RV Shore Power Calculator
Work out what a 30 or 50 amp RV shore power connection can actually run at once, why 50 amp gives more than three times the capacity, and how to balance the two legs.
How to use this calculator
- 1Pick the connection you actually have at the site, not the one your RV is wired for. A 50 A rig on a 30 A pedestal has 3,600 VA, and an adapter does not change that.
- 2Measure the pedestal voltage if you can — a surge protector with a display makes this easy. It sags at busy parks and directly reduces capacity.
- 3Tick only what will genuinely run at once, then look at the headroom. Filling it leaves nothing for a compressor restart.
- 4On a 50 A service, check the heavy-leg figure rather than only the total. Half your capacity being on the wrong leg is the most common cause of a trip that makes no sense.
- 5Remember the propane option: switching the water heater and fridge off electric frees around 1,800 W, which is half a 30 A service.
How the calculation works
Capacity per leg (VA) = service amps x supply voltage x 0.8 continuous derate
Total capacity = capacity per leg x number of 120 V legs
30 A service: 1 leg — 30 x 120 = 3,600 VA
50 A service: 2 legs — 50 x 120 x 2 = 12,000 VA
Heavy leg load = total watts x leg balance share
Fits if total ≤ total capacity AND heavy leg ≤ per-leg capacity- Legs
- How many separate 120 V circuits the connection provides. TT-30 has one; 14-50 has two, which is the whole reason for the capacity difference
- 0.8
- The continuous-load derate. A breaker is not rated to carry its full nameplate indefinitely, and a load above 80% will eventually trip it
- Leg balance
- How the RV’s circuits divide between the two legs of a 50 A service. Each leg has its own breaker, so an unbalanced rig trips one while the other idles
- Supply voltage
- What the pedestal actually delivers, which sags under load. Capacity in volt-amperes falls with it
The 30-to-50 difference is the single most misunderstood thing about RV electrical systems. Both figures describe amps per leg; the plugs differ in how many legs they provide. Fifty amps at 120 volts on two legs is 12,000 VA against 3,600 — a factor of 3.3, not 1.67.
On a 50 A service the two legs are genuinely independent, each with its own breaker in the RV’s panel. Total capacity is only reachable if the load divides evenly, and RVs are rarely wired to divide evenly. An air conditioner on each leg is the sensible arrangement; both on one leg is a design fault that shows up as a tripping breaker on the hottest day.
Voltage matters as much as amperage. Capacity in volt-amperes is amps times volts, so a pedestal sagging to 105 V has lost 12% of its capacity — and low voltage is itself hazardous to motor loads, which draw more current to compensate and overheat.
Worked example
One air conditioner, water heater, fridge, converter and electronics on a 30 A service
- 1.A 30 A TT-30 is a single 120 V leg. At the measured 118 V that is 30 x 118 = 3,540 VA.
- 2.Applying the 80% continuous derate: 2,832 VA usable.
- 3.Selected load: air conditioner 1,500 + water heater 1,400 + fridge 400 + converter 500 + electronics 250 = 4,050 W.
- 4.That is 34.3 A at 118 V — well over the service.
- 5.It exceeds the derated capacity by 1,218 W, so the breaker will trip.
- 6.Switching the water heater to propane frees 1,400 W, bringing the load to 2,650 W and inside the limit with 182 W to spare.
- 7.A 50 A service would give 11,800 VA before derate — three and a third times the 30 A figure — and would run all of it plus a second air conditioner.
Result: Exceeds a 30 A service by 1,218 W
Why 50 amp is more than three times 30 amp
This is the single most common misunderstanding in RV electrics, and it comes from the plugs being labelled in a way that invites the wrong comparison. Fifty is 67% more than thirty, so 50 amp service ought to be 67% more capacity. It is not; it is 233% more.
The reason is the number of circuits. A 30 amp RV connection uses a TT-30 receptacle with three prongs: one hot, one neutral, one ground. That is a single 120 volt circuit rated at 30 amps, giving 30 × 120 = 3,600 volt-amperes.
