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Old 11-21-2013, 07:40 PM   #11
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Originally Posted by clubhouse View Post
Thanks. You first paragraph is what I always understood, and that is what got me thinking about the OPs original question and wondered if that temporary start up current would require heavier guage wire.
Good observation. Startup is one of those times when you don't want the voltage dipping down. A lower voltage slows the rotor coming up to speed. Depending upon the switching methods used for the start windings that can stress the contacts, or components unnecessarily.

The OP indicated that this is an additional A/C unit. I a$$-u-me the trailer has an existing 30 amp supply, so that indicates a separate supply for the second A/C unit is necessary. So-o-o, that may mean often coming from a 20 amp receptacle.

The OEM A/C unit is wired from the circuit breaker box in the trailer. That circuit breaker box is supplied by a 30 amp cord. That larger wire feed means not much voltage drop at the trailer circuit breaker panel. The branch circuit from the circuit box to the OEM A/C unit is relatively short so wire size is less critical. This additional A/C unit will have a supply which will likely be derived from a 20 amp circuit and I a$$-u-me a "normal" extension cord. That's a different situation.

One needs to be careful using generic circuit calculators. In the calculator cited, the instructions mention "We allow a maximum voltage drop of about 3% before the wire specification increases.". The voltage supplied in a campground could be more drop than that with no problem. It also just asks for current. Assuming 14 amp running current doesn't really give all the conditions including cycling load.

My experience is that regularly cycling loads like air conditioners, sump pumps, etc. can be very hard on branch circuits. Many of the <few> branch circuit breakers and connections which I have found failed have been in that service.

#12 AWG wire is likely just fine, even though a larger size for short distance is not much additional cost. Sorry to butt in. vic
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Old 11-21-2013, 08:15 PM   #12
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12 gauge romex

My plan is to use another 30 amp power cord to connect to the pedestal. The power will go into a breaker box that will have two 20 amp breakers. One feeding the new a/c and another to feed a another outlet to give a little more power in the trailer for space heater or whatever we need. I would plug the original trailer plug into the 50 amp plug and the aux a/c to the 30 amp plug leaving me with the 20 amp plug on the pedestal for any thing on the outside. Does this sound okay. I checked with the rv park electrician and he said that the reciprocals are all wired separate with 3 different wires going into pedestal. My brother is doing this same thing and has had no problems.
Thanks
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Old 11-21-2013, 08:23 PM   #13
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... Does this sound okay. ...Thanks
That is a good plan. Given those details a #12 AWG branch to the A/C is more than enough. vic
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Old 11-21-2013, 08:31 PM   #14
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My plan is to use another 30 amp power cord to connect to the pedestal. The power will go into a breaker box that will have two 20 amp breakers. One feeding the new a/c and another to feed a another outlet to give a little more power in the trailer for space heater or whatever we need. I would plug the original trailer plug into the 50 amp plug and the aux a/c to the 30 amp plug leaving me with the 20 amp plug on the pedestal for any thing on the outside. Does this sound okay. I checked with the rv park electrician and he said that the reciprocals are all wired separate with 3 different wires going into pedestal. My brother is doing this same thing and has had no problems.
Thanks
Is your normal TT plug 50amp or 30amp?

If it's 30amp, then I would suggest this.
If you're going to supply a breaker box with 2-20amp breakers, than your supply cord has to be rated to carry 40amps.
(Probably just as easy to use a 50amp factory style cord and plug.)

I would connect my new breaker box with 2-20amp breakers to the 50amp pedestal outlet using a cord rated to at least 40amps.

Then I would plug my 30amp factory cord to the 30amp pedestal outlet.

Best regards,
Doug
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Old 11-21-2013, 08:49 PM   #15
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Sorry to butt in again.
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Originally Posted by Doug View Post
...
If it's 30amp, then I would suggest this.
If you're going to supply a breaker box with 2-20amp breakers, than your supply cord has to be rated to carry 40amps.
(Probably just as easy to use a 50amp factory style cord and plug.)

I would connect my new breaker box with 2-20amp breakers to the 50amp pedestal outlet using a cord rated to at least 40amps.

...Best regards,
Doug
There should be no problem with feeding the 2 ea. 20 amp circuits with a 30 amp cord. If it is plugged into a 30 amp receptacle then the total current is limited to 30 amps by the pedestal breaker. The individual 20 amp breakers protect the branch circuit #12 AWG. The A/C unit described and an occasional electric heater should never exceed the 30 amp cord.

As to plugging into a 50 amp receptacle with a 50 amp to 30 amp adapter. That is done all the time isn't it? The connected loads within the trailer may exceed the 30 amp cord for short bursts, but the trailer circuit breaker panel and connected loads are sized for the 30 amp cord feed. The branch circuit breakers are sized to protect the branch conductor.

A 50 amp cord and service would add expense and be much more difficult to deploy and store.

Just my thinking. vic
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Old 11-21-2013, 08:58 PM   #16
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Quote:
Originally Posted by Doug View Post
Is your normal TT plug 50amp or 30amp?

