Emergency Power from Generator or Vehicle Inverter

[IT_Architect]

Normally, the house is connected to the grid.  My thoughts are to make it manually transferable to power from a generator or vehicle-mounted inverter, depending on the circumstances.  I understand that connecting to mains requires an inverter that supports a neutral such as Samlex PST-1500 & 2000 or Xantrex Prosine 1000 or 1800.  (It's not clear from the documentation if the newer Xantrex Pro SWs support this)  In the c panel, I was planning to add a 240V breaker and a piece of metal that makes it impossible for the mains and this 240V breaker to be on at the same time.  Then run 4 lines to a receptacle outside of the house:
- 2 lines for hot, connected to the added breaker.  When connected to a generator, it would supply 240V in the conventional way to the c panel bus.  When connected to an inverter, the two would be in-phase, so no 240 would be available.
- 1 line for neutral
- 1 line to connect the inverter or generator ground & earth ground to the c panel ground.

In this configuration, it would not be possible for a generator or inverter to sense current on the ground.  On the other hand, there is nothing to sense it at all when connected to the grid.  GFCI receptacles would work the same in either case.  So how much of a problem would this configuration actually be?

Thanks!

[dnix71]

Any genset made to connect to a transfer switch on a home for backup power would have the legally required fault detection built in. If you buy new, pull a permit and install to code that's the electrician's problem. House wiring is neutral-bonded ground, where most gensets under 5kw are floating neutral. Both are inherently safe, they just happen to be incompatable for an interconnect.

If you don't want to go that route, then don't do it at all. An improperly connected genset could run your meter backwards (which will be note as 'tampering' by your utility and subject you to fines and a disconnect). You could backfeed the grid if done incorrectly and injure or kill a lineman doing repairs during a power outage. An you could damage your home electronics or set fire to your home.

I have a 600 watt sine inverter for backup in my van, a 900 watt 2-stroke portable genset and a 1kw battery-powered sine inverter inside. If I need them out comes a 12 gauge extension cord.

[IT_Architect]

Thanks for the reply.

House wiring is neutral-bonded ground, where most gensets under 5kw are floating neutral. Both are inherently safe, they just happen to be incompatable for an interconnect.

I'm not willing to buy a generator or inverter that has a floating/live neutral configuration.

An improperly connected genset could run your meter backwards (which will be note as 'tampering' by your utility and subject you to fines and a disconnect). You could backfeed the grid if done incorrectly and injure or kill a lineman doing repairs during a power outage. An you could damage your home electronics or set fire to your home.

...and a piece of metal that makes it impossible for the mains and this 240V breaker to be on at the same time.

Makes that not possible.  My thoughts are a PB30 or larger coming inbound to the added 240V breaker that cannot be turned on when the mains are on, and vice-versa.

I have a 600 watt sine inverter for backup in my van, a 900 watt 2-stroke portable genset and a 1kw battery-powered sine inverter inside. If I need them out comes a 12 gauge extension cord.

For me, splicing plugs for the furnace, and when there is an outage, getting behind built-in refrigerators, microwaves, telephone/internet boxes is not practical.  In that situation I would probably use the approach used on aircraft where we have essential and non-essential buses.  If I have an in-flight electrical subsystem emergency, I can elect to power only the essential bus.  I could have a PB30 connection to the house's "essential bus", and plug either the mains, or emergency power to the "essential bus".  The benefit for me would be is the neutral bonding would occur where it was designed to during both grid and off-grid operation, and also it would work for someone that needed a floating/live neutral configuration because on emergency power, the circuit would no longer terminate at the c panel.

Thanks!

