Wiring Transfer Switch (My expanation may be confusing. Sorry.)

Lin · January 02, 2009, 07:15:00 PM

If things go as planned, which they usually don't, I will be installing my inverter and transfer switch soon. On my last bus, I wired the transfer switch so that shore power energized the solenoid and loss of power defaulted to the inverter. Hence, whenever there was shore power, the relay was hot. At the time, I figured that, if I had shore power, I had power to spare, and it would be better that way then wiring it so that I wasted battery juice to keep the inverter on. As a result, since we lived in it for some years, that transfer switch solenoid was energized for very long periods of time. Can the system work in the reverse--ie: the switch defaults to shore and switches to inverter when there is power?

Sean · January 02, 2009, 07:38:12 PM

Lin,

I can't follow your question as stated.

Let me ask you a couple of my own:

1. What make and model of inverter do you have?

2. Are you only needing transfer between shore and inverter, as you described, or is there also a generator involved?

3. Do you intend for all your loads to be connected to the inverter when away from shore power, or only a select few?

-Sean
http://ourodyssey.blogspot.com

Lin · January 02, 2009, 08:22:35 PM

It is a Trace U2512 inverter. My breaker panel is wired to a plug that is moved by hand from the shore power outlet to the generator outlet depending on where I wish to draw the power. Therefore, the transfer switch is only needed to switch from generator/shore mode to the inverter. I have planned to use an external transfer switch so that the entire panel is energized by the inverter rather than use the units internal transfer switch which would require isolating inverter loads. I realize that the inverter will not power everything, but I can manage the loads as I want.

What I am getting at is that the transfer switch accommodates two inputs. The unit defaults to one and switches to the other when the coil is energized. Can I use it either way? In other words, wire it so that switch defaults to shore/gen instead of defaulting to the inverter as I had it in the past? Has anyone done it that way? Is that any better?

Rick 74 MC-8 · January 02, 2009, 08:35:34 PM

in short YES just change inputs

Sean · January 03, 2009, 06:03:38 AM

Quote from: Lin on January 02, 2009, 08:22:35 PM
... Therefore, the transfer switch is only needed to switch from generator/shore mode to the inverter.

Except the U2512 already has a built-in, 60-amp transfer switch for this purpose, so no external switch is needed. In fact, using an external switch downstream of the U2512 will defeat the battery charger entirely, so you'd need to have a different way to charge batteries while on shore or generator.

Quote
I have planned to use an external transfer switch so that the entire panel is energized by the inverter rather than use the units internal transfer switch which would require isolating inverter loads.

I'm not sure I follow this statement. Is this because you have a 240-volt shore/generator setup, and you want to bridge the legs and run both from the inverter?

Quote
What I am getting at is that the transfer switch accommodates two inputs. The unit defaults to one and switches to the other when the coil is energized. Can I use it either way? In other words, wire it so that switch defaults to shore/gen instead of defaulting to the inverter as I had it in the past?

Well, yes and no. You could certainly wire the switch so that the "default" is the cord, and the "priority" is the inverter. However, if you intend to run the inverter all the time, the switch will then always be on inverter (and the coil will always be energized), and the only way it will switch back to cord is if you turn the inverter off. I'm not sure why you'd want this.

To wire it as you suggest, you'd also be using battery power just to keep the transfer switch coil energized.

It would seem to me that the system you are proposing would be better achieved either with a manual transfer switch, or just by adding a receptacle to the inverter output and using your cord to select it instead of either shore or generator. But, again, you'd be foregoing the battery charger on the inverter.

-Sean
http://ourodyssey.blogspot.com

Lin · January 03, 2009, 11:28:11 AM

Thanks Sean for going into detail. You are making me re-think and re-analyze my assumptions, which seems like a real good idea.

"In fact, using an external switch downstream of the U2512 will defeat the battery charger entirely, so you'd need to have a different way to charge batteries while on shore or generator."

As far as the battery charger goes, my plan was to supply power to the inverter from the shore input to the transfer switch. I thought this would let the charger work without creating a loop situation that would destroy the inverter. One of the wiring diagrams in the Trace manual looked like it was doing this. I should probably study it a bit more to be 100% sure.

"I'm not sure I follow this statement. Is this because you have a 240-volt shore/generator setup, and you want to bridge the legs and run both from the inverter?"

You hit the nail on the head re the shore/generator setup. Currently (no pun...) the generator puts out real 240 so its receptacle has two 120 legs. The shore, on the other hand is faux 240. It has one 120 leg, but there is a jumper in the receptacle energizing the second leg. Hence, the panel sees it as 240. So there can be no jumper in the panel. I guess I could use the inverter to fire up one leg in the panel and put all the inverter loads on that leg. However, since the only circuits I don't intend to run through the inverter would be the water heater, the two AC's, and a couple of built-in electric heaters, it seemed that I would end up with a very imbalanced panel. Therefore, I wanted to energize the whole panel and try to think before I turn something on. The external transfer switch would do this since it would open/close both legs without caring if it were genuine 240 or not.

