Priority of 120 volt Sources

Topic: Priority of 120 volt Sources (Read 1452 times) previous topic - next topic

Priority of 120 volt Sources

July 30, 2021, 08:40:51 pm

I am stuck at a Cummins service center for the weekend. My coach is just outside one of the work bays and they allowed me to plug into a 15 amp outlet inside before they closed the bay door and left. I figured I could run my generator periodically when I needed the air conditioners, otherwise the 15 amps would keep the batteries charged and run our other meager requirements. However, I cannot get into the building to unplug my cord and it just occurred to me that shore power has priority over generator. Is this actually the case? Or, mercifully, does the generator trump the shore power?

Does anyone have a solution short of overloading their circuit and blowing their breaker? I would not want to do that for fear of causing damage or even a fire in their shop.

Richard

Re: Priority of 120 volt Sources

Reply #1 – July 30, 2021, 08:51:42 pm

On most all of the Foretravel's I have worked on if you are plugged in and start the generator then the generator is what is supplying your 120 volts. I haven't seen any that weren't that way. I guess that someone could rewire it the other way. Hope this helps.

Re: Priority of 120 volt Sources

Reply #2 – July 30, 2021, 08:57:30 pm

Easy to test.

Turn off all large loads (all loads off are even better on the points of the ATS).

Turn on the generator and let it try to supply power. After the normal delay, you MAY be able to hear the ATS "click" as it changes sources.

IF NOT:

Listen carefully as someone turns on one roof A/C. If the generator is taking the load, you should hear it's RPM change as it takes the load.

Re: Priority of 120 volt Sources

Reply #3 – July 30, 2021, 09:01:44 pm

Quote from: red tractor – July 30, 2021, 08:51:42 pm

On most all of the Foretravel's I have worked on if you are plugged in and start the generator then the generator is what is supplying your 120 volts. I haven't seen any that weren't that way. I guess that someone could rewire it the other way. Hope this helps.

There were definitely some that gave shore power priority. One needs to look at the schematics for the coach to determine which is the primary power source.

Re: Priority of 120 volt Sources

Reply #4 – July 30, 2021, 10:31:33 pm

Per Michelle's suggestion I checked the wiring diagrams that I had but could not be sure that they specifically applied to my build number. I read the owner's manual and it seemed to suggest that the generator had priority but it was not definitive.

I followed Bret's suggestion and it was obvious that the generator never took over control of the A/C power system. So, at least on my coach, the shore power line has priority over the generator.

So, even if I had large liFePo batteries, lots of solar panels and soft start air conditioners as well as a 10kw generator, as long as I am plugged into a 15 amp outlet I can omly run one air conditioner or the battery charger or the 120 volt heater coil on the AquaHot but not much else.

BUMMER.

I should not have plugged in at all. But such is life.

Richard

Re: Priority of 120 volt Sources

Reply #5 – July 30, 2021, 10:37:41 pm

You could check the exterior doors for a "In Case Of Emergency" sign that lists name and number to call and see if they can come out and let you unplug.

Re: Priority of 120 volt Sources

Reply #6 – July 30, 2021, 10:47:49 pm

Just another example of a situation where a simple manual switch would be superior to the ATS. On our coach, I can have shore power connected (any amperage) and the generator running at the same time. My manual switch allows me to select either source of AC power as desired, but will NEVER allow both sources to be connected simultaneously.

Simple...convenient....and even more importantly, SAFE.

Transfer switch questions

Re: Priority of 120 volt Sources

Reply #7 – July 30, 2021, 11:36:29 pm

Quote from: master2301 – July 30, 2021, 10:37:41 pm

You could check the exterior doors for a "In Case Of Emergency" sign that lists name and number to call and see if they can come out and let you unplug.

In fact there is such a number posted and also I made sure I had a contact in case of trouble. However, so far I haven't felt that this is something I wish to bother someone about. If someone shows up over the weekend to work, I will ask to unplug.

Richard

Re: Priority of 120 volt Sources

Reply #8 – July 31, 2021, 09:17:14 am

If you fire up your AC units, wouldn't they blow the breaker on the 15 amp line? Then run your generator.

Re: Priority of 120 volt Sources

Reply #9 – July 31, 2021, 09:23:31 am

Am I missing something here?
On our coach, all I would have to do would be to unplug the power cord from the plug in inlet in the engine compartment.
Isn't there a plug in in their coach?
What about a disconnect from power input in the power cord compartment?

