Voltage Drop at an RV Park

philbullock122 · Aug 11, 2020

First, I really appreciate any feedback you can provide. Its awesome to have a group of people that have a passion for this!

Our RV Park has a transformer dropping service down into three 200-amp meters (single-phase 240V service X 3). From each of these three meters, we have an emergency disconnect (3 in total), and then we daisy-chain 4/0 wire to each of our RV Pedestals. We have 6 campsite RV pedestals on each of the 3 lines (total of 18 RV pads). Each pedastal has in and out lugs, then have a 20-amp, 30-amp, and 50-amp breaker.

We are experiencing a voltage drop on only one leg. One leg (on all three services) remains fairly constant at 120V. The other leg (on all three services) drops to 118 at the emergency disconnects, and then looses voltage quickly. We are seeing 114V at the shortest run, 111V at the end of the middle run, and 103V at the end of the longest run. Again, the other leg remains constant 120V throughout the park.

We have tried to balance the amperage between the 2 phases. We have some 30-amp RVs and some 50-amp RVs. Initially we were seeing more amp draw from the bad leg. Now that we have moved some of the breakers to the other pole, we are getting a balance (within 10-amps). This has not changed the Voltage Drop.

We have tried to attribute this to something. We have cut power to individual service meters with no change. When we pull off the RVs from the service, we get less of a drop but not that much. We were told that maybe our wire was undersized or that the wire was too long, but we don't see any Voltage drop on the other leg, and we have an aluminum 4/0 URD wire running 500 LF to the furthermost set of pedestals.

The electricity company came out and told us they thought it was our problem. We did all of the wiring ourselves, so I'm working to get an electrician to come out. I'm pretty good with this stuff, so I thought I would be able to track down the problem by now.

We don't even know what to check at this point... any suggestions would be super helpful.

Also, does a 200-amp service meter drop in 200-amps total or 400-amps (i.e. 200 on each 120V leg)? The electrical disconnects have 200-amp fuses; do they need to be 100-amp fuses instead?

telectrix · Aug 12, 2020

suspect high resistance of the faulty leg. could be corrosion at a terminal, can youmeasure the resistance of each section of the leg?

pc1966 · Aug 12, 2020

Do you have a clamp ammeter to allow you to check what the current in each leg actually is?

If the current is similar to the good leg, then it has to be high resistance: either bad terminations (as suggested above), or someone had put in the wrong cable. E.g. one size down or something, but some capliper and POWER OFF AND VERIFIED AS OFF would allow the size to be checked, but if fake (e.g. Cu layer on Al inner pretending to be solid Cu) it would need a bit more investigation.

Lucien Nunes · Aug 12, 2020

The fact that the drop is thought to be excessive on the same leg across all three runs but consistently low on the other leg, suggests it is something systematic that is wrong in the same way everywhere, rather than localised loose connections etc. A loose connection dropping a few volts at 100A gets quite hot, so although that possibility must not yet be eliminated, I would expect some signs of distress that you are not seeing.

My first strategy would be to estimate the expected voltage drops based on the measured load and the length and size of the cables. A cigarette-packet calc shows that with a completely balanced load (discounting the drops that occur in the neutral) on 4/0 Al cable you should get about 40-45mV drop per amp load on each leg at the end of the 500ft run depending on temperature. With 100A load per leg spread evenly along the cable that would give around 2-3V drop at the far end. I would double-check this and the distance pedestal-pedestal, to validate the expected drop per amp to within say 20% as a basis for comparison of the actual readings

I would then check the drops at each pedestal, to confirm that it is progressive along the cable sections and not all concentrated in a few places, and also make sure to measure the voltage hot-hot to confirm whether it is close to the sum of the drops from hots to neutral. What we do not know at this stage how much of the neutral current, if there is one, will return via parallel paths in the actual ground. Given that the measured voltage drop is an order of magnitude higher than expected, and that the neutral is a 4/0 conductor, I don't think this will be a major factor, but it will have at least some influence on the measurements if there is neutral current.

The most striking thing about your findings was the lack of change when you reassigned load from one leg to the other. There is a remote possibiity (and I hesitate to suggest it, but it's the best fit for the evidence so far) that a circulating current in your grounding system, possibly originating from another nearby transformer and service not part of your installation, is passing a stray current into your grounded neutral that is causing some drop along it in the absence of any load from the park. This pushes the neutral towards one leg of your supply, making the drop from your own loads higher than expected in one leg, but lower in the other. It would not affect the drops measured hot-hot, so if these came up consistenly close to the estimated value but the drops hot-neutral were askew, it would be worth checking. If you can attach a clamp meter around the neutral at each pedestal, you can probably eliminate this 'externally driven neutral drop' possibility very quickly, by confirming that the current is nowhere near high enough relative to the load current in the hots to be a key factor.

