5th wheel living issues
We have a fifth wheel that we live in full-time (at least in the summers). We are running power to it from a shop my husband owns. I don't think the a.c. is big enough, it probably needs 2 a.c.s, but...
Forum Discussion
About the only way 2 AC's can be run from a 120Vac 30 amp connection is if they are both using (good) "soft starts". 120Vac 30 amps is only 3600 watts and a 15k btu roof AC will need over 2500 watts to start the compressor. That only leaves 1100 watts available, not enough to run a portable AC.
To make matters even more confusing, the 30 amp rating is the ampacity rating. This means that the actual amp rating is 24 amps or 2880 watts. The breaker will generally trip after several minutes at about 25 to 27 amps.
This is why larger RV's have 240Vac split phase 50 amp systems. These are capable of up to 12,000 watts based on 50 amps ampacity. The actual amp rating is 40 amps or 9,600 watts. This is enough to run 3 roof AC units with a small amount left over. If each AC unit draws 2000 watts while running, that is 6000 watts, leaving 3600 watts for other devices. With 3 roof AC units it is best to stagger start them so that the don't trip a breaker (or better yet use soft starts).
To make matters even more confusing, the 30 amp rating is the ampacity rating. This means that the actual amp rating is 24 amps or 2880 watts. The breaker will generally trip after several minutes at about 25 to 27 amps.
a 30 amp breaker is actually rated for 30amps continuously at 120V so 3600watts. the wire and fixtures are rated even higher but the breaker is your cut off point. for design purposes if you are going to run a load for 3 hours strait then you size it to 80% of the household circuit, that's what you might be referring to. the reason for this is eventually over time it will prematurely wear out the breaker. you see a lot of this in old houses with the 30amp breaker the stove/oven is on. newer ranges use pretty much that full capacity so if you do a lot of baking while cooking on the top eventually your breaker gets week and you start popping one leg. you can replace it with a brand new 30 amp breaker and be good for a couple years or so then replace it again, or you can pull a new 40 amp rated wire and install the 40 amp breaker and be good for the foreseeable future. if the breaker won't hold hours at a full 15 amps you have a defective or worn out breaker. but yes household design for circuits is to only load them to 80% of the capacity, but the reason is so there is room in the future to tap into runs with out having to pull new wire for the whole circuit.
AC's are not a continuous load as the compressor turns on and off as needed, only the fan is constant, so ya what could be happening is when they start up and the other one is running it is spiking over the 30 amps and tripping the breaker. then the more this happens the weaker the breaker gets and the lower and/or faster it will trip.
I have been having fun with my 1970's house and previous owners that did their own electrical with out having a clue. I think I have moved 12 circuits off the main panel onto a sub panel now and replaced 60% of the breakers. as well as replacing probably 300 feet of shoddy wiring that doesn't come close to meeting code but made it through all the inspections over the years 🤔
RV roof AC's are not very efficient and RV's are not insulated well, even "4 season" RV's. If the RV is somewhere where the ambient temp is 90+F and in direct sun it is likely that the AC compressors will run continuously for most of the day.
Ampacity is tricky. According to code it is 80% of the wire's full load capacity. Many people just look up the amp capacity and call it good, but that's not how it works. Normally the first part of the wire to fail is the insulation. The insulation will have a temperature rating and usually specify how and where it must be run. That temperature rating is based on 25C/77F ambient temperature. If the wire is in a location where the ambient temperature is higher, it's amp rating will be less than specified. Somewhere in the wire specification will be a temperature coefficient. The temperature coefficient will tell you how much to reduce the amperage per C of temperature increase above 25C.
As wires and cables tend to be in places without good airflow it is common for the ambient temperature in that location to be much higher than the ambient air temperature. This is especially true in RV's. Many RV's have most of the wire runs in the ceiling. If the air ambient temperature is 90F or 32C, those wires may be 120+F or 49C. The wire run to the air conditioner is ROMEX, which is rated for 90C/194F. An ambient temperature of 49C means that the ROMEX is derated by about 82%.
The ampacity rating has nothing to do with "so there is room in the future to tap into runs with out having to pull new wire for the whole circuit".
The ampacity rating is a safety buffer to help with wires that are not connected well, with ambient temperatures above 25C, with wires that are bent at too sharp of an angle producing localized hot spots. It's there to help prevent the wires from failing which will cause a fire. It also has nothing to do with residential, it's industry standard and national code. In the US, it's the National Electric Code or NEC.As for why the breakers trip "early", I believe that many cheaper breakers list their amp rating, not their ampacity rating. So often they trip at about 80% of the expected value. I've seen brand new 240Vac 50 amp breakers trip at about 40 amps. I haven't been to any campground that can handle the full rating for more than a couple of minutes. It doesn't matter if it's 240Vac 50 amp or 120Vac 30 amp.
I have a large solar system installed in my RV with (6) 48v 100ah batteries and I can charge them at a high rate. I've never been able to pull more than about 10,000 watts from a 240Vac 50 amp circuit, and about 3000 watts from a 120Vac 30 amp circuit. At home, I have a 240Vac 30 amp circuit breaker. It will trip at about 24 amps after about 15 minutes. If I was going to stay in the house longer, I'd replace it with a 240Vac 50 amp circuit breaker and replace the cabling to hander the higher amperage.
Ampacity is tricky. According to code it is 80% of the wire's full load capacity. Many people just look up the amp capacity and call it good, but that's not how it works. Normally the first part of the wire to fail is the insulation. The insulation will have a temperature rating and usually specify how and where it must be run. That temperature rating is based on 25C/77F ambient temperature. If the wire is in a location where the ambient temperature is higher, it's amp rating will be less than specified. Somewhere in the wire specification will be a temperature coefficient. The temperature coefficient will tell you how much to reduce the amperage per C of temperature increase above 25C.
not tricky at all, people like to make it sound tricky, but it isn't. but if you take a basic 14/2 wire it is rated for 20 amps capacity in the testing, but used for 15 amp circuits because the safety factor it gives. everything in the electrical code is geared to make a 15amp circuity carry 15 amps. a 20amp carry 20amps and a 30amp carry 30 amps and so on. like I said the only time you need to size the breaker bigger than the rating is for continuous loads at the maximum of the circuit that run for more than 3 hours, which is why the standard for kitchen outlets has moved to 20amp circuits because of all the new countertop appliances. in some places the plug even has to be split between two different 20 amp runs.
now if you want to know the real issue, the efficiency of the roof top ac has nothing to do with it, its wattage is it wattage. RV manufactures do not follow the building code, and the other issue is the building code is different in every state and or province. yes you have the national codes to set the baseline, but that's all it is, is a baseline. so where the national code might say you need 14/2 for a 15 amp 120V circuit and stuff like only 3 wires can be run through the same hole and yadeyada, rv manufactures just do what they want. do you think the types of plugs they use in RV's would ever be approved in a household application? or the way they do there wire management (or should I say lack of wire management) I am sure you get the point.
as a result of the "cheep" fixtures, connectors and probably even wire, they use (all in attempt of making the cheapest rv they can) what should be able to handle 15 amps for a minimum of 3 hours might not be able to, or as it goes over the years they wear out faster. plus nothing on a rv needs an actual trade to work on so even repairs in the rv shops are ok at best.
