Power Pedestal Test Box

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Power Pedestal Test Box

February 20, 2011, 09:27:38 pm

I am thinking about constructing a box I'll use to test power pedestals. I'll probably break down and spend the ~$400 for the EMS box but may still want this tester.

[] I can buy a 6' 10/4 dryer cord with a 14-50P plug for less than $20 that will plug into a 50amp campground socket.
[] A 3-way electrical switch (heavy duty - ~$6) is normally used to switch an incoming hot wire to one of two outgoing hot wires. I will wire it to switch either the red or the black hot legs in the 125/250 cord set to the other contact on the switch. I'll wire this output to the hot side of a standard duplex plug. The green ground and the white common wires will be wired to this duplex plug. Say $4.00.
[] I already have a polarity tester that plugs into a standard grounded socket and a plug-in volt-meter.
[] I can buy a gray plastic duplex box (or if I'm not too concerned about water proof, a blue plastic existing-construction electrical box) and a wall plate that has cutouts for one switch and one duplex receptacle for probably another $5.

What I wind up with is a box that will plug into a 50a campground socket. It will display the voltage and polarity status of one leg of the connection. Throwing the switch will show the same for the other leg.

If I use a 30a-50a dog bone it still works.

An additional 'benefit' is that it gives me a 50a to 15a adapter.

I would appreciate (maybe) comments on things I have missed, dangers I have created, etc. Do I need to worry about fuses?

TIA

Re: Power Pedestal Test Box

Reply #1 – February 20, 2011, 09:43:14 pm

I made a similar tester and generally followed the design shown at this link:

50-amp Tester

I tried using the voltage gauge specified, but it was too fragile and I now use an inexpensive meter. I posted a couple of photos in my album here if you are interested. I think the cost was about $60 total. I use it every time we come into a new campground.

George

Re: Power Pedestal Test Box

Reply #2 – February 20, 2011, 10:36:49 pm

Thanks George..
Almost identical to what I have in mind. I'll have to think about the second duplex vs a switch.
I do like the 220 test.

Re: Power Pedestal Test Box

Reply #3 – February 21, 2011, 01:09:45 am

George, I noted your reference, in the reference, to a fake 50 amp source. Perhaps that needs clarifying. Most of our coaches will work fine using a 30 amp to 50 amp adapter which does just what you describe as fake 50 amp. Using the adapter the only limitation is how much power we can draw. We are limited to the 30 amp source and can run only 1 AC at a time and otherwise be careful. A pedestal wired with a 50 amp plug should have the full 50 amp capability whether the two hot leads are in phase or out of phase.
Put another way, our coaches in general, take in two 115 volt lines that are used independently on only 115 volt appliances including the ACs. Perhaps someone with a newer all electric coach can clarify whether this is still true.

Re: Power Pedestal Test Box

Reply #4 – February 21, 2011, 09:36:32 am

Harvey,
The 'Real' 50 amp feed coming into our coaches is either one 250v 50a circuit or two independent 125v 50a circuits, depending on how the two feeds are wired once they get into the coach. My coach sees them as two 125v feeds. In either event we get 1,250 watts (watts=volts X amps) of usable power.
The voltage potential between the white and black wires is 125v and the potential between the red and white is 125v, 50 amps going through each with separate breakers. Between the black and the red there is a potential of 250v if they are of different phases.

The 'Fake' will take a single 125v 50amp feed and tie the black and red (the hot) wires together at the pedestal. Thus, we really only have the equivalent of one 125v 50a circuit and will get only 6,250 watts. As you mentioned, the 30a to 50a adapter also ties the two hot wires together but delivers only 125 X 30 = 3,750 watts.

Usually my eyes glaze over about halfway through discussions about electricity and it only took me 7 years to get a handle on this..

HTH

Re: Power Pedestal Test Box

Reply #5 – February 21, 2011, 09:47:10 am

I may not understand what you are trying to accomplish, but it seems as though my portable EMS does the same thing and offer continuous protection as well. When I plug in to the pedestal it checks voltage per line, phase and polarity before it allows the electricity to flow into the coach. Is there any other advantage to your custom built box?

Re: Power Pedestal Test Box

Reply #6 – February 21, 2011, 09:50:28 am

Now get a handle on your 10KW on board power unit, rated @ 10,000 watts

(45 X 120=5400 watts) X2 = 10,800 watts.

Can that be true

Re: Power Pedestal Test Box

Reply #7 – February 21, 2011, 11:52:49 am

This is a good reference on the 50amp service...... RV Electric

Here is a quote from the 50amp tab...

"This 50-amp service has 4 wires with two 120-volt HOT feeds. It is a misconception that this 50-amp RV service is something special. This service is a STANDARD 120/240 50-amp 3 pole with 4 prongs used for numerous applications.
From this common service we can draw 120 or 240 volts. Each leg is 50 amps @ 120 volts. 50-amp X 120-volt = 6000 watts. But since there are 2 HOT 120-volt legs at 6000 + 6000 = 12,000 watts to use in the RV or 50-amp X 240-volt = 12,000 watts when used as a 240-volt service.

