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[Mike Sokol]

Yes, I know this is a pro-sound forum, but power is power so I thought it would be interesting for all of you to get your heads around voltage drops in branch circuits. Plus I suspect this reader has been plugged into something I call a bootleg 240-volt service at a campground. That's when only one leg of 120-volts is connected to both hot legs of a 240-volt outlet. In that case the neutral currents will be additive, which up to 100 amp of currents on the 50 amp wire and connector. But I'll update you all once I talk to the transfer switch manufacturer.

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Dan,

Of course, it's normal to read 6 volts between the ground and neutral if there's a load sufficient to cause a 12 volt drop in the Hot to Neutral voltage. But a 12-volt drop on a 20-amp branch circuit suggests a VERY long run of 12-gauge. Whenever I do a branch circuit longer than 100 ft from the panel I always increase the size of the wire to 10 Gauge to prevent just this amount of voltage drop in the line. Do you know how much current is being drawn when you see this 6 volt neutral to ground difference?

Also, I have a few ideas about what burned up the first four transfer switches, but have sent a query to Progressive Dynamics to talk to their engineering department about it. 

Mike Sokol

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On May 7, 2014, at 8:20 AM, Dan Mayer wrote:

Hi Mike,

I wonder if you could give me your opinion on something. 

I've had a series of problems with the 50amp Progressive Dynamics Auto Transfer Switch in my RV.  In February, the unit failed completely on both shore power and generator.  There was significant heat damage on both sides of the shore power path with significant damage to the insulation and probably to the wire.  The inside of the unit looked pretty smoked.   I got a replacement unit, cut back about 6" of wiring to get rid of any damaged wire, and added a new 2' length of wire for each conductor, connecting the new wire to old using aluminum butt splices.  The splices are contained in metal junction boxes and are wrapped with shrinkwrap and electrical tape.  The original wiring and new wire is all copper and I've been told that copper to aluminum is ok.

The replacement switch #2 failed on the generator side about 2 months later.  The neutral contact was completely open.  It was about the 4th time the generator had been used with the new switch - all fairly short runs of a few minutes.

Progressive sent another switch #3 which chattered when generator power was applied and failed to hold the contactor.  At the switch, power measured from generator input wires L1 to neutral or to ground was 120V.  Also L2 was 120V.  L1 is used to activate the contactors.

Progressive sent switch #4.  Before installing, I checked resistance on each set of connections with contactor at rest and then manually activated.  All were open when at rest and 0 ohms when the contactor was manually depressed.  However, after installing, I recheced and found the generator L2 contact was at 0 ohms whether at rest or when depressed.  I did not apply power.

Progressive sent switch #5 after bench testing it.  It checked out with the ohm meter before and after installation.  I fired up the generator and checked voltages.  120V from hot to ground and from hot to neutral.  0V from neutral to ground.

A few days later I brought the RV home to check the shore power operation.  Here's where I ran into trouble.  Plugged in the 50amp shore power cord through a 50-30 adapter and a 30-15 adapter and a 50' #12 extension cord.  It's plugged into a GFCI circuit in my house.

Measuring voltage at an outlet in the RV, (which is dedicated to a 20amp circuit breaker), I saw 125V from hot to ground, but 119V from hot to neutral, and 6V from neutral to ground.  There was no voltage on metal parts of the RV.

After going through so many transfer switches, I'm wondering if #5 has got some kind of problem.  But I'm also wondering if I'm losing some voltage through the adapters and extension cord, and whether this would account for the 6V neutral to ground in the RV.  Or perhaps there's a poor neutral connection somewhere in the collection of cords and adapters.

Do you have any ideas on how abnormal this is and if it's a serious problem, how I might go about finding and correcting the cause?

Thanks for taking the time to read this and for any advice you might have.

Dan Mayer

[Jonathan Johnson]

Plus I suspect this reader has been plugged into something I call a bootleg 240-volt service at a campground. That's when only one leg of 120-volts is connected to both hot legs of a 240-volt outlet. In that case the neutral currents will be additive, which up to 100 amp of currents on the 50 amp wire and connector.

In the tests that Dan reported he had done, I don't see an L1 to L2 voltage test on the shore power (and for good measure, on the generator power too). This will be telling. If that test returns 0V (or anything <10V) then it's quite likely both L1 and L2 are connected to the same hot leg, either at the shore power connection, the breaker/fuse panel, or somewhere in between. If the voltage is higher, but something other than ~240V, this is a red flag that L1 and L2 are on separate phases. Either situation can result in additive currents that exceed the ampacity of the neutral conductors, even if the currents on L1 and L2 are safe.

Another meaningful test will be to place a clamp-on ammeter on each of L1, L2, and neutral to measure the current flow on each conductor. For this test, I would recommend a calculated load be place on both L1 and L2. For example, you could connect two 500W halogen work lights -- one each on L1 and L2. Ensure that no other loads are present. You should then be able to determine if the observed currents are what you expect they should be. (To avoid doing the math, plug one of the worklights into an outlet in your shop and measure the current there as a reference.)

