50 amp versus 30 amp

[raspivey]

Originally Posted by tomrob1161

It's really amazing how these RV'ers can run an A/C, 15 amps, microwave, 12.8 amps, electric water heater, 12.5 amps - total 40.3 amps on a 30 amp circuit, plus all the other parasite draws without any problems.

Probably because those numbers are BS. My water heater draws 11A, My 15K AC draws 11 - 14A, my microwave draws 9A. That's 31 - 34A IF everything is running at the same time. 90% of the time that wouldn't be the case. I'd expect to trip a breaker if I were running all three at the same time.

[pmsherman]

As stated in a prior response, everything (except possibly a drier) in an RV is designed to run on 110V. Your 50A power cord provides 100A of 120V power to the RV because there's 50A on each of two hot leads. A 30A connection provides only 30% of the power available from a 50A connection.

Circuit breakers are designed to work properly when providing continuous current equal to 80% of their rating. A 30A breaker should not be used for continuous loads greater than 24A. This doesn't mean that you can't pull 30A through the breaker without it tripping but you shouldn't be surprised if you're pulling 30A through it for hours at a time and it eventually trips.

Most 110V devices have two current ratings; surge and run. The power line feeding them must be able to meet the surge demand which normally lasts for a very short interval, usually less than a second for motors. Your water heater's electrical heating element draws X watts where X will be documented on the heater's label, a UL requirement. When the water is hot, the element is shut off, when additional heat is needed, it's turned on and draws its full power. Your air conditioner works the same way, when it's cool inside, the compressor shuts off and just the fan runs but as the air warms up, the compressor starts up creating a surge draw then it runs with the AC drawing its run mode power. Like the water heater electric element, the compressor is on or off, never partially on.

Your coffee maker works the same way. When you start making coffee, the heating element is full on. Once it stops making coffee, the heater shuts off and assuming it will keep the coffee hot for hours, a small secondary heater under the pot will keep the coffee hot.

You microwave does a similar thing. Heat a cup of water in it with the power level set to 5 and listen to it. You'll hear the klystron tube (microwave generator) turn on and off. It draws full power when on while the microwave only draws enough power for the fan, turntable, and electronics when it's off.

Total power usage is the sum of all power consumed by whatever is running in your rv. Since high power consumption devices tend to cycle on and off, it's not uncommon to be able to run 40A or more of devices on a 30A circuit. As long as all of them aren't drawing high current at the same time, you won't trip a breaker. Eventually though, you'll hit a moment when multiple devices are drawing high power and the breaker will trip. A 30A breaker also does not trip immediately if you draw 31A through it. It will trip after some interval of time which shortens as the current draw increases.

Follow the general rule of keeping loads to 80% of the breaker's rating and you'll never have an issue tripping a breaker, even with startup surge loads.

Phil

[rockfordroo]

Originally Posted by pmsherman

Your coffee maker works the same way. When you start making coffee, the heating element is full on. Once it stops making coffee, the heater shuts off and assuming it will keep the coffee hot for hours, a small secondary heater under the pot will keep the coffee hot.

Phil

Some might do this, but definitely not all (and maybe not most). I've torn several of these apart over the years, and they've had one heating element. It is located under the pot warmer. The water runs through a tube in the heater so it heats the water and the pot warmer together. Once all the water's run through, it only heats the water. But there's a single pair of wires running directly from the ON switch to the heater. Nothing that would shut it off and turn on a secondary heater.

[Brewhedd]

Originally Posted by wmtire

Many people don't realize that a 50 amp 120/240 volt split phase service is capable of providing a total of 100 amps of 120 volt power.

I've seen this before but still don't believe it, it is a 50 amp service. I just put in a 100 amp breaker for running 220v to a subpanel. It has two legs with 100 amp printed on each leg. That does not mean each leg is capable of 100 amps, no way no how. The breaker's total capacity is 100 amps, by the spec's.
It would be interesting to know what wire size is running to each leg of the breaker panel in the trailer. In order to have a 100 amp rating each leg would have to be #2 wire, that's some large wire.