I take it that it would be very inconvenient to de-power the whole system. Can you get hold of a loop-impedance tester? This can be used with the system energised, to verify the approximate resistance of each conductor, from one pedestal to the next, by subtracting each reading from the previous one. By measuring each hot leg to neutral and then between the hots, you can discover whether the resistance of the three conductors in each cable section is equal, to within reasonable limits. If you can shut the whole system down and take genuine resistance measurements of the cables they will be more accurate, but on 4/0 you will have to use a 4-wire continuity tester, not just a regular multimeter, to get a reliable reading.

I am very interested to hear how this unfolds...

Note for UK readers - 4/0 URD is 141mm² alumin(i)um service cable, resistance 82μΩ/foot at 20°C, 100μΩ/foot at 75°C.

philbullock122 · Aug 12, 2020

OMG! Thank you all so much for these suggestions. We are going to spend the day trying to figure it out again. We are going to try to measure resistance on each leg. I don't have a loop impedance tester unfortunately. If I can't figure it out today I'll make sure an electrician comes out with one.

Lucien, I'm suspecting more and more that the voltage drop might be a normal occurrence due to the length of the 4/0 and the amps used. Your suggestion might be correct, so I did the math on a calculator I found online: 500FL of 4/0 aluminum URD pulling 100 amps will have an 8 volt drop at the end... so now I'm questioning everything. I might need to up-size the wire out to the first pedestals.

Does a 200-amp service meter drop in 200-amps total or 400-amps (i.e. 200 on each 120V leg)? The electrical disconnects have 200-amp fuses; do they need to be 100-amp fuses instead?

What is a good range for acceptable voltage drop? Is 115V OK? What about 110V? When does it start to become an issue?

I really appreciate the responses. This has been very helpful.

Lucien Nunes · Aug 12, 2020

I don't know what calculator you used or what data you fed in, but I can guess two possible things might not have been allowed for, that has resulted in the answer being four times what I made it (apologies if you have in fact got this right and I am in error.)
a) If the 100A load is spread along the cable instead of all at the end, the effective current over the full length is only half the total current, or vice versa. So you would calculate based on say 50A over 500 feet or 100A over 250 feet, which give the same result.
b) On a 3-wire circuit with balanced loading across the two legs, there is no neutral current therefore the drop seen by a 120V circuit using one leg is only half what it would be on a 2-wire circuit. (If this seems like magic, it's what makes the 120/240 3-wire system popular. The power is taken from the circuit at 120V but the circuit behaves like a 240V one, and the drop in the 240V circuit is shared between the two legs when in balance)

Therefore I think my figure of 2V (100A spread along one leg of a balanced 3-wire circuit) has come out of the calculator as 8V (100A at the end of a 2-wire circuit). I said 2-3V because the two legs won't be in perfect balance and will have some harmonic currents that don't cancel, so there will be some drop in the neutral, plus I have allowed for a worst-case conductor elevated temperature of 60°C, as this increases the resistance relative to room temperature.

AFAIK a 200A service means 200A on each leg (it does in the UK and generally in electrical engineering, but there can be strange legal definitions of things that vary by locale).

Also AFAIK the NEC recommends a maximum of 5% voltage drop total at the point of use, with up to 3% in either the feeder or the branch circuit. If you count the cable under consideration as the feeder and everything beyond the pedestal as the branch circuit, then 3% of 120V is 3.6V, which the cable should achieve even when significantly unbalanced loads are in use.

Double-check my numbers, I've got to go now as a storm is starting...

philbullock122 · Aug 12, 2020

Thanks again Lucien.

I have attached the calculator where I derived an 8.5V drop in our line at the end of the 6 pedestals. If you get a chance, take a look. I'm not sure what the "Number of Conductors" means. We do have ~100-amps of use, but it is spread out along the line at 6 different RV pads. The calculator doesn't take that into consideration.

Again, thank you so much for this information! I really appreciate it. Its a huge help to discuss this with someone that knows this a lot better than me!

Based on your numbers, 5% of 120V is 6V. So 114V at the end pedestal is an acceptable voltage. Thanks so much for this. Also, I'm glad to hear we have 200 Amps per leg.