A 50 amp RV connection uses a 14-50 receptacle with four prongs: two hots, one neutral, one ground. The two hots come from opposite legs of a split-phase supply, so the RV receives two entirely separate 120 volt circuits, each rated at 50 amps. That is 50 × 120 × 2 = 12,000 volt-amperes.
The 14-50 also provides 240 volts between the two hot legs, which is how the same receptacle serves an electric range or a car charger. RVs almost never use the 240 volt capability — they treat it as two independent 120 volt supplies — but the receptacle is the same one.
This is why the practical difference is so stark. A 30 amp rig runs one air conditioner and manages everything else around it. A 50 amp rig runs two air conditioners, an electric water heater, a residential fridge and a microwave without anyone thinking about it.
The two legs are separate, and that catches people out
Twelve thousand volt-amperes on a 50 amp service is not a single pool that any appliance can draw from. Each leg has its own 50 amp breaker in the RV’s distribution panel, and each is limited to 6,000 volt-amperes independently.
An RV whose circuits are unevenly divided will therefore trip one breaker while the other sits half idle. The classic case is both roof air conditioners wired to the same leg: 3,000 watts on one side, a few hundred on the other, and the loaded leg struggling on the day both units are needed.
Well-designed rigs split the heavy loads deliberately — one air conditioner per leg, the water heater on one side and the microwave on the other. Whether yours does is worth knowing, and it is discoverable: turn off one leg’s main breaker and see what stops working.
This also explains a trip that otherwise makes no sense — a rig drawing 5,000 watts total on a 12,000 volt-ampere service, and still tripping. The total was never the constraint; the balance was.
Voltage is the other half of the problem
Capacity is amps times volts, so everything above assumes the pedestal actually delivers 120 volts. Frequently it does not.
Campground distribution is often marginal, particularly at older parks that were wired when RVs drew a fraction of what they do now. On a hot afternoon with every site occupied and every air conditioner running, pedestal voltage can sag into the low 100s. At 105 volts, a 30 amp service delivers 3,150 volt-amperes rather than 3,600 — 12% of the capacity simply gone.
Worse, low voltage is actively dangerous to motor loads. An air conditioner compressor tries to maintain torque as voltage falls, and does so by drawing more current, which heats the windings. Sustained operation below about 108 volts is a recognised cause of compressor failure, and it is an expensive one.
This is the real argument for an RV surge protector — not the surges, which are rare, but the low-voltage and open-neutral protection, which disconnects the rig before the damage is done. A unit with a voltage display also lets you watch the pedestal sag and shed load before the protection has to act.
Managing a 30 amp service
Thirty amps is 3,600 volt-amperes, or about 2,880 after the continuous derate. An air conditioner takes half of that, which leaves enough for lights, electronics and one more moderate appliance — and not much else.
- 1Switch the water heater to propane — the single most effective move, freeing around 1,400 W instantly. Most RV water heaters do both, and the propane consumption is trivial next to the electrical freedom.
- 2Run the fridge on propane too — an absorption fridge uses very little gas and frees several hundred watts. Between the two, propane recovers half the service.
- 3Sequence the heavy loads — the kettle, the microwave, the hair dryer and the toaster are each 1,200 to 1,500 W and each is brief. Using them one at a time, rather than while someone else is making coffee, is the whole discipline.
- 4Watch the converter — a battery charger works hardest immediately after you arrive with a depleted battery — which is also when the air conditioner goes on and everyone plugs in. It is the hidden load in most first-evening trips.
- 5Consider a soft start on the air conditioner — it does not reduce running watts, but it cuts the startup surge dramatically, which is what trips a service that is already near its limit. It also makes running the air conditioner from a modest generator possible.
Adapters, dogbones and what they cannot do
Adapters between RV connections are everywhere, and they are widely misunderstood. An adapter changes the shape of the plug. It does not change the capacity of the supply.