[R3Jones]

Originally Posted by Brewhedd

I've seen this before but still don't believe it, it is a 50 amp service. I just put in a 100 amp breaker for running 220v to a subpanel. It has two legs with 100 amp printed on each leg. That does not mean each leg is capable of 100 amps, no way no how. The breaker's total capacity is 100 amps, by the spec's.

He's correct there are two legs (phase 1 & 2) each rated at 50 amps.




-R.Jones
Nitro 31FQSL

[wmtire]

Originally Posted by Brewhedd

I've seen this before but still don't believe it, it is a 50 amp service. I just put in a 100 amp breaker for running 220v to a subpanel. It has two legs with 100 amp printed on each leg. That does not mean each leg is capable of 100 amps, no way no how. The breaker's total capacity is 100 amps, by the spec's.
It would be interesting to know what wire size is running to each leg of the breaker panel in the trailer. In order to have a 100 amp rating each leg would have to be #2 wire, that's some large wire.

It's all explained in the links you quoted me from. It does take a little to get your head around that:

50 amps X 240 volts = 12000 watts

or how RV's utilize it:

100 amps X 120 volts = the same 12000 watts.

You are confusing the way 240 volts works vs 120 volts, which is also explained in the links I provided. It does kind of take a little , but once you get your head around the way 240 volts work and the common neutral wire that carries the load imbalance, it's not complicated.

This is probably the best one as far as that goes, and may help you get a handle on it.

http://www.rvtechmag.com/electrical/chapter3.php

[WY Husker Fan]

A 100 amp 2-pole breaker does indeed provide for 100 amps in each leg of the circuit and hopefully you did use wire capable of 100 amps for your sub panel feeder. A 50 amp 4-wire circuit for an RV feeder is exactly the same as a 4-wire 50 amp circuit for a range receptacle. The way the RV utilizes the circuit is what leads to the statements about 100 amps at 120 volts. It is essentially two 50 amp circuits sharing a neutral. Because the neutral currents from the two hot legs are 180 degrees out of phase with each other the maximum current in the neutral conductor is 50 amps. The wire then only has to have a capacity of 50 amps.

[wmtire]

These diagrams may help too, as I use these in a lot of threads:

[Steeljag]

Originally Posted by Mr. Dan

R. Jones speaks for me. When we have our 50a Wildcat on 30a, we run 1 A/C at a time but don't change any other thing... meaning we leave water heater on electricity and use the μ-wave whenever we want.

X 3 ! On our 30 amp TT we ran AC, microwave, water heater, coffee pot, etc all on electric. I do recall one morning when we tripped the breaker (twice). It was caused by DW using the blow dryer with some of those other items being used !

[Brewhedd]

Originally Posted by WY Husker Fan

A 100 amp 2-pole breaker does indeed provide for 100 amps in each leg of the circuit and hopefully you did use wire capable of 100 amps for your sub panel feeder. A 50 amp 4-wire circuit for an RV feeder is exactly the same as a 4-wire 50 amp circuit for a range receptacle. The way the RV utilizes the circuit is what leads to the statements about 100 amps at 120 volts. It is essentially two 50 amp circuits sharing a neutral. Because the neutral currents from the two hot legs are 180 degrees out of phase with each other the maximum current in the neutral conductor is 50 amps. The wire then only has to have a capacity of 50 amps.

The breaker is rated for 100 amps total per the specs. and #2 wire is needed for 100 amps. Residential wiring is as follows:
120/240 VOLTS / 1 PHASE The primary of a single-phase transformer is connected to one phase of the three-phase distribution system. The secondary contains two coils connected in series with a midpoint tap to provide a single-phase, three-wire system. The two ‘hot’ wires, or lines, provide 240 Volts, and either of the ‘hot’ lines to neutral provides 120 Volts.

A center tapped transformer can provide 2 hot points that are 180 degrees out of phase with respect to the center tap (neutral) but obviously a home supply is in phase. There is something in the configuration I am not understanding here. I'm an electronics guy, not an expert regarding electric.

[raspivey]

I think the problem here is that we keep throwing 100A into the mix. For RV use...one double pole 50A breaker (essentially two 50A breakers tied together) using 6AWG on both sides. Easier to think of that as two 50A lines than 100A total. It's kept separate from the breaker all the way to the RV panel so that both lines separately carry 110V at 50A max.