the generator.  When the generator stops the coil de-energizes and the relay switches back to shore power.

It's output feeds a second relay connected to the inverter in the same way and either passes the power from the first relay to the loads or switches to the inverter when it's turned on.

https://www.amazon.com/gp/aw/d/B07FCJFGL9
 

Edited Saturday at 03:29 PM by Lou Schneider

[Deezl Smoke]

23 minutes ago, Lou Schneider said:

Absolutely backfeeding power into an inverter or generator will destroy it.  The rule is one and only one AC source may be connected at a time.  The only exceptions are inverters with built-in transfer switches that disconnect incoming power when they're switched on and inverter generators that are designed to load share with a second, identical unit.

Manual transfer switches are available, but they're not any cheaper or simpler than a double pole relay wired so it's coil is energized when a generator or inverter produces voltage and switches the load from external power to that source.  

For 3 sources (shore power, generator, inverter) you'll need two relays connected in series.  Relay 1 has it's coil connected to the generator and when it's coil is not energized passes shore power to it's output.  When the generator starts it energizes the coil and the relay disconnects from shore power and connects to the generator. 

It's output feeds a second relay connected to the inverter in the same way and either passes the power from the first relay on to the loads or switches to the inverter when it's turned on.

https://www.amazon.com/gp/aw/d/B07FCJFGL9
 

 Awesome. Thank you.

 I've watched several videos on the auto switches and how they work. It's really an amazing invention. I had been just opening the door and plugging the outgo cord into whichever incoming source I had available to use. That works too, definitely no accidents. But since I'm "upgrading" the compartment that houses the batteries and inverter, and putting the generator in a less accessible spot, i figured I'd ask and learn the easy way rather my usual way......hard and expensive way.

[Lou Schneider]

10 minutes ago, Deezl Smoke said:

 Awesome. Thank you.

 I've watched several videos on the auto switches and how they work. It's really an amazing invention. I had been just opening the door and plugging the outgo cord into whichever incoming source I had available to use. That works too, definitely no accidents. But since I'm "upgrading" the compartment that houses the batteries and inverter, and putting the generator in a less accessible spot, i figured I'd ask and learn the easy way rather my usual way......hard and expensive way.

The one thing you'll have to watch is it will be best not to have two sources energized at the same time.  The relays will switch but since the sources aren't synchronized changing from one active source to another can send surges into your electronics.  Commercial transfer switches work the same way but have additional circuitry (time delays, etc.) to minimize the surges if both sources are active when switching.

Edited Saturday at 03:39 PM by Lou Schneider

[Deezl Smoke]

 Ok, that makes perfect sense. The way I set up in travel, I think the only mistake I could make, would be to forget that the inverter is on. Pretty much no chance of the generator and shore service ever operating at the same time. But if I do not have some system that warns me the inverter was left on, I may make that mistake. Have to think about this some more.

[Kirk W]

Lou has given you some very good advice. The other thing that can be dangerous without any transfer device is that the person who forgot might get electrocuted! That is the reason such devices are required by all electrical codes.

There is a very easy and inexpensive way to avoid any sort of problem between the use of the generator and shore power. In the early days of generators being installed into RVs it was very common to wire the output from the generator into and outlet that matches the RV's shore-power cord. In order to use the generator the owner just plugged the shore-power cord into the outlet from the generator. We had a class A that was set up in that way. It had a 50A shore power cord and in the bay where the cord was stored was an outlet that the plug was placed into for storage, thus keeping it handy and that outlet was connected to the output of the 4.5KW Onan generator. That way I could just start the generator for 120V power any time that we were not connected to shore-power. It worked great and I never missed the automatic transfer device. 

Edited Saturday at 08:08 PM by Kirk W
correct typo

[Deezl Smoke]

6 hours ago, Kirk W said:

Lou has given you some very good advice. The other thing that can be dangerous without any transfer device is that the person who forgot might get electrocuted! That is the reason such devices are required by all electrical codes.

There is a very easy and inexpensive way to avoid any sort of problem between the use of the generator and shore power. In the early days of generators being installed into RVs it was very common to wire the output from the generator into and outlet that matches the RV's shore-power cord. In order to use the generator the owner just plugged the shore-power cord into the outlet from the generator. We had a class A that was set up in that way. It had a 50A shore power cord and in the bay where the cord was stored was an outlet that the plug was placed into for storage, thus keeping it handy and that outlet was connected to the output of the 4.5KW Onan generator. That way I could just start the generator for 120V power any time that we were not connected to shore-power. It worked great and I never missed the automatic transfer device. 

 Ok, I'm following. So if we were to throw in an inverter into the scenario, that shore cord would be pulled from the outlet in the storage compartment, and plugged into an adapter to a plain 115V AC? That way there's only 1 cord and it can only draw power from 1 source. Like. Very simple.

[Chad Heiser]

If you have a pass through inverter, it also simplifies the process.  When shore/generator power is present, it simply passes through the inverter with no issues.  When shore/generator isn’t present, the inverter supplies the power.

[Deezl Smoke]

4 hours ago, Chad Heiser said:

If you have a pass through inverter, it also simplifies the process.  When shore/generator power is present, it simply passes through the inverter with no issues.  When shore/generator isn’t present, the inverter supplies the power.

 Never heard of it. Learn something new every day. My inverter is not pass through, but it is pure sine.

 Any good brands of pass through inverters that are known to be reliable?

