mworl avatar image

Auto Transformer split phase, neutral bond question

I have a 230v MP2 feeding an auto transformer for 120/240v split phase to the panel. It is currently connected to the hot legs of 120/240 shore power, shore neutral not connected to anything.

I have a 3200w iso transformer on the way to step up 120v shore power and remove the incoming neutral-ground bond.

In either of the above scenarios I do not take on the shore neutral. Am I correct that I need a N-G bond after the AT 100% of the time, not just when the MP2 is inverting and activates the relay?

Next question:

I noticed that after the AT my legs have different voltages, ~120 and ~126. L1-G reads ~3v higher voltage than L1-N as expected, however L2-N is ~3v higher than L2-G. N-G is ~3v.

Edit to add: As I am passing through shore power with the inverter in charge only mode the internal ground-neutral relay is not functioning. When my ve bus to usb adapter arrives I will disable the relay entirely if my solution of a hardwired bond is acceptable.

To experiment I created a N-G bond with a jumper at the AT output. Now L1-N = L1-G and L2-N = L2-G. N-G = a few mV with the jumper in place. However L2 still 6v higher than L1. Is this normal?

Anyone see any safety issues here? I can not think of any situation where I would not want N-G bonded on board with this setup.

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2 Answers
Kevin Windrem avatar image
Kevin Windrem answered ·

Your problems occur because the incoming feed has neutral bonded to safety ground before you get it. The autotransformer essentially makes a "different" neutral based on incoming offset between L1 and L2 relative to INCOMING neutral. Any L1/L2 load differences could also offset your OUTGOING neutral.

There should never be a connection between incoming neutral and outgoing neutral or between either neutral and safety ground as long as shore power is in use. There is no direct bond between outgoing neutral and safety ground nor should you attempt one.

The internal safety ground/neutral will connect L2 or L1 to safety ground and should be disabled.

When shore power is not being used (AC input relay open), then you need to provide a neutral to safety ground at the output of the autotransformer.

Adding an isolation transformer changes everything since it will sever the shore power neutral/safety ground. With the isolation transformer at the input of the multi, you will need to bond the autotransformer neutral tap to safety ground and insure the Multi's internal safety ground relay is disabled. The bond needs to be active when on shore power or not.

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Ah that makes sense! Thank you for this explanation.

So 120v through the isolation transformer will work as I expected but when I am pulling 240v off a split phase (even without the neutral) the existing load imbalances between L1 and L2 before it gets to me are offsetting my created neutral.

But why does my on board L2 read a higher voltage to neutral than ground while L1 is the reverse? Is this also a result of the upstream N-G bond?

It seems that my best option may be to simply pull from only one leg through the isolation transformer even when 240v is available. I dont actually need the full 12kva, especially with power assist available from the MP2. I have enough room for a 5kva isolation transformer.

If I need to run my welder at high amperage or decide to add an on demand water heater I can plug them directly into shore power independently of the on board grid.

I suspect the voltage difference you see IS due to the up-stream N-SG bond. It could also be due to imbalance in your loads since a higher load on one leg will result in a lower voltage on that leg and offset the neutral.

The best approach would be to use an isolation transformer that can be reconfigured for 120 volt or 240 volt shore power and always output 240 volts. Maybe you can exchange what you just ordered for one that has split primary. You may also wish to look for a transformer that has taps so you can accommodate 208 volt (2 legs of 120/208 3-phase) power. That is 208 in / 240 out.

mworl avatar image mworl Kevin Windrem ·

I have been exploring the idea of a reconfigurable transformer. My biggest concern is making it accident proof as I have my wife and kids with me. No one else will be opening the transformer though so its just a matter of correctly wired shore power adapters. I can leave it on 120v most of the time for convenience but have the flexibility to reconfigure if I want to.

I found this: Hammond Power C1F010WES. 10kva PRI:120/208/240/277V, SEC:120/240V. I have just enough room to mount it too. Most other manufactures transformers over 5kva are too physically large for my install.

I think this is the way to go.

I think this may take more than just correctly wired shore power adapters. You want to take in 240 volts when available and ignore the neutral. But when 120 volts is all that's available you need 1 hot leg plus a neutral.

Let's assume you use the typical RV 120 volt 30 amp plug and convert it to a 120/240 volt 50 amp receptacle. This "dog-bone" connects the hot of the 120 volt plug to BOTH hot legs of the 50 amp.

A switch could be used safely to select 120 volt and 240 volt shore power since you'd be switching between using both hots which would be either 240 volts or 0 volts, and one hot and the neutral which would always be 120 volts.

A light between L1 and L2 could signal "240 volts available" to help you make the wisest choice.

A relay between L1 and L2 could also automate this selection.

Adjusting for 208 gets a bit more complicated and setting a switch to the wrong position puts out of spec voltage on the inverter input. What happens in this case is unknown: reject with a waring, passing the voltage to your loads, smoke?

mworl avatar image mworl Kevin Windrem ·

I have a 4 conductor shore cable (including ground) with a twist lock ss2-50 connector. Before installing the inverter and AT I used a dog bone from ss2-50 to 14-50 to plug directly into split phase power.

Now I will disconnect the N in that dog bone and also make another dogbone for 120v/30a outlets that leaves L2 unconnected in the shore cable. Then that 120v dogbone can be adapted to 15/20a outlets as well. Likewise the 240v 14-50 dogbone can be adapted to other 240v outlets.

This way if the transformer is configured for 120v input and someone plugs into 240v using the 240v dog bone nothing happens because L2 is disconnected at the transformer. If they then switch to the dog bone compatible with 120v outlets L1 and N will be correctly connected. Probably be easier to make a diagram but you get the point.

Wife and kids can operate the system safely because selecting the wrong dog bone is harmless. I considered using a big rotary switch to configure the transformer but then its too easy to make a mistake. Better if I am the only one who can reconfigure the transformer.

Johannes Boonstra (Victron Energy Staff) avatar image
Johannes Boonstra (Victron Energy Staff) answered ·


Whats needed in your system is that the isolation transformer is essential here.

The transformer will isolate the system from the grid which allows you to create your own grounding system

The secondary side from the isolation transformer (the side which is connected to the multi) should NOT have the ground bounding as you want to threat the two lines as 2 x phase 240V floating from ground.

In the Multi you do the same, so ground relay OFF, as in inverter mode also the 2 x phase keeps floating from ground.

On the At you create the system N, that's the centre tab. So that centre tab needs to be bounded to ground. Then the whole system has a common N and L1/L2 with each 120V to ground. Rcd and such should work fine. So NOT connect loads before the Multi !!

second question, do NOT use the transformer with a centre tab to ground bound when you don't have an isolation transformer as then the AT will try balance the grid, which will be a recopy for smoke.

there can be a voltage difference depending on current flowing through the windings, 1-5 Vac isn't an issue I would say )

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Thank you for the response. I'll be putting a 10kva isolation transformer at the shore input.