[IT_Architect]

While a common configuration, there is a fly in the ointment with my plan A.  I found a good explanation here http://members.rennlist.org/warren/generator.html as to why it can be a problem.  To make that work, I could tie the earth grounds together, but would need to float the neutral on the generator or inverter, and it won't work for low-cost inverters at all because they have 2 hots and no neutral.  If one would try to wire a low-cost inverter, it would blow up because the c panel would tie the ground and neutral together.  I one uses just the two hots, and doesn't connect the ground, it would work fine, but then there would be ~80 volts AC between the battery and ground.  If one would ground the battery, he would again blow up the inverter, because the AC ground is tied to the negative battery terminal, would again complete the circuit through the c panel where neutral and ground are bonded.  Thus, modified Plan A will work fine as long as you have an inverter that is capable of being hard wired.  Plan B would work fine in any case, but electrically, switching plugs is the same as a 3-pole transfer switch, which would work in any scenario.  Sooo it appears that a modified plan A, or plan B are my only two good choices.

There is some potential for something really bad to happen, but the much more likely scenario is there are some not-so-apparent paths that can easily hose some expensive equipment.  I need to make this as Murphy proof as possible.

Thanks!

[dnix71]

Not to repeat myself, but you need to hire someone. Backup gensets with transfer switches are common, but you won't save doing it yourself. You need to either know the max load or have a subpanel that only feeds selected appliances, and you will need a genset over 5kw nameplate made for the US market to insure it is neutral-bonded ground. US regs allow floating neutral for portable gensets under 5kw, but require neutral-bonded ground for permanent and over 5kw.

It isn't expensive to do this. My church has a 12kw Briggs gasoline genset with a twistlock cable that goes to a lockout breaker on a panel in the kitchen. Fire up the gen, plug in the cable, flip the breaker and it just works. Nothing fancy or expensive, but we did it to code and our insurance company knows about it and is okay with it. We had a kitchen fire from a failed microwave oven earlier, the genset and transfer switch were added during the repairs. It won't run the ac, but those are on a separate panel so no chance of overloading the Briggs.

[XeonPony]

Isolation transformers preform many miracles, one of them is emptying your wallet, so how bad do ya want it?

[IT_Architect]

My logic has taken me full circle to where I can better appreciate what dnix71 has suggested.  If the generator and inverter will be used in multiple scenarios, from a human-factors engineering viewpoint, it doesn't make sense to set up a system that requires configuration perfection of the source each time one needs it for temporary backup power.  Either a transfer switch, which provides the "essential bus" I referred to earlier, or running cords, which is electrically the same as the transfer switch, are the only methods that I can envision being suitable for temporary backup power.  That enables the grounding method to be automatically controlled by the source's current grounding configuration, rather than requiring the source to be configured perfectly to match the load's grounding method each time it is deployed, to prevent destroying equipment or creating an unsafe situation.  That method also greatly expands the number of devices suitable for temporary backup power to almost anything out there, which is something that dnix71 mentioned, but took me a while for me to fully appreciate.

Using a second 240V breaker with mechanical lockout that I mentioned earlier, would be more suitable for a permanent standby power installation where the source could be configured one time to match the load's ground configuration.  This method also restricts the number of suitable devices to those that can optionally utilize a bonded neutral.

This discussion gave me a much needed broader perspective as to why the transfer switch/cord strategy is heavily favored for temporary emergency power setups, and might alter my strategy.

Thanks!

[joestue]

An inverter that puts out 120v modified sine wave but then couples that to the battery side isn't an inverter in my opinion.
its a uh, <censored> <censored for folks with lack of humor> detector.

In any case, mixing inverters and generators is almost always impossible.

So anyhow, seems to me that even if the code requires a generator that has a neutral ground bond, meaning you're running a 4 wire cord from the "professionally installed transfer box" to the generator, if the generator has a 3 wire plug it should be just as easy to ground the generator and fufill the intent of the code.

"Any genset made to connect to a transfer switch on a home for backup power would have the legally required fault detection built in"
this is likely only the case for generators intended for permanent installation and are relatively new, i've not run into any sort of intelligent fault detectors, other than programmable circuit breakers but that is for overload only. i don't know that the code required GFCI fault detection in the states, unless they changed it in the last 5 years.