"Well, yes and no. You could certainly wire the switch so that the "default" is the cord, and the "priority" is the inverter. However, if you intend to run the inverter all the time, the switch will then always be on inverter (and the coil will always be energized), and the only way it will switch back to cord is if you turn the inverter off. I'm not sure why you'd want this."

This is why I posted the question. It seemed that it would not work for the reasons you describe. But, not being an expert, I thought that I might be misunderstanding things. Thanks for clearing that confusion.

I guess I could do a "combination" here and use the internal transfer switch to light one leg, put all the inverter loads along with some of the heavy loads on that leg to avoid a general imbalance paner, but, using my same faulty memory, remember not to turn the big ones on. This would have the benefit of being able to isolate the water heater, which is the most likely one to be left on by accident. Does that sound better?

Sean · January 03, 2009, 03:04:20 PM

Quote from: Lin on January 03, 2009, 11:28:11 AM
As far as the battery charger goes, my plan was to supply power to the inverter from the shore input to the transfer switch. I thought this would let the charger work without creating a loop situation that would destroy the inverter. One of the wiring diagrams in the Trace manual looked like it was doing this. I should probably study it a bit more to be 100% sure.

You can't supply the inverter input from one side of the transfer switch, and, at the same time, feed the inverter output to the transfer switch. That will ultimately short the input to the output (or at least create that possibility), which can destroy the inverter and also have other bad consequences.

You'd need to have two separate transfer switches -- one upstream of the inverter, and one downstream.

Quote
You hit the nail on the head re the shore/generator setup. Currently (no pun...) the generator puts out real 240 so its receptacle has two 120 legs. The shore, on the other hand is faux 240. It has one 120 leg, but there is a jumper in the receptacle energizing the second leg. Hence, the panel sees it as 240. So there can be no jumper in the panel. I guess I could use the inverter to fire up one leg in the panel and put all the inverter loads on that leg. However, since the only circuits I don't intend to run through the inverter would be the water heater, the two AC's, and a couple of built-in electric heaters, it seemed that I would end up with a very imbalanced panel. Therefore, I wanted to energize the whole panel and try to think before I turn something on. The external transfer switch would do this since it would open/close both legs without caring if it were genuine 240 or not.

OK, well 120 into 240 is a legitimate issue. Your choices really come down to putting a transfer switch upstream of the inverter, and splitting your panels into inverter/leg1 and non-inverter/leg2, or putting in transfer switches on both sides of the inverter.

Incidentally, we've essentially done both. We have our main transfer switch upstream of the inverter -- it selects between generator and shore. If 240 is coming in, we feed the inverter and all its loads with one leg, and everything else with the other. This gives us somewhat of a balance between the legs (and you have to do this anyway, to balance your 240-volt generator). We have a second "transfer switch" downstream of the inverter, which moves several of the loads from the non-inverter side to the inverter side when 120-volt shore power is available.

Quote
I guess I could do a "combination" here and use the internal transfer switch to light one leg, put all the inverter loads along with some of the heavy loads on that leg to avoid a general imbalance paner, but, using my same faulty memory, remember not to turn the big ones on. This would have the benefit of being able to isolate the water heater, which is the most likely one to be left on by accident. Does that sound better?

Without a load-sharing inverter, I would tend to keep the water heater, air conditioners, etc. on leg2 in any case. But you have to look at your total load situation. I don't ever want the possibility that things like a water heater will accidentally be running from batteries (although I have an override system to force exactly that if I need to).

HTH,

-Sean
http://ourodyssey.blogspot.com

Lin · January 03, 2009, 04:25:29 PM

This is the diagram that I referred to. Will this work or is there some risk?

Sean · January 03, 2009, 07:10:00 PM

That diagram, from the U2512 manual, works fine, so long as you add a second set of contacts for the neutrals (only hots are shown in this drawing).

Note, however, the following:

1. This scheme involves two switches, not one (as I suggested earlier). One is upstream of the inverter, and one downstream. In your case, the first switch can be substituted with the cord-connection mechanism.

2. You will, in this instance, need to add an additional relay to effect ground-neutral bonding, unless your unit is equipped with that option. The neutral bond can not simply be hard-wired at the inverter, because the inverter is connected full-time upstream.

3. This drawing is not line-for-line compatible with the Iota switch you have. You will need to be careful in mapping the Iota wiring.

