Thirty amp and fifty amp are not the same kind of number
This is the single most useful thing to understand about RV electrics, and it is almost always explained wrongly. A 30 amp RV service is one 120 volt leg protected at 30 amps. That is 30 times 120, or 3,600 volt-amps. A 50 amp RV service is not one 50 amp leg; it is two separate 120 volt legs, each protected at 50 amps. That is 50 times 120 times two, or 12,000 volt-amps.
So the step from 30 to 50 is not from 30 to 50, a factor of 1.67. It is from 3,600 to 12,000, a factor of 3.33. A coach on 50 amp service can run two air conditioners, an electric water heater and a microwave at once and still have headroom. The same coach on a 30 amp pedestal is choosing between the air conditioner and the microwave, and gets a reminder from the breaker if it fails to choose.
The practical consequence is that a 50 amp coach on a 30 amp pedestal through an adapter has not merely lost some capacity, it has lost roughly seventy percent of it, and it has lost the ability to spread loads across two legs at the same time. Adapters change the plug. They do not change the pedestal.
Balancing the two legs
On a 50 amp service the panel splits circuits between the two legs, and the breaker that trips is the one on the busier leg regardless of what the other leg is doing. It is entirely possible to trip a leg at 50 amps while the total draw across both is well under 12,000 volt-amps.
This is why the calculator shows the split rather than only a total. If the imbalance line shows several thousand watts, the fix is usually not to switch anything off but to move something. A second air conditioner on the same leg as the first is the classic case: separate them and the load fits, keep them together and it does not. Which circuits sit on which leg is set by the panel and is not something to rearrange casually, but knowing the split tells you which combinations of appliances are actually available to you.
Running watts, surge watts and the eighty percent habit
| Load | Typical running watts | Amps at 120 V |
|---|---|---|
| Roof air conditioner, 13,500 BTU | 1,300-1,700 | 11-14 |
| Microwave | 1,000-1,500 | 8-13 |
| Water heater on electric | 1,200-1,500 | 10-13 |
| Converter or charger | 200-900 | 2-8 |
| Residential fridge | 150-400 | 1-3 |
| Portable space heater on high | 1,500 | 12.5 |
| Coffee maker or toaster | 800-1,500 | 7-13 |
These are orientation figures for filling in the list; use the data plate on your own appliance where you have it. Two things then matter beyond the total. Motors draw far more current for the first second or two than they do running, which is why an air conditioner can start-trip a breaker that carries it happily once it is turning. And breakers are thermal devices not meant to be held near their rating for hours, which is the origin of the eighty percent working figure used above. A 30 amp leg carrying 28 amps continuously is not comfortable even though it has not tripped yet.
Pedestal voltage, which nobody watches
Campground distribution is often long, old and shared with dozens of other coaches all running air conditioning on the same hot afternoon. Voltage at your pedestal sags accordingly, and appliances that draw constant power draw more current when the voltage drops. Set the voltage field to 112 or 108 and watch the amp figures rise for exactly the same appliance list; that is why breakers trip in the late afternoon and not in the morning.
Sustained low voltage is worse for compressor motors than it is for breakers. An air conditioner or fridge compressor asked to start at low voltage draws heavily, heats up and can fail. This is the reason people fit voltage monitors and protective devices, and the reason it is worth glancing at a pedestal reading before plugging in rather than after something has stopped working.
Generators, in one paragraph
A generator substitutes for the pedestal and the arithmetic is identical: running watts divided by volts gives the amps available. Use the running figure from the data plate rather than the surge figure, because the surge figure exists for the two seconds an air conditioner compressor is starting and not for the hours it then runs. The non-negotiable part is placement: a generator must never run inside a vehicle, in a garage, under an awning against the coach, or anywhere near an open window, door or vent. Its exhaust carries carbon monoxide, which has no smell and no warning, and it accumulates in enclosed and semi-enclosed spaces faster than people expect. Distance and open air are the only things that work.
Related pages
Generator fuel duration is on the generator fuel runtime calculator and sizing on the generator sizing calculator. For the battery side of the same coach see the RV and van solar calculator, the off-grid load audit and the battery runtime calculator. Household branch circuits work the same way on the circuit load calculator, and cord runs on the extension cord size calculator.
Questions people ask
Can I plug a 50 amp RV into a 30 amp pedestal?
With the proper adapter the plug fits, but you have 3,600 volt-amps available on a single leg instead of 12,000 across two, which is under a third of the capacity. Both legs of your panel are fed from that one 30 amp source, so loads that would have been split now add together. In practice this means one air conditioner rather than two, and choosing between the microwave, the electric water heater and the coffee maker. Set the service to 30 amp above and put your whole appliance list in to see exactly which combinations survive it.
Why does my breaker trip in the afternoon but not the morning?
Usually voltage sag. Campground distribution is shared, and on a hot afternoon when everyone is running air conditioning the voltage at your pedestal drops. Appliances that deliver constant power compensate by drawing more current, so the same appliance list that measured 27 amps at 120 volts measures over 28 at 114. Add a compressor start on top of that and a 30 amp breaker that was fine all morning lets go. The voltage field above exists for exactly this; enter what your monitor actually reads rather than the nominal 120.
What does the 80 percent allowance mean and should I use it?
Breakers are thermal devices. They tolerate their rated current briefly but are not intended to be held near it for hours, and the conventional working figure for a sustained load is eighty percent of the rating, which is 24 amps on a 30 amp leg and 40 on a 50. Leaving the field at 80 shows headroom against that working figure; setting it to 100 shows the raw point at which the breaker would eventually open. Use 80 for anything running for hours such as air conditioning, and treat the 100 figure as information about how close to the edge you were rather than as a target.
Does it matter which leg an appliance is on?
On a 50 amp service, very much. Each leg is protected separately, so the leg carrying 45 amps trips even if the other is carrying 5. Two air conditioners on the same leg is the common case that fails while the same two on opposite legs fit comfortably. Which circuit sits on which leg is determined by the panel and is not something to change casually, but the leg column above lets you model your actual arrangement and see whether an imbalance rather than a shortage is what is stopping you.
Is this calculator a substitute for having the coach checked?
No. This adds up loads against a supply rating, which tells you what you can run at once. It says nothing about the condition of the cord, the pedestal, the connections, the panel or the bonding, and those are where the failures that matter actually happen. Overheated shore cord ends and worn pedestal receptacles are common and are a fire risk rather than a convenience problem. Anything involving how the coach is wired, protected or connected is work for a qualified electrician.