Lucien Nunes · Aug 12, 2020

As I suspected, the calculator is giving you a figure approaching four times too high; twice on account of the load not all being at the far end, and twice on account of it being a balanced 3-wire circuit not a 2-wire single-phase circuit. So it does agree with my 2-3V estimate.

Megawatt · Aug 13, 2020

philbullock122 said:
First, I really appreciate any feedback you can provide. Its awesome to have a group of people that have a passion for this!

Our RV Park has a transformer dropping service down into three 200-amp meters (single-phase 240V service X 3). From each of these three meters, we have an emergency disconnect (3 in total), and then we daisy-chain 4/0 wire to each of our RV Pedestals. We have 6 campsite RV pedestals on each of the 3 lines (total of 18 RV pads). Each pedastal has in and out lugs, then have a 20-amp, 30-amp, and 50-amp breaker.

We are experiencing a voltage drop on only one leg. One leg (on all three services) remains fairly constant at 120V. The other leg (on all three services) drops to 118 at the emergency disconnects, and then looses voltage quickly. We are seeing 114V at the shortest run, 111V at the end of the middle run, and 103V at the end of the longest run. Again, the other leg remains constant 120V throughout the park.

We have tried to balance the amperage between the 2 phases. We have some 30-amp RVs and some 50-amp RVs. Initially we were seeing more amp draw from the bad leg. Now that we have moved some of the breakers to the other pole, we are getting a balance (within 10-amps). This has not changed the Voltage Drop.

We have tried to attribute this to something. We have cut power to individual service meters with no change. When we pull off the RVs from the service, we get less of a drop but not that much. We were told that maybe our wire was undersized or that the wire was too long, but we don't see any Voltage drop on the other leg, and we have an aluminum 4/0 URD wire running 500 LF to the furthermost set of pedestals.

The electricity company came out and told us they thought it was our problem. We did all of the wiring ourselves, so I'm working to get an electrician to come out. I'm pretty good with this stuff, so I thought I would be able to track down the problem by now.

We don't even know what to check at this point... any suggestions would be super helpful.

Also, does a 200-amp service meter drop in 200-amps total or 400-amps (i.e. 200 on each 120V leg)? The electrical disconnects have 200-amp fuses; do they need to be 100-amp fuses instead?

Phil what it sounds like to me is your problem is starting at where your primary voltage is coming in which I Can’t understand What size wire you used to feed 3-200 amp services And how you did it. What really should have taking place that the power company feeds a 400 amp panel and then you install 3-200 amp double pole breakers to feed each disconnect and that would have made it easier for you to determine is it really a power company problem or your problem. What size wire and how did the power company deliver it to you? At what point are you having a voltage drop and if it’s all 3 disconnects then it has to be coming from tha actual service point. 110vac is well within your threshold of voltage but no lower and 220 is fine but also no lower. You mentioned that you was having higher amperage draw at the end of these services, that’s because simply the lower the voltage the higher the amp draw

marconi · Aug 13, 2020

Dear Megawatt: You may want to revise with your reasoning - 'You mentioned that you was having higher amperage draw at the end of these services, that’s because simply the lower the voltage the higher the amp draw'

before others comment.

philbullock122 · Aug 13, 2020

So just to check in with you again, the electric service provider is going to install a voltage tracking device this week. We are still experiencing higher than normal voltage drops in the lines. Everything remains the same.

At the box, the voltage is fairly normal, 120V and 119V in. These don't appear to be dropping out of regulation (5%) anymore. We still have a significant drop on each of our runs.

We have balanced the amps to make it easier to determine if we are experiencing a realistic voltage drop. Right now, we are pulling about 70 volts per leg on one of our runs. On one leg at the end, we have 118V and the other leg 111V. I can't explain why one leg would drop lower than the other if they have the same amps and are the same length. At each pedestal, the voltage drop steps down, so we are not seeing a large hit on voltage at one location.

Still very odd. Thank you all for the suggestions.

Lucien Nunes · Aug 13, 2020

Good stuff, let us know the findings. If you like the posts, feel free to hit the happy buttons

philbullock122 · Aug 13, 2020

Lucien Nunes said:
Good stuff, let us know the findings. If you like the posts, feel free to hit the happy buttons

Will do! Didn't know it was important

marconi · Aug 13, 2020

Philbullock122: LN has probably already rigged up a scaled down electrical equivalent of your installation in his electrical emporium, fully instrumented, with his electronic acolytes running tests to develop a computer model of your set-up - so feed him with data - you will not be disappointed. I don't joke.