A 50 amp rig plugged into a 30 amp pedestal through a dogbone adapter receives 3,600 volt-amperes, not 12,000. Both of the RV’s 120 volt legs are fed from the same single pedestal leg, so the two 50 amp breakers in the RV panel are now sharing one 30 amp supply upstream. The RV’s own breakers will not protect you here, because they are each rated well above what the pedestal can give — the pedestal breaker trips instead.
The reverse is harmless: a 30 amp rig on a 50 amp pedestal is limited by its own 30 amp breaker and simply uses part of what is available.
The dangerous case is the household adapter. Running a 30 amp rig from a 15 or 20 amp domestic socket through an adapter gives 1,800 or 2,400 volt-amperes, which is not enough for an air conditioner and a great deal else. People compensate with extension leads, and a long, thin, coiled extension lead carrying 15 amps continuously is a genuine fire risk — it is one of the more common causes of RV electrical incidents at home rather than at a campground.
What this assumes, and where it stops
Assumptions
- Capacity is amps times the measured supply voltage, times the number of 120 V legs, with an optional 80% continuous derate.
- Appliance figures are running watts. Startup surges, which are what actually trip breakers, are not modelled.
- On a 50 A service the load divides between the two legs at the balance percentage entered.
- Power factor is treated as 1, so watts and volt-amperes are used interchangeably. Motor loads have a power factor below 1 and draw slightly more volt-amperes than watts.
- The RV’s internal breakers and wiring are assumed adequate for the service it is designed for.
Limitations
- Startup surge is not included, and it is the most common cause of a trip. An air conditioner compressor draws several times its running current for a fraction of a second unless fitted with a soft start.
- Power factor is ignored. Motors and some electronics draw more volt-amperes than watts, so a real load is slightly higher than the watt figure implies.
- Leg balance is entered rather than derived. Which circuits sit on which leg is a property of your specific RV’s panel wiring.
- Pedestal voltage varies through the day and with park occupancy. A single figure cannot capture the afternoon sag that causes most problems.
- Nothing here assesses the pedestal’s own wiring, the campground’s distribution, or whether the ground and neutral are correctly connected — all of which a proper RV surge protector checks before it connects.
Common questions
Is 50 amp RV service really more than three times 30 amp?
Yes. A 30 amp TT-30 connection is a single 120 volt leg, giving 3,600 volt-amperes. A 50 amp 14-50 connection is two independent 120 volt legs of 50 amps each, giving 12,000. That is 3.3 times the capacity, not the 1.67 the amp numbers suggest, because the plugs differ in how many circuits they provide.
Can I run two air conditioners on a 30 amp service?
No. Two rooftop units draw around 3,000 watts running, which is more than the 2,880 usable volt-amperes a 30 amp service provides after the continuous derate — and that is before the fridge, converter or anything else. Two air conditioners is the specific reason 50 amp service exists.
Why does my breaker trip when the total load seems fine?
On a 50 amp service, almost always leg imbalance. The two legs are separate breakers with 6,000 volt-amperes each, so a rig with both air conditioners on the same leg trips that one while the other sits idle. The total was never the constraint. Startup surge is the other common cause — a compressor draws several times its running current for an instant.
Does an adapter let a 50 amp RV use full power on a 30 amp pedestal?
No. An adapter changes the plug shape, not the supply. A 50 amp rig on a 30 amp pedestal gets 3,600 volt-amperes total, with both of its 120 volt legs fed from the same single supply leg. You have to manage load exactly as a 30 amp rig would — the RV’s own breakers are rated far above what the pedestal can deliver.
What voltage is too low at a campground pedestal?
Below about 108 volts, air conditioner compressors are at real risk — they draw more current to maintain torque as voltage falls, and the windings overheat. Sustained low voltage is a recognised cause of compressor failure and is common at busy parks on hot afternoons. A surge protector with low-voltage cutoff disconnects before the damage happens.
Sources
- NFPA 1192, Standard on Recreational Vehicles — National Fire Protection Association
- NFPA 70, National Electrical Code — free public access — National Fire Protection Association
- Electrical safety and portable power — US Consumer Product Safety Commission
Formula and content last reviewed on .
Results are estimates for information only, not professional advice.
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