If it's 30amp, then I would suggest this.
If you're going to supply a breaker box with 2-20amp breakers, than your supply cord has to be rated to carry 40amps.
(Probably just as easy to use a 50amp factory style cord and plug.)

I would connect my new breaker box with 2-20amp breakers to the 50amp pedestal outlet using a cord rated to at least 40amps.

Then I would plug my 30amp factory cord to the 30amp pedestal outlet.

Best regards,
Doug
30 amp is what my trailer uses.
Doug so with what you said then i should buy a 50 amp power cord? How would that wire into breaker box? Would i use both hot legs or just one?
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Old 11-21-2013, 09:02 PM   #17
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Originally Posted by VicS1950 View Post
Sorry to butt in again.

There should be no problem with feeding the 2 ea. 20 amp circuits with a 30 amp cord. If it is plugged into a 30 amp receptacle then the total current is limited to 30 amps by the pedestal breaker. The individual 20 amp breakers protect the branch circuit #12 AWG. The A/C unit described and an occasional electric heater should never exceed the 30 amp cord.

As to plugging into a 50 amp receptacle with a 50 amp to 30 amp adapter. That is done all the time isn't it? The connected loads within the trailer may exceed the 30 amp cord for short bursts, but the trailer circuit breaker panel and connected loads are sized for the 30 amp cord feed. The branch circuit breakers are sized to protect the branch conductor.

A 50 amp cord and service would add expense and be much more difficult to deploy and store.

Just my thinking. vic
That's true Vic if the 30amp breaker doesn't malfunction.
I am just always used to sizing things based on the potential.
Two 20 amp breakers have the potential to draw a combined 40 amps, so I always size everything upstream accordingly.

It definitely could work as you say. I guess the 50amp wouldn't protect the 2-20amps either. You'd be relying on them to not malfunction.

Just be sure to check the functionality of your breakers. I guess that's something we should always keep am eye on.


Best regards,
Doug
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Old 11-21-2013, 09:24 PM   #18
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Originally Posted by lx22f/c View Post
30 amp is what my trailer uses.
Doug so with what you said then i should buy a 50 amp power cord? How would that wire into breaker box? Would i use both hot legs or just one?
After our discussion, I have to agree with your 1st option. Feeding your breaker box with 2-20 amp breakers with the 30 amp cord and outlet. I apologize for not looking that diagram up to confirm my thinking.
I forgot the 50amp cord is going to bring in 240 volts. Sorry, I just wired a 30 amp plug for my sister and was thinking it was 120 volts.

It can be done, but I think you were on the right track.

Here's an article on how it could be done, but I would just stick to where you were.

Each leg would have to be split off like a house breaker box to supply each breaker with 120V.
I guess this is how they do it in big rigs with 240V appliances.

Quote from article:
There is a lot of misunderstanding about how MOST 50 Amp RV's are wired. Some of the larger bus type diesel pushers are all electric and in fact, do have 250 volt appliances in them. In that case, both sides of the 50 amp circuit are used to power those devices (just like your home) and those coaches MUST have 50 amp, 250V service in order to run things like the 250V stove, AC and the electric water heater. In the case of the rest of the 50 amp RV's, 250 volts is still supplied to the coach just like your home electrical service but typically one side of the line is used for all the 125 volt appliances except the rear AC and the other side is used for the rear AC only. The service coming into the RV is still 50 amp, 250 volts. Unquote:

Your good to go like you said. Just check your breakers periodically.

Best regards,
Doug
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Old 11-21-2013, 09:37 PM   #19
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Thanks Doug i feel better leaving it on a 30 amp plug. Just wanting to get it right and not endanger my family.
Thanks again for all replies. This forum is great!!!
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Old 11-22-2013, 07:43 AM   #20
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Heavier gauge wire is not required because of startup. LRA is a lot more than operating amperage, but it occurs for a very brief time, usually less than a second and is irrelevant in sizing wire for a given load. The wire length and size coming from the pedestal is also irrelevant as long as it is sized correctly and there's no need to take that into consideration either. If that's not sized right, you are going to get voltage drop regardless of what you use for the AC.

That said, use #12 wire. The rated amp draw of the AC is based on an ambient operating temperature of 95 degrees. That amperage will increase at a rate of one amp per 10 degree shift in ambient temperature. In fact that's how you check an AC for proper charge without tapping into the system, by measuring amperage and checking amp draw relative to ambient temp. As an example a unit that draws 14.5A in 95 degrees will draw15.5A in 105 temperatures, so you would exceed the capacity of the 15A breaker. When I do my marine installations, I use #12 wire and a 20A breaker for all but the smallest units Along with factoring in the possible changes in ambient temperature, you also have to realize other factors that will affect current draw, condition of bearings and components at the unit ages, dirty filters, lower than normal CG pedestal voltages, so bottom line, go with #12 wire and protect it with a 20A breaker. You can use #10, as no one ever got in trouble using larger wire, but it's more expensive, harder to work with, and just isn't needed.
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