[Kirk W]

Quote

That way there's only 1 cord and it can only draw power from 1 source.

You could do it that way but you might not need to.  What inverter do you have, make/model?

Edited yesterday at 12:22 PM by Kirk W

[Lou Schneider]

13 hours ago, Deezl Smoke said:

 Ok, I'm following. So if we were to throw in an inverter into the scenario, that shore cord would be pulled from the outlet in the storage compartment, and plugged into an adapter to a plain 115V AC? That way there's only 1 cord and it can only draw power from 1 source. Like. Very simple.

Just make sure you turn off the converter charging the batteries when you're using the inverter.  If they're both on at the same time you'll set up a perpetual motion loop (batteries-inverter-converter-batteries) that wastes power.  Safest way to prevent this is to put the converter on a circuit that's fed from the generator/shore power relay before the inverter.  So it's only on when the generator or shore power is active, not when power is being supplied by the inverter.

[Lou Schneider]

22 hours ago, Deezl Smoke said:

 Ok, that makes perfect sense. The way I set up in travel, I think the only mistake I could make, would be to forget that the inverter is on. Pretty much no chance of the generator and shore service ever operating at the same time. But if I do not have some system that warns me the inverter was left on, I may make that mistake. Have to think about this some more.

When I was servicing remote transmitter sites we had a warning light next to the building's door that lit up to remind us to restore remote access to the transmitter before we left the site. Saved having to drive back up the mountain to flip a switch if we left the remote control disabled.

You could do the same with a small LED that comes on when the inverter is on.

[Chad Heiser]

11 hours ago, Deezl Smoke said:

 Never heard of it. Learn something new every day. My inverter is not pass through, but it is pure sine.

 Any good brands of pass through inverters that are known to be reliable?

Victron is the top of the line and their Multiplus inverter chargers offer pass through and additional features like a 120 amp charger, power assist (for when shore or generator is limited so it can make up the difference from the batteries to power a heavy load) and others.

[Deezl Smoke]

8 hours ago, Lou Schneider said:

Just make sure you turn off the converter charging the batteries when you're using the inverter.  If they're both on at the same time you'll set up a perpetual motion loop (batteries-inverter-converter-batteries) that wastes power.  Safest way to prevent this is to put the converter on a circuit that's fed from the generator/shore power relay before the inverter.  So it's only on when the generator or shore power is active, not when power is being supplied by the inverter.

 That's interesting. By converter, you are basically referring to a battery charger? I worked on the rig today and got the inverter mounted and the shore inlet mounted. What I have been doing is charging the batteries when on shore power with a 25 amp smart charger. Just a plain ol' Viking battery charger that floats when topped. I would never have a reason to run the inverter when I have the generator going or access to shore service, but say I did mess that up, would running my battery charger bring a similar scenario as the converter?

[Chad Heiser]

19 minutes ago, Deezl Smoke said:

... would running my battery charger bring a similar scenario as the converter?

You do not want to power any battery charger with an inverter.  This is in reference to a battery charger that is trying to charge the batteries the inverter is drawing power from.  It is a loosing proposition.  The inverter will draw more energy from the batteries just to run the battery charger than the battery charger can replace.  So even if there was nothing else drawing power from the batteries, eventually you would run out of power. The loop of the inverter drawing DC power from the batteries, inverting it to AC power and the charger using that AC power to convert back to DC to charge the batteries is a downward spiral.  There are conversion losses at each step and it will drain the batteries.

Edited 15 hours ago by Chad Heiser

[Deezl Smoke]

1 hour ago, Chad Heiser said:

You do not want to power any battery charger with an inverter.  This is in reference to a battery charger that is trying to charge the batteries the inverter is drawing power from.  It is a loosing proposition.  The inverter will draw more energy from the batteries just to run the battery charger than the battery charger can replace.  So even if there was nothing else drawing power from the batteries, eventually you would run out of power. The loop of the inverter drawing DC power from the batteries, inverting it to AC power and the charger using that AC power to convert back to DC to charge the batteries is a downward spiral.  There are conversion losses at each step and it will drain the batteries.

 My apologies. What I was referring to would be running the battery charger from shore or generator, but pulling coach power "from" the inverter at the same time as charging from shore or generator.

I would never have a reason to do so, as if I am running the generator or on shore service, then the coach would pull power from that. But if for some reason I did pull power from the inverter and charge batteries on shore service or generator, would the charging and drawing at the same time cause issues? In an automobile, no. You can run lights from the battery while driving and charging with the alternator. But I know too little about inverters to know if a 115vold AC charger would ever spike the input of the inverter or what.

[Chad Heiser]

2 hours ago, Deezl Smoke said:

 My apologies. What I was referring to would be running the battery charger from shore or generator, but pulling coach power "from" the inverter at the same time as charging from shore or generator.

I would never have a reason to do so, as if I am running the generator or on shore service, then the coach would pull power from that. But if for some reason I did pull power from the inverter and charge batteries on shore service or generator, would the charging and drawing at the same time cause issues? In an automobile, no. You can run lights from the battery while driving and charging with the alternator. But I know too little about inverters to know if a 115vold AC charger would ever spike the input of the inverter or what.

You can have a charge source charging the batteries at the same time an inverter is drawing from the batteries.  This is done all the time.  Typically it is solar charging the batteries while the inverter is drawing from them, but your example would work as well.