Re: Priority of 120 volt Sources

Reply #10 – July 31, 2021, 09:28:09 am

When you have a shore power cord reel (as I assume the OP has) the cord is generally hard wired into the electrical system. No easy way to "unplug" the cord at the coach end.

Re: Priority of 120 volt Sources

Reply #11 – July 31, 2021, 09:39:24 am

Sorry, never liked the word "ass-u-me"

Re: Priority of 120 volt Sources

Reply #12 – July 31, 2021, 10:29:02 am

Quote from: Want2rv – July 31, 2021, 09:17:14 am

If you fire up your AC units, wouldn't they blow the breaker on the 15 amp line? Then run your generator.

It MIGHT trip the breaker. But in a shop environment, it may be well larger wire and breaker to handle some of their high-amp tools. It might melt your adapter, might take voltage so low that you damage your roof A/C units, etc.

If shore power was off (may not help this time), another option if you want generator to have priority, would be to reverse the wiring on the ATS IN side-- wire generator where shore power is and visa versa.

You should be able to run ONE roof A/C on 15 amps if nothing else is on. Monitor voltage, but I would sure try it.

Re: Priority of 120 volt Sources

Reply #13 – July 31, 2021, 10:42:16 am

As Chuck said, I have a cord reel which is hard wired into the coach electrical system. The cord reel is very convenient but does not allow me to disconnect at the coach. At the moment the cord runs under the garage type door and is plugged into an outlet inside the locked shop building. I do not wish to blow the circuit breaker in the shop, so I will just deal with it. Thankfully the next day or two are predicted to be cooler.

And I have now learned a valuable lesson that I had never thought about before. Live and learn.

Richard

Re: Priority of 120 volt Sources

Reply #14 – July 31, 2021, 10:51:07 am

Thanks Bret. I agree about intentionally trying to trip the shop breaker. I did run one A/C last evening. I made sure that I had no other large loads turned on. The voltage did drop from the 116 range down to 110 volts but it was steady there.

Re: Priority of 120 volt Sources

Reply #15 – July 31, 2021, 10:54:25 am

Quote from: Jan & Richard – July 31, 2021, 10:42:16 am

And I have now learned a valuable lesson that I had never thought about before. Live and learn.

To avoid this scenario in the future, as Brett suggested above, it is a fairly simple matter to change your shore/generator ATS priority. You simply need to swap the two input leads to the ATS (with all AC power sources turned OFF, of course). Once that is done, the ATS will default to GEN.

NOTE: See Reply #24 below before making the changes suggested in this post!

Re: Priority of 120 volt Sources

Reply #16 – July 31, 2021, 12:10:23 pm

Quote from: Chuck & Jeannie – July 31, 2021, 10:54:25 am

To avoid this scenario in the future, as Brett suggested above, it is a fairly simple matter to change your shore/generator ATS priority. You simply need to swap the two input leads to the ATS (with all AC power sources turned OFF, of course). Once that is done, the ATS will default to GEN.

Thanks Chuck.

I may make this change after I can disconnect the power. I will have to ponder whether I want shore power priority (as it is now) or generator priority. Either way, I will make a notation in my Foretravel manual as to which way it is wired. That may help some future owner.

Richard

Re: Priority of 120 volt Sources

Reply #17 – July 31, 2021, 07:43:08 pm

Someone must have rewired yours if the shore power is priority. I have worked on many motorhomes and can't remember ever having one where shore power was priority

Re: Priority of 120 volt Sources

Reply #18 – July 31, 2021, 07:46:43 pm

Quote from: red tractor – July 31, 2021, 07:43:08 pm

Someone must have rewired yours if the shore power is priority. I have worked on many motorhomes and can't remember ever having one where shore power was priority

Ron,

There were a couple late 2002/early 2003 IIRC that were wired shore power as priority. There was some discussion of this years ago back on the old Yahoo group. For some reason, I'm thinking Barry B's coach was one of them.