4. You need to have an intervening panel ahead of the inverter, to properly protect the inverter input.

Lastly, this method absolutely requires that the static default of the output relay will be the inverter, and the "priority" will be the cordset whenever energized.

Does that make sense?

-Sean
http://ourodyssey.blogspot.com

Lin · January 03, 2009, 09:17:56 PM

The only thing I am not sure of is #2, "an additional relay for ground-neutral bonding." The power supply system is only 3-wire--2 hots and a neutral and uses three prong 240 plugs and receptacles. So, in my case, there would be the hot from one leg of the supply going to the inverter plus a neutral. The neutral is grounded in the panel. To my understanding, this 3 prong type of system is not used anymore. Could I ground the equipment separately?

From this and others, it looks like you are getting a workout today.

belfert · January 03, 2009, 09:31:17 PM

If shore power is 120 volt, why not change the generator to 120 volt if possible?

I have 240 volt shore power, but the generator is set up for 120 volt. Powertech feeds the 120 out of the generator as two hots so I hook both hots to my 50 amp transfer switch.

Sean · January 04, 2009, 06:27:58 AM

Quote from: Lin on January 03, 2009, 09:17:56 PM
The only thing I am not sure of is #2, "an additional relay for ground-neutral bonding." The power supply system is only 3-wire--2 hots and a neutral and uses three prong 240 plugs and receptacles.

Ok, you need to fix this. There needs to be a ground wire for your shore cord; if it's 240 you need four wires, for 120 you need three.

Quote
So, in my case, there would be the hot from one leg of the supply going to the inverter plus a neutral. The neutral is grounded in the panel.

And you really need to fix this, once you add the ground for the shore. Neutral and ground in a coach should not be tied together at the panel. That connection happens at the shore outlet, in the generator, or in the inverter when it is running. If it's also in the panel, you'll be running current through your chassis, creating a hazardous condition.

Quote
To my understanding, this 3 prong type of system is not used anymore.

Actually, ungrounded connections have never been permitted on RVs since the earliest days of bus conversions.

Quote
Could I ground the equipment separately?

No, code requires a direct ground in the shore cord. Only two types of plugs are acceptable for RVs -- the TT-30P for 30-amp, 120-volt service, and the NEMA 14-50P for 50-amp, 240/120-volt service.

-Sean
http://ourodyssey.blogspot.com

Sean · January 04, 2009, 06:31:36 AM

Quote from: belfert on January 03, 2009, 09:31:17 PM
If shore power is 120 volt, why not change the generator to 120 volt if possible?

I have 240 volt shore power, but the generator is set up for 120 volt. Powertech feeds the 120 out of the generator as two hots so I hook both hots to my 50 amp transfer switch.

Brian -- this could be a dangerous situation if the generator produces more than 25 amps per leg. That's because your neutral return current, on a system like this, will be the sum of the two hot currents. Since your transfer switch, and probably some of the wiring, will be rated for only 50 amps per wire, pulling, say, 30 amps per leg will overload the neutral by 20%.

On a 240-volt system, the neutral carries the difference between the hot currents, not the sum, so the neutral can never carry more than either hot alone.

-Sean
http://ourodyssey.blogspot.com

Lin · January 04, 2009, 11:49:33 AM

Sean,

I was afraid you were going to say that. I have to get motivated to do this soon. It won't be too bad. The shore cord is already four wire with the ground on the chassis. The rest is just wire, some hardware, and time.

This question is not offered as an alternative, but as a semi-related electrical question. I once read that it would be good to also ground the bus chassis directly to the ground when parked. This could be done by having a ground cable from the chassis to a removable metal peg in the ground, or even a jumper cable from the bus to the metal water supply pipe. Is there anything to this?

Sean · January 04, 2009, 12:09:07 PM

Quote from: Lin on January 04, 2009, 11:49:33 AM
... I once read that it would be good to also ground the bus chassis directly to the ground when parked. This could be done by having a ground cable from the chassis to a removable metal peg in the ground, or even a jumper cable from the bus to the metal water supply pipe. Is there anything to this?

There is no need to do this when connected to shore power. The ground on the shore cord will provide the necessary safety mechanism. If you are concerned about a lightning strike, my strongest recommendation is to disconnect the shore power cable as well as all other cords (phone, cable TV, etc.) during an electrical storm, thus isolating the bus entirely from the ground.

If you intend to run loads outside of the bus using an on-board generator, there is some benefit to providing a "driven ground" for the generator by pounding a steel rod at least 6' into the ground. But if all the loads are entirely on-board, again the ground-neutral bond in the generator or inverter and required chassis bonding for the ground system provides sufficient protection.

-Sean
http://ourodyssey.blogspot.com