Are these figures published standards or rules of thumb?

Both, and the page labels which is which. Conductor ampacity, resistance, voltage drop limits, overcurrent sizing factors and device ratings come from NEC, ABYC and NEMA and are labeled as published standards. Depth of discharge, solar derate factors and inverter efficiency are planning conventions with no standards body behind them, and are labeled as conventions.

Why does this calculator sometimes say there is no answer?

Because there genuinely is not one within the published tables. Rather than extrapolating past the end of an ampacity table or inventing a figure to fill a gap, the calculator returns nothing and explains why. A run that is too long for the current at 12 volts needs a shorter run or a higher system voltage, not a made up wire size.

Do I still need an RV technician if I use this?

For DC work many owners do their own installation, and these figures are the ones that work is sized from. Permanent AC wiring, anything connected to the shore power inlet and anything behind the distribution panel is work for a qualified RV technician or a licensed electrician. Sizing a circuit correctly and installing it safely are two different jobs.

Shore power calculator

30A vs 50A Shore Power Load Budget Calculator

Quick answer

A 30A RV service is a single 120V leg at 30A, which is 3,600 watts. A 50A service is two 120V legs at 50A each, which is 12,000 watts. A 50A pedestal supplies more than three times the power of a 30A one, not 67 percent more.

Result

Total available 3600 W

Available per 120V leg 3600 W Published standard

Headroom left 1200 W

Within the service Yes

The formula

30A: 120V x 30A = 3,600 VA on one leg. 50A: 120V x 50A x 2 legs = 12,000 VA

An adapter never increases available amperage. A 30A rig on a 50A pedestal still has 30A, and a 50A rig on a 30A pedestal has 30A total for the whole coach.

The figures shown above are computed with these exact inputs: Pedestal service tt-30, Planned load 2400 W.

What this calculator assumes

Some of these are published standards and some are conventions. They are not the same kind of number, so each one is labeled.

Assumption	Kind	Where it comes from
Tt30 Va: 3600	Published standard	NEMA TT-30, 30A at 120V
Nema1450 Va: 12000	Published standard	NEMA 14-50, two 120V legs at 50A each

Components that match this result

Every card below is in the page source. The calculator re-ranks them against your numbers and marks anything that falls short of what you just calculated.

Expert

Progressive Industries

Progressive Industries EMS-PT30X, 30A

$135.89

An EMS disconnects on low voltage, high voltage, open ground and reverse polarity. A plain surge protector only handles a spike.

Best for: Anyone plugging into parks they have not vetted.

Check price on Amazon

Intermediate

Power Watchdog

Power Watchdog PWD30 Bluetooth, 30A

$139.99

Reports actual pedestal voltage and current to a phone, which is how you catch a sagging pedestal before it damages a compressor.

Best for: Watching pedestal quality over a long stay.

Check price on Amazon

Expert

Power Watchdog

Power Watchdog PWD30EPO, 30A, IP65

$305.31

An IP65 body, which matters because a portable unit sits on the ground at the pedestal in the weather.

Best for: A portable unit left outdoors.

Check price on Amazon

Frequently asked questions

How does the 30a vs 50a budget calculation work?: 30A: 120V x 30A = 3,600 VA on one leg. 50A: 120V x 50A x 2 legs = 12,000 VA. An adapter never increases available amperage. A 30A rig on a 50A pedestal still has 30A, and a 50A rig on a 30A pedestal has 30A total for the whole coach.
Are these figures published standards or rules of thumb?: Both, and the page labels which is which. Conductor ampacity, resistance, voltage drop limits, overcurrent sizing factors and device ratings come from NEC, ABYC and NEMA and are labeled as published standards. Depth of discharge, solar derate factors and inverter efficiency are planning conventions with no standards body behind them, and are labeled as conventions.
Why does this calculator sometimes say there is no answer?: Because there genuinely is not one within the published tables. Rather than extrapolating past the end of an ampacity table or inventing a figure to fill a gap, the calculator returns nothing and explains why. A run that is too long for the current at 12 volts needs a shorter run or a higher system voltage, not a made up wire size.
Do I still need an RV technician if I use this?: For DC work many owners do their own installation, and these figures are the ones that work is sized from. Permanent AC wiring, anything connected to the shore power inlet and anything behind the distribution panel is work for a qualified RV technician or a licensed electrician. Sizing a circuit correctly and installing it safely are two different jobs.

Researched guidance, not professional advice. These results are planning figures from published standards and manufacturer specifications. Never exceed a conductor's rated ampacity, fuse every positive conductor close to the battery, never run a generator in an enclosed space because carbon monoxide is lethal and odorless, and have permanent AC wiring done by a qualified RV technician or licensed electrician.