Guys,

I am building a heavy "extension cord" intended to carry 50 amps not
more than 100' from our 200 amp power box to my van. Inside the van
will be a couple of welders and a plasma cutter.

The idea will be to move the van around the yard to where I need to do
welding or cutting so that I don't have to load and unload the
equipment constantly.

I am running AWG #4 four conductor wire. The power supply end of the
cord will plug into a 4 conductor (two hots, ground, and neutral) 50
amp plug protected by a 50 amp breaker located at the main power box.

The cord will terminates at a 100 amp weatherproof box that is screwed
to a 4' piece of 2x12. This "panel board" will be semi-permanently
affixed to the interior of the van - perhaps fastened to the back of
one of the bucket seats.

The panel box only has spaces for three 240v circuits. One of these
three will be a 50 amp plug controlled by a 50 amp breaker inside the
panel box. Another will be a 20 amp 220v circuit for my inverter
welder. It's the same plug configuration as for a 220v air
conditioning unit.

I wish to use the last of the three spaces for 2 pole circuit breaker
energizing a two gang box that will have two duplex 120v receptacles
(i.e. four 120v receptacles).

Rather than put these 120v plugs on 120v single pole breakers I would
prefer to split each leg of the double pole breaker to feed two of the
outlets.

I believe I'm correct that every time you double the voltage you
reduce the amperage by 50%?

Does this mean that a 20 amp double pole breaker will give me 40 amps
of 120 volt protection? In other words, if I have 4 outlets (two on
each leg) protected by a single 20 amp breaker will each duplex 120v
receptacle be good for 20 amps?

Also, from a code standpoint is it permissible to protect more than
one circuit by the same breaker? In other words, and by way of
example, is it kosher to run a single 240v plug PLUS two 120v outlets
by simply doubling up the hot wires under the breaker lugs?

Since I work alone it is not gonna happen that I'm using the inverter
welder and any 120v power tool at the same time. I just wanna know if
anybody remembers what the codes say about this and can give me a
title reference. To the extent that it is not inconsistent with good
electrical practice I would prefer to have enough outlets mounted on
the panel board to keep all of the equipment permanently plugged in -
or at least one of the big welders AND the plasma cutter. That would
save me having to crawl in and out plugging and unplugging machines.

Finally, should I ground the truck frame to the box?

Thanks,

Vernon