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Old 06-03-2013, 07:18 PM   #1
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Energy Management System Versus Two Air Conditioners

This is probably a pretty basic question - but it is driving me crazy and I can not really find an answer posted on line.

50 Amp Service: I get that 50 amp service is actually two separate 120 volt supplies that (ultimately) end up feeding separate circuits on the coach: for example one circuit supplies one air conditioner and the other circuit supplies the other air conditioner (though I am not sure exactly additional loads are divided...).

If I am parked at a site with only 30 amp service, and I use my pigtail to adapt my 50 amp plug to the 30 amp service, then my guess is that only one of the circuits from the "50 amp" supply gets passed in to the transfer switch (and ultimately in to the coach). Some previous discussions had led me to think that when so hooked-up the Energy Management System would allow "toggling" back and forth between running of the two different air conditioners. However, in looking at the wiring diagrams for the transfer switch and the energy management systems (precision Circuits EMS in my case) the separation between the "two circuits" look pretty hard-wired and set in stone with no ability to "toggle" a single supply leg back and forth to the two separate air conditioners.

Have I been running with a mis-understanding, and the truth is that when adapting a 50-amp house to a 30-amp pedastal supply, only a single one of the air conditioners can ever be run?

(A side question: is there a standard for how the two circuits for the 50 amp supply are split? when so plugged in to a 30 amp supply, my refrigerator (residential electric) runs, as do all of the internal outlets and televisions. I have not tried the electric element on the water heater though.....)

Thanks!
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Old 06-03-2013, 07:54 PM   #2
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I could certainly be wrong on this but I don't think that your two AC units are split between the two 50 amp 120v legs into your coach. You probably know that each of those legs are actually 50amps for a total of 100 amps available when connected to a 50 amp service.

However, 30 amps usually won't supply enough power to run both ACs at the same time. I've read where some folks pull it off but I don't think it's that common because you actually have only 30% of the power available from a 50 amp connection.

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Old 06-03-2013, 08:13 PM   #3
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Just to add to the puzzle, when I use 50/30 and 30/20 adapters and plug into a 20 amp outlet at my house to keep the batteries up, I get a amp reading on both legs PI MS-LCHW50C. Based on this, it would appear like 50/30 adapter parallels the two 50 amp legs so that you are feeding both legs when using a 50/30 adapter.
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Old 06-03-2013, 09:04 PM   #4
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The EMS in my coach does take the 30 amp circuit and splits it to both sides of the electrical. When on 50 amp each a/c is on different legs, one on L1 the other on L2, I can see the current draw on my pannel as they turn on. When on 30 amps, the EMS still shows voltage on both legs and both a/c's run fine at around 13 amps each, one on L1 and the other on L2. When I turn on the water heater or washing machine, the rear a/c is cycled off automaticly so I don't exceed 30 amps. Works great for me.
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Old 06-03-2013, 10:42 PM   #5
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Splitting at the pigtail makes some sense, and solves part of my mystery. However, from what I have read the EMS determines that it is seeing a 30 amp circuit if it sees power in only the L1 leg. If the 30 amp circuit is actually being used to energize both the L1 and L2 legs, then just how does the EMS auto detect that it is on a 30 amp circuit????????
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Old 06-03-2013, 11:01 PM   #6
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My EMS (Intellitec’s Smart Energy Management System - 50Amp) will auto detect detect whether it is 50 or 30 amp circuit but I must manually set it when on a 20 amp circuit. My guess is that if it reads input on both legs it defaults to 50 amps other wise it switches to 30.
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Old 06-03-2013, 11:03 PM   #7
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The 30 - 50 amp Dogbone takes the 30 amp 120 VAC load and distributes to both L-1 & L-2 of your 50 amp shore power cord and then takes the neutral and ground to their appropriate locations.

Now you have ONLY 3600 watts of power to use in your coach., 120 x 30 = 3600.

Your EMS system will now decide how you can use that power. I am not familiar with your brand of EMS, Precision Circuits EMS. My system is the Intellitec EMS 750 and it has a specific shedding order when the 3600 watts of power is getting close to being maxed out.

My shedding order is, Hot Water Heater, Rear A/C, Washer/Dryer, Front A/C.

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Old 06-04-2013, 08:17 AM   #8
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Quote:
Originally Posted by MN_Traveler View Post

Splitting at the pigtail makes some sense, and solves part of my mystery. However, from what I have read the EMS determines that it is seeing a 30 amp circuit if it sees power in only the L1 leg. If the 30 amp circuit is actually being used to energize both the L1 and L2 legs, then just how does the EMS auto detect that it is on a 30 amp circuit????????
The EMS system has a current sensor on the neutral of both L-1 and L-2.

See diagram from manual Page 15.

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Attached Files
File Type: doc EMS Page 15.doc (219.0 KB, 54 views)
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Old 06-04-2013, 06:13 PM   #9
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Another manual on line indicates that the unit senses the presence of 30 A service when it detects "no voltage difference between the L1 and L2 legs". This seems consistent with the presence of the current sensor, and the discussion above that the 30A adaptor feeds both the L1 and L2 legs (but hence both are in-phase and there is no voltage difference between them). Cool: this discussion has helped a lot: thank you all for helping to clear up my mystery!
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Old 06-04-2013, 06:34 PM   #10
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And now lets add this one to the mix. If you are running both AC's and other items so your current is right up against the 30 amp max for three hours it will load shed to 24 amps so my rear AC will come offline at that time.

So while on 30 amps I keep the refer on propane and the HWH too. I also turn the charger way down to try and keep two ac's running.

Just learned this trick after 5 years. Putting my AC fans on High versus the Auto mode really helps. The fans never kick off even when the compressors cycle. This gets the maximum cooling available and the extra air flow really helps. Yeah they are not quiet but I don't have to hear the wife complain when it is 100 outside. It is going to set a new record here in sunny Northern Calif this week. Supposed to get to 110 earlier that it ever has. Glad I just built a 50 amp extension cord. The 30 amps just does not cut it when it gets that hot.
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Old 06-04-2013, 06:54 PM   #11
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...It is going to set a new record here in sunny Northern Calif this week. Supposed to get to 110 earlier that it ever has. Glad I just built a 50 amp extension cord. The 30 amps just does not cut it when it gets that hot.
Maybe send some of that heat this way: in MN we have been in the 60's recently
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Old 06-05-2013, 05:10 PM   #12
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30A vs 50A is decided based on the voltage between L1 & L2. In a standard 50A supply, the voltage difference will be 240v; in a 30A connection it will be zero volts because both L1 and L2 are fed from the single hot line of the 30A outlet.

Most EMS also have a signal line that will force the EMS to NOT do any power management. This override signal is used for generator models that produce more than 30A but do not show 240v across L1 & L2, e.g. the popular Onan 7500 and 8000 models.
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