Almost ALL 50-amp wired RV's use both sides of the service separately as 120 volt on each leg. Only a few mostly high-end coaches utilize the 240-volt from this same service.

The 50-amp 3-pole 4-wire service is superior to the 30-amp service because of the total amperage available.

30-amp 120-volt service = 3,600 watts
50-amp 120/240-volt service = 12,000 watts"

George

Re: Power Pedestal Test Box

Reply #8 – February 21, 2011, 02:09:32 pm

I like that test box - nice and clean.

I would guess the built in OEM voltmeter/reverse polarity device on our (older) coaches only picks up one of the legs? I wonder if that leaves the other unchecked? Seem like there should be a second device for the other leg.
But this box is actually the ideal: Check before you plug in!! (I think I'll make one too, thanks for bringing this topic up)

Quote

A pedestal wired with a 50 amp plug should have the full 50 amp capability whether the two hot leads are in phase or out of phase.

This is what I understand about 50 amp service regarding this issue:
If the pedestal really has two 50 amp circuits that are "in phase" the return current on the neutral white wire would be the sum of the current being pulled in both legs - potentially 100 amps through a wire designed for a maximum of 50 - this would not be good (understatement). Worst yet there is no circuit breaker on the neutral to prevent a fire with this condition. But I have never heard of warnings about this. I would guess the most likely scenario is the splitting of a single 50 amp circuit into both legs (mentioned above) which would be just like 30 amp setup but higher by 20 amps - this wouldn't cause an overload danger (at least with a 50 amp chord).

Re: Power Pedestal Test Box

Reply #9 – February 21, 2011, 02:15:57 pm

George, excellent explanation. This is what i always presumed but had never confirmed.

Re: Power Pedestal Test Box

Reply #10 – February 21, 2011, 03:52:25 pm

I am planning to check the power pedestal using a multimeter prior to plugging into the outlet. In addition, I plan to utilize a 50A EMS. Is this test box of benefit based on doing both of these?

Thank you.

Re: Power Pedestal Test Box

Reply #11 – February 21, 2011, 05:51:29 pm

Quote from: ncaabbfan – February 21, 2011, 03:52:25 pm

I am planning to check the power pedestal using a multimeter prior to plugging into the outlet. In addition, I plan to utilize a 50A EMS. Is this test box of benefit based on doing both of these?

Thank you.

The only particular advantage is that Mary can plug it in and see if the proper lights come on...
She wouldn't do that with a voltmeter.

And... it only cost me $52 and I had an hour's worth of fun.

I'll eventially hardwire in an EMS but will continue to use this to initially test the pedestal.

Re: Power Pedestal Test Box

Reply #12 – February 21, 2011, 07:46:59 pm

I am in the processing of making a test box too. I have a 50A plug and will wire it to a duplex 15/20A recepetacle so that each duplex socket gets a different leg. I have a Kill-A-Watt meter with a idiot light tester (three neon lights) plugged into the Kill-A-Watt. All I need to do is plug the Kill-A-Watt into one then othe other receptacle to check for voltage and grounding. As for phasing, the rig doesn't care.

I originally planned to wire a switch into a single receptacle (and may still do so) that allows the Kill-A-Watt to meter either one leg or the other, or across both legs via a resistor voltage divider in order to check for phasing.

BTW, this is not a high priority project as I am comfortable with a DVM.

Re: Power Pedestal Test Box

Reply #13 – February 28, 2011, 05:24:45 pm

We replaced our one-outlet with 3-way switch, test box with this new dual outlet no-switch, test box.

Foretravel Owners' Forum For owners and Fans of Foretravel Motorcoaches September 12, 2025 ...

Power Pedestal Test Box

February 20, 2011, 09:27:38 pm

Re: Power Pedestal Test Box

Reply #1 – February 20, 2011, 09:43:14 pm

Re: Power Pedestal Test Box

Reply #2 – February 20, 2011, 10:36:49 pm

Re: Power Pedestal Test Box

Reply #3 – February 21, 2011, 01:09:45 am

Re: Power Pedestal Test Box

Reply #4 – February 21, 2011, 09:36:32 am

Re: Power Pedestal Test Box

Reply #5 – February 21, 2011, 09:47:10 am

Re: Power Pedestal Test Box

Reply #6 – February 21, 2011, 09:50:28 am

Re: Power Pedestal Test Box

Reply #7 – February 21, 2011, 11:52:49 am

Re: Power Pedestal Test Box

Reply #8 – February 21, 2011, 02:09:32 pm

Re: Power Pedestal Test Box

Reply #9 – February 21, 2011, 02:15:57 pm

Re: Power Pedestal Test Box

Reply #10 – February 21, 2011, 03:52:25 pm

Re: Power Pedestal Test Box

Reply #11 – February 21, 2011, 05:51:29 pm

Re: Power Pedestal Test Box

Reply #12 – February 21, 2011, 07:46:59 pm

Re: Power Pedestal Test Box

Reply #13 – February 28, 2011, 05:24:45 pm

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