50 amp Service

[Cracker]

I've got a question for the electrical gurus. Using the equation P=IE, a 30 amp service yields 30 x 120 or 3600 watts. It would appear that 50 amp service would provide 50 x 120 or 6000 watts, however, a new twist is thrown in with the provision of the 4th blade on the plug - i.e. - we can now provide 240 volts to some appliances. So, for us electrical dummies, just how many net watts are available in a 50 amp service and just how is it distributed??? Or, in other words, how do you calculate the available wattage??? Is the 50 amp service what you would call "dual phase" - as opposed to the single phase 30 amp service???

[HiHoAgRV]

Our local campgrounds are 50A, 2 phase, netting 220 if needed. A pal installed a 220 dryer in his toy hauler. The 50A service I installed at my weekened place is also on 2 separate phases, also netting 220. From what I gather, it is possible for a site to have 2, 50A lines installed for a 50 A service but both to be on the same phase, tossing out the 220V possibilties. I have not read the code but this is what my electrican bud told me.

[HowieE]

Can't answer the math but a 120 volt system is not single phase. You only have one hot leg in a 120 volt system. Single phase circuit is achieved by connecting 2 legs of a 3 phase system. Any 2 hot legs coming off a Y system will give you a single phase and 240 volts.

[thecatsandi]

Originally Posted by Cracker

I've got a question for the electrical gurus. Using the equation P=IE, a 30 amp service yields 30 x 120 or 3600 watts. It would appear that 50 amp service would provide 50 x 120 or 6000 watts, however, a new twist is thrown in with the provision of the 4th blade on the plug - i.e. - we can now provide 240 volts to some appliances. So, for us electrical dummies, just how many net watts are available in a 50 amp service and just how is it distributed??? Or, in other words, how do you calculate the available wattage??? Is the 50 amp service what you would call "dual phase" - as opposed to the single phase 30 amp service???

Same formula applies. Power = VA Power= 240*50. 12,000 Watts or 6,000 per leg.

[wahoonc]

Need to clear up the terminology...I think. You have multiple poles but it is all single phase. ie; the 30 amp is single pole 110/120 volt, the 50 amp is double pole or 220/240 volts. Typically you won't run into phases until you hit the 480volt heavy commercial/industrial stuff. I am not an electrician, but work around the stuff a fair bit. Last project involved working around 1600volts DC and 25,000 amps Then there were the 30kv lines running along the side of the building in armored housings...

Aaron

[markdoane]

Rather than dual phase, I think the common terminology is to call this "split phase", although it doesn't mean the same as a 'split phase motor'.

You can call it three wire single phase, or 120V/240V, to distinguish it from 120V two wire.

Michelle has the math all figured out, you have 12 Kw (or 100 amps 120V) available.

[Cracker]

I'm getting there! So, with a 6 Kw generator that has a 3-prong twist lock for 120 volts and a 4-prong twist lock for 240 volts, the maximum current is either 6000/120 = 50 amps or 6000/240 = 25 amps. With that in mind, if a trailer is equipped with a 50 amp plug, and an adapter is used to fit the 3-prong 120 volt twist lock, do you still have 50 amps or 6 Kw available??? When we originally tried to run off the generator with a 50 amp plug, using an adapter to plug into the 4-prong outlet, we had a definite loss of current while the voltmeter (at the trailer) continued to show 120 volts. Switching to the 3-prong adapter cured the problem - but I'd like to know why??? Incidentally, the problem involved a commercial exhaust hood fan in a portable kitchen trailer. There are currently no 240 volt appliances in the unit. On shore power, using the provided 50 amp female plug, everything works good.

[markdoane]

I think the problem is that the generator puts out the maximum power when the windings are in parallel, which is what you have when using the 120v power tap.

When you switched to the series windings configuration for 120V/240V operation, you have only half the power available at each leg of the tap.

For more information see this site.

[moosetags]

I have another question along this same line. I carry in the trailer an electrical pig tail adapter with a 4 prong 50 amp male plug and a 3 prong 30 amp female outlet on the other end. I bought this when I started running into campgrounds with only 50 amp outlets.

When I hook-up at a campground that has both a 50 amp and a 30 amp outlet, is there any advantage to using my adapter and plugging into the 50 amp outlet?

Brian

[markdoane]

There should not be any difference.

The 50a-to-30a adapter has one hot disabled. The remaining leg will have 50 amps available, but the 30 amp circuit breaker in your main panel will limit current to 30 amps.

I suppose you could add a 30 amp + 20 amp panel between the power inlet and the main panel so that you could run another airconditioner, but you would need to get rid of the 50a/30a dogbone because the wire size is only rated for 30a.

[garry]

A 240V 50 amp plug has 4 prongs the 2 outside flat blades are hot and have 240V measured between them, the top flat blade is neutral and the round one is ground.