(But I know YOU know that, Mike! I'm just posting this comment for the benefit of other readers.)

[Mike Sokol]

(But I know YOU know that, Mike! I'm just posting this comment for the benefit of other readers.)

Perhaps, but I can always use another set of eyes (and brains) on a problem. I'm mostly posting this here just to get everybody to exercise their current troubleshooting skills. RV owners are a little like pro-sound guys. We're all plugging into unknown power every week, and we often have lot's of dollars invested in the gear. Many Class-A motorhomes cost upwards of  few hundred thousand dollars, and I've been in a few of them pushing 1 million bucks. And it's those same guys who use a free Harbor Freight meter to test an unknown campsite power pedestal for proper voltage, if they test anything at all.   

[Matthew Knischewsky]

"Measuring voltage at an outlet in the RV, (which is dedicated to a 20amp circuit breaker), I saw 125V from hot to ground, but 119V from hot to neutral, and 6V from neutral to ground.  There was no voltage on metal parts of the RV."

Any time I see a difference between ground and neutral I start to consider loose conductors, or corroded connections somewhere in the system. In this case I'd suspect the neutral...could be in his RV but it also could be in any of his string of extension cords and adapters. And I'm not sure if it would cause his transfer switch to blow up.

In any case, something happened to cause the first switch to blow up and that's the real cause of the problem. Find that problem, rectify it and then go from there.

Matt

[Jonathan Johnson]

Another meaningful test will be to place a clamp-on ammeter on each of L1, L2, and neutral to measure the current flow on each conductor. For this test, I would recommend a calculated load be place on both L1 and L2. For example, you could connect two 500W halogen work lights -- one each on L1 and L2. Ensure that no other loads are present.

Just to clarify, the reason for testing with two 500W work lights is that the current should be high enough to produce measurable differences in voltage and current (if, for some reason, there are differences), but the maximum possible current shouldn't exceed the ampacity of any circuit or conductor. FYI, at 120V, a 500W load should result in about 4.17A current. The two together should not exceed about 8.33A. However, the current flow for each lamp should be first tested against an independent circuit of known voltage, because variations in construction of the lamp filament could result in slight variations of load and, therefore, current.

Another question I have is does the "blow up" ONLY happen on a particular shore power connection (such as at home) or has it also happened at other sites? That would be a big, fat clue that the problem has something to do with the shore power wiring. (Has the shore power ever been connected elsewhere?)

[Mike Sokol]

Any time I see a difference between ground and neutral I start to consider loose conductors, or corroded connections somewhere in the system. In this case I'd suspect the neutral...could be in his RV but it also could be in any of his string of extension cords and adapters. And I'm not sure if it would cause his transfer switch to blow up.

Actually, and time I DON'T see a voltage between ground and neutral on a branch circuit under load, I get worried. And here's why. Consider a branch circuit or extension cord that started with 120-volt at the service panel or feed outlet but now has a 10-volt drop over the length of the run due to current draw, ending up with 110 volts metered at the outlet/recept. Since the black (hot) and white (neutral) wires are exactly the same gauge, there will be an equal but reverse phase voltage drop end-to-end for the the hot (5 volts) and neutral (5 volts) wires. That 5 + 5 volts adds up to the total 10 volt drop over the length of that run. So now consider the ground wire (EGC - Equipment Grounding Conductor). Since it's not allowed to carry load current during normal operation, the ground should NOT have any voltage drop at all. So if you measure that same outlet that started at 120-volts and dropped to 110-volts over the run, you'll read 5 volts between the neutral and the ground. If you take the load off of that circuit, it should drop closer to zero volts. If the service or sub-panel itself has any other loads, then it will likely have a small difference (maybe a volt) between the ground and neutral bus bars.

If you DON'T see any voltage on a branch circuit between the Neutral and Ground while there's a load on the circuit, then you've likely found a bootleg ground or double bonded G-N connection. A bootleg ground is where the ground wire didn't exist in older wiring, so an electrician jumpers between the green (ground) and white (neutral) screws on the outlet. A double-bonded ground occurs in a sub-panel where an electrician incorrectly leaves the green panel-bonding screw in place, creating a condition where the ground wire between the main and sub-panel  is now carrying load current.

I have a table-top demo of this G-N voltage thing using a 100-ft extension cord and a 600-watt space heater. It's really easy to mock up this demonstration yourself and do some measuring to get your head around it.   

[Stephen Swaffer]

I hope that he understands that when using a double adapter-50 to 30 and 30 to 15 that you cannot expect to run everything!  If the adapters could compensate for that, we save a lot of copper on high power circuits!

Also, the statement "copper to aluminum is OK" is not correct that is asking for problems.  You need to make sure the connectors you use are designed and listed for both Cu and Al and that the Cu and Al are not in contact when the connection is finished.  I doubt that that is the problem, as usually it will take some time for a Cu to Al (or any dissimilar metal for that matter) to cause a problem-and it is likely his butt connectors are designed for copper in any case.

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