Re: Priority of 120 volt Sources

Reply #19 – July 31, 2021, 07:52:13 pm

Looks like 1994-2002 was primarily shore-priority

Run Generator While on 30 Amp Shore Power

110 volt help needed. I'm stumped. 2002 U270

Shore Power Issues

Transfer switch Question

Transfer Switch Issues

What did you do to your coach today VIII

50 Amp Cable on Reel

Do it yerself

Re: Priority of 120 volt Sources

Reply #20 – August 01, 2021, 12:05:03 pm

Quote from: Jan & Richard – July 30, 2021, 08:40:51 pm

However, I cannot get into the building to unplug my cord and it just occurred to me that shore power has priority over generator. Is this actually the case? Or, mercifully, does the generator trump the shore power?

Richard

The whole purpose of the automatic transfer switch is to keep you from trying to power the Cummins shop with your generator. When the generator comes on the shore power is automatically disconnected from the coach at the ATS and transferred to the generator.

Another way to state this is the shore power contacts are normally closed, and when the ATS is energized by the generator the normally open generator contacts close* leaving the shore power contacts open. It's an OR switch.

*After a suitable time delay to allow the generator to stabilize before assuming the load.

Re: Priority of 120 volt Sources

Reply #21 – August 01, 2021, 02:33:37 pm

Old Toolmaker suggested in his post that the ATS provides a 'delay' function that, after detecting voltage on the generator inputs, waits for a short period of time before switching from shore to generator power so that the generator has stabilized and is ready to assume the electrical load. My question is ... does that delay function work on both the shore and generator inputs?

Several comments have suggested that you can just reverse the shore power and the generator inputs to the ATS and that will switch which source has priority. That makes sense. However, if the delay function only works on one ATS input, and if you switched the ATS inputs on a coach that was originally wired for shore priority, the generator would now have priority, but it would also pick up the load the instant the ATS saw voltage from the generator inputs. You might lose the delay function when switching to generator power.

Re: Priority of 120 volt Sources

Reply #22 – August 01, 2021, 02:44:14 pm

Very good point from Eric, and one which I did not consider when I offered my (unqualified) opinion about swapping input leads.

See the first check mark under "features" in the product sheet linked below. On this particular ATS it would NOT be advisable to swap input cables.

I don't know if every ATS includes this "time delay" feature, but certainly something to ascertain before making any wiring changes.

PD52 240 VAC 50-Amp Automatic Transfer Switch

https://www.progressivedyn.com/wp-content/uploads/Support/manuals/PD52-PD52DCSD-PD53-Manual-english.pdf

https://www.youtube.com/watch?v=WGSFBlB75DA

Re: Priority of 120 volt Sources

Reply #23 – August 01, 2021, 05:28:50 pm

Unless someone specifically reconfigured your transfer switch, they all (for all rv's and boats) are wired to default generator priority. I fire up my generator with a 15amp ext cord plugged in all the time when I'm doing periodic driveway warm ups...it's important to force these systems to exercise if you want them to work..

Re: Priority of 120 volt Sources

Reply #24 – August 02, 2021, 09:39:25 am

Quote from: EricFromPA – August 01, 2021, 02:33:37 pm

Old Toolmaker suggested in his post that the ATS provides a 'delay' function that, after detecting voltage on the generator inputs, waits for a short period of time before switching from shore to generator power so that the generator has stabilized and is ready to assume the electrical load. (A) My question is ... does that delay function work on both the shore and generator inputs?

Several comments have suggested that you can just reverse the shore power and the generator inputs to the ATS and that will switch which source has priority. That makes sense. However, if the delay function only works on one ATS input, and if you switched the ATS inputs on a coach that was originally wired for shore priority, the generator would now have priority, (B) but it would also pick up the load the instant the ATS saw voltage from the generator inputs. You might lose the delay function when switching to generator power.

(A) No. Normally closed contacts id est shore power are the un-energized state, so you plug it in, you got your power.

(B) Qualified No it won't. If you guys swap the shore and generator inputs at the ATS, you will have instant power from the NC contacts on the ATS, but only when the coil isn't being energized by the shore power line. That being said, what makes more sense:
(a) Energizing the coil in the ATS only when using the generator as a source of power, or
(b) Keeping the coil in the ATS energized for all of the time that shore power is hot?

I'd rather not burn up the coil before I must. When off-grid in the winter I run my generator 2 hours in the morning, 1 hour for lunch and 1 hour for supper giving me 4 hours of charging time - enough time to bring flat batteries up to 80% state of charge. And I try not to go below 50% state of charge.

Affiliate Notice