[VAC] Re: 50 amp service ?

[Dr. Gerald N. Johnson, electrical engineer]

Actually the 50 amp receptacle can supply two 50 amp 120 volt loads
because its a 50 amp maximum, 120/240 volt receptacle.

That does NOT mean that hooking up a wire or load rated for 15 amps will
have 50 amps forced to it. Much to the contrary.

This electrical system is a constant voltage system where the source
maintains the voltage, while the resistance of the load determines the
current. The current rating is the limit of the circuit breaker and the
wiring to the receptacle. A 3 watt night light draws 0.025 amp whether
connected to a 15 amp receptacle, a 20 amp receptacle, a 30 amp
receptacle or a 50 amp receptacle so long as each is supplying 120
volts.

The current rating of the pedestal and each circuit in a circuit breaker
or fuse panel is the current that when exceeded will trip the breaker
before the wire rated for that current is damaged. Most of the time
smaller wires in extension cords have been presumed protected by the
receptacle, which hasn't always been true so that I'm seeing the
smallest extension cords are made of heavier wire than they used to be.
That is good for safety.

Using an adapter to plug a 20 amp cord into a 30 or 50 amp receptacle is
fairly safe, most of the time there is a main breaker in the trailer to
protect the feed cable so that it won't be overloaded by any combination
of loads in the trailer. But that 30 or 50 amp circuit breaker may not
prevent cable damage if the cable itself gets shorted. It takes physical
damage to cause the short, so the whole cable may need replacement
anyway. The 20 amp cord could be damaged by 30 or 50 amps allowed by the
larger circuit breaker.

Ideally an adapter that allowed plugging a 20 amp plug into a 50 amp
receptacle would include a 20 amp fuse or circuit breaker to best
protect that 20 amp cable. Ditto a 30 amp cable to 50 amp receptacle.
Just for fire protection purposes, it won't affect the longevity or
performance of appliances. Though the 50 amp circuit should have less
voltage drop under load than a 20 amp circuit and so the light blink
when the air conditioner starts should be much less noticeable and so
beneficial to motor operated appliances.

Some parts of the world, especially Great Britain, install fuses in each
plug rated for the load and cord capacity to customize the cord
protection. Similar plugs to fit US receptacles have been available in
the US at times, but have never proven very popular.

I hope this mud is clearer than the Iowa countryside today.

Gerald J.



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