Lucien Nunes · Aug 13, 2020

Haha, reminds me of the method apparently by Edison's team when laying out and sizing distribution cables. They had physical analogs of the network made with resistance wire that they could proble with a meter to discover the voltage drops under certain load conditions. Their aim was to equalise the drop to every consumer so that they could serve as a large an area as possible from a single point while keeping the voltage within tolerance. Definite echoes of that here.

Megawatt · Aug 13, 2020

marconi said:
Dear Megawatt: You may want to revise with your reasoning - 'You mentioned that you was having higher amperage draw at the end of these services, that’s because simply the lower the voltage the higher the amp draw'

before others comment.

@marconi that’s what I told him the lower the voltage the higher the amp draw
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philbullock122 said:
So just to check in with you again, the electric service provider is going to install a voltage tracking device this week. We are still experiencing higher than normal voltage drops in the lines. Everything remains the same.

At the box, the voltage is fairly normal, 120V and 119V in. These don't appear to be dropping out of regulation (5%) anymore. We still have a significant drop on each of our runs.

We have balanced the amps to make it easier to determine if we are experiencing a realistic voltage drop. Right now, we are pulling about 70 volts per leg on one of our runs. On one leg at the end, we have 118V and the other leg 111V. I can't explain why one leg would drop lower than the other if they have the same amps and are the same length. At each pedestal, the voltage drop steps down, so we are not seeing a large hit on voltage at one location.

Still very odd. Thank you all for the suggestions.

Phil that’s what we are here for is to help you figure out where your problem is originating from

marconi · Aug 16, 2020

Philbullock122: Only of you feel confident and have suitable test equipment and PPE eg a Fluke voltmeter and Fluke ampclamp could you measure some voltages and currents?

The points for measurements I have shown in the second diagram. The table to fill is in the third attachment.

Could you also clarify the wiring just before the 3 meters - see my first diagram.

Last please indicate the places where the neutral is grounded.

marconi · Aug 16, 2020

Philbullock122: In an attempt to flush out further information you could:

1. Turn off all the disconnectors D1-D3. Then turn one and only one of these disconnectors on in turn and measure the L1-L2, L1-N and L2-N voltages and if you have an ampclamp I1, I2 and IN.

2. If you can do it - I suspect your POCO would have to do it for you though - swap over L1 and L2 at the inputs to the meters and see if the 'drooping leg' switches over too. I'd be keen to know if the 2V drop along the drooping leg line conductor between the transformer and disconnector swaps over to the other line too - See your #1 third paragraph.

3. See attached diagram. Find out if the RV have a battery charger and what type they are and whether they are connected permanently and to which leg ie: are they all 110V types and connected to the same drooping leg? (Battery chargers have power factor less than 1. Plus so many such chargers energised from the same supply may be generating harmonic currents which might add up in the neutral causing further voltage drop and heating of the N conductor thence further voltage drop).

Try turning all the battery chargers off and doing the voltage and current measurements.

Note: Maybe, 'cos it does not matter much for one RV - the manufacturer of RVs always put the 110V supply battery charger as standard on one leg which happens to be your drooping leg.

4. Do any of the vans have solar PV systems which have a grid-tied inverter to feedpower back into the 120-0-120 system?

marconi · Aug 17, 2020

Philbullock122: In #1 you mentioned you did all the wiring yourself/selves. When you daisy-chained a run of 6 pedestals did you simply daisy chain or did you introduce a cross-over between pedestals as well as a method to achieve some distribution of RV loads across and along Leg1 and Leg2? Please see my attached diagram.

philbullock122 · Aug 17, 2020

Just to update everyone, the problem was discovered to be on the electricity provider side. The wires running from the transformer into the gutter and divided between the three meters was not large enough to carry the load of the three meters (or a wire was bad).

The electricity company added a second 4/0 copper wire running from the transformer to the gutter and into our meters. The voltage has now returned to normal and we are getting normal readings at our boxes.

We are getting a voltage drop of about 2 or 3 volts down the line, and we expect this to be normal according to Lucien's comments.

I really appreciate everything you helped with and will reach out if we have any other issues. Thanks so much everyone!

Phil

marconi · Aug 17, 2020

Magnolia — Triple-B RV Park - https://www.3brv.com/magnolia

Your RV parks look superb places to stay.

PS: I am following avidly the run up to the Presidential election. Do you have a prediction for the outcome? From what I have read it is hard to say.