L1 ___ 120V___|
N ----- | 240V
L2 ___ 120V___|

Between any one hot blade and the neutral blade is 120V@50A. Most RV's have breaker panels that are wired so the neutral and one hot (called L1) feed 1/2 of the appliances and the the other hot (called L2) and neutral feed the rest of the appliances I say most since there are very few RV's with 240V appliances.

So the 50 amp RV is powered with 240V split into two 120V@50A circuits. The down side to this set-up is if the neutral wire ever fails all appliances in the RV will be destroyed.

A 30A RV is 120V @30A power is provided through the neutral leg and one hot leg limited by a 30A breaker at the power pole and one in the RV.

When you use a 50A to 30A adapter at the CG you only use 1/2 of the available power 120V @ 50A and that is limited to 30A by the RV 30A circuit breaker.
So really there should be no advantage for a 30A RV to use the adapter if there is a 30A outlet provided.
Having said that there are some occasions if the CG has low power using the adapter might give you an edge.

For the generator question:

A 6000W generator is limited to 6000W period and the math shows;

I (amps)=P/E 6000/120=50A or 6000/240=25A

However; It is not really that simple things like center taps and phase differences become involved.

From a power standpoint the big deal between 120V & 240V

Things like 240V electric motors and other 240V high amp draw devices will run cooler (less amps) and you can use less wire since a true 240V device needs no neutral wire.

Hope this helps & I hope I got it right.

Garry

[teagues]

Thanks, Garry. That was very helpful.

Originally Posted by garry

So really there should be no advantage for a 30A RV to use the adapter if there is a 30A outlet provided.
Having said that there are some occasions if the CG has low power using the adapter might give you an edge.

Interesting idea there. I am not sure I can wrap my head around exactly where the advantage comes from, but I will certainly try my 50a-30a adapter the next time I am at a camp with low power.

[HowieE]

Originally Posted by teagues

Interesting idea there. I am not sure I can wrap my head around exactly where the advantage comes from, but I will certainly try my 50a-30a adapter the next time I am at a camp with low power.

Unless the CG has balanced its load on each side of the panel, and that is a function of which sites are in use at that point in time, just using an adapter may not be an advantage. You may end up on the side that was low to begin with.

Even if you are on he low side of the panel plugging into the 50 circuit may give you a couple of extra volts because hopefully the CG used proper size wire on the newer install and the line lose will be less.

[Cracker]

The down side to this set-up is if the neutral wire ever fails all appliances in the RV will be destroyed.

Garry[/QUOTE]

Garry:

You lost me on that. In simple terms, what happens electrically if the neutral in the 50 amp setup fails. Are the appliances suddenly exposed to 240 volts??? I would have thought that you just had an open neutral.

Thanks for the explanations!

[HowieE]

No this will not happen. A 240 volt supply in a RV has 2 hot legs. If a 240 volt appliance, a heavy motor, is used there is no current in the neutral. All the 120 volt appliances are supplied voltage via one leg and the neutral. Under ideal conditions the loads in the RV would be balanced and again there is no current in the neutral. If the neutral fails everything would just shut down.

There is current in the neutral when the loads are unbalanced and is limited to the current supplied by either leg if there is nothing on the other leg.

[Cracker]

Originally Posted by HowieE

No this will not happen. A 240 volt supply in a RV has 2 hot legs. If a 240 volt appliance, a heavy motor, is used there is no current in the neutral. All the 120 volt appliances are supplied voltage via one leg and the neutral. Under ideal conditions the loads in the RV would be balanced and again there is no current in the neutral. If the neutral fails everything would just shut down.

There is current in the neutral when the loads are unbalanced and is limited to the current supplied by either leg if there is nothing on the other leg.

OK - so if the neutral fails, does the 240 volt unit continue operating? Further, is there any increased risk of shock with a failed neutral (---at the unit) if the ground remains intact?

I have one of those plug-in circuit testers with the two amber and one red light and I've often wondered what the different light combinations meant. Although I've yet to have a faulty connection, I'll bet that a lot of others often wonder the same thing.

Thanks again for your patience!

[garry]

In a 240V application;

If the neutral opens the two 120V lines now become a series circuit and depending on what's on will be subject up to the full 240 V.

If you have say a TV that has power applied even when turned off (most TVs have a warm start circuit) and it plugged in the TV could be subject to high voltage and be damaged.

The amount of damage to the appliances depends on what is plugged in and turned on.

It is a dangerous situation since the ground wire is now a hot wire and tries to act like a neutral and in an Airstream that means the skin will be hot.

On a 120V RV you may not notice an open neutral since the ground will carry the load
the tester you have should have a label explaining what the different light configuration means and once again you could get a shock from the skin on the RV.

Garry