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Old 03-31-2014, 02:02 PM   #1
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208V 3 Phase to 240V 2 Phase

My marina has 50AMP shore power which provides 2 x 120V legs (taken from 3 phase) and a neutral to each slip.

I've just fitted a MS4448PAE 240V inverter which needs 240V two phase power. I cant connect the shore power as I need two 120V legs that are 180 degrees out of phase, not 120 degrees like the marina provides.

Can anyone recommend an isolation transformer that might help with this problem? I believe a delta-wye transformer might output 2 x 120V legs that are 180 degrees out of phase but not sure? The inverter needs around 5KVA.
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Old 03-31-2014, 02:08 PM   #2
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Just a terminology nit, but it is typically called "split phase" not 2 phase.

What you are encountering is quite common and most likely not an issue. The two 120v legs will give you 208V rather than 240V, but most appliances are rated down to 208V for just this reason. Check the rating on the inverter, and I bet you will be fine.
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Old 03-31-2014, 02:16 PM   #3
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Are you sure the legs are 120deg out? Put meter between L1 and L2 and read volts. Also measure between L1 and N. And L2 and N. If from L1 to L2 is exactly twice the other two, it is 180deg out. Pretty uncommon in my experience to see 208.
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Old 03-31-2014, 02:41 PM   #4
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Originally Posted by Ski in NC View Post
Are you sure the legs are 120deg out? Put meter between L1 and L2 and read volts. Also measure between L1 and N. And L2 and N. If from L1 to L2 is exactly twice the other two, it is 180deg out. Pretty uncommon in my experience to see 208.
The voltage for both L1 and L2 to N = ~120V.

Voltage between L1 and L2 is ~208V (not 240V) which is why I think I have 120 degree phases here at Sausalito Yacht Harbor.

Everything on the boat runs just fine on 208V except the new inverter. The way it is wound it will only run either on 1 leg @ 120V or 2 legs split phase. If I connect the marina shorepower with 2 legs at 120 degree phases I get a dead short between L1 and L2. If I connect the split phase legs from my genset I get no problems at all.

I think the answer is either just to run my inverter on 1 leg and make do with 50% of the power or somehow convert my 3 phase shorepower to split phase on the boat. I am needing a isolation transformer anyway so I was hoping this could be used to solve the problem.

However I note that most 'marine' inverters are only single phase input and that if you want to use both L1 and L2 of shorepower on the boat then you need to buy 2 inverters, one for each leg. I guess some marina's offer split phase and some offer 3 phase and so I am being too 'clever' to avoid buying two inverters. It certainly is not possible to get 2 legs in and out from the one inverter unless you can convert to split phase first.
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Old 03-31-2014, 03:00 PM   #5
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Why would you need to run an inverter when you're connected to shore power?
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Old 03-31-2014, 03:11 PM   #6
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Why would you need to run an inverter when you're connected to shore power?
The MS4448PAE is a charger/inverter with a pass through. So when on shorepower it passes both legs through to the boat sub-panel and simultaneously charges the battery bank. If I can only connect 1 leg to the inverter then I can only pass through half the power that I normally would be able to when at the dock.

The only way to pass both legs through is to bypass the inverter entirely when on shorepower but then I lose the charger and nice autoswitching that the inverter would offer if only I had split phase.
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Old 03-31-2014, 05:00 PM   #7
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If it were me I'd run the inverter in 120V mode when on shore power.

Thats a problem with marina power, my marina is the same, you can get a 50 amp source that they say is 240V single phase, but its actually two phases of a three phase 208V wye system.
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Old 03-31-2014, 05:14 PM   #8
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Originally Posted by lelievre12 View Post
The MS4448PAE is a charger/inverter with a pass through. So when on shorepower it passes both legs through to the boat sub-panel and simultaneously charges the battery bank. If I can only connect 1 leg to the inverter then I can only pass through half the power that I normally would be able to when at the dock.

The only way to pass both legs through is to bypass the inverter entirely when on shorepower but then I lose the charger and nice autoswitching that the inverter would offer if only I had split phase.
I didn't realize it was an "inverter/charger" not just an "inverter". That makes a difference.

My question now is: Is it possible to buy an inverter/charger that will work with your marina's power rather than trying to convert a brand new product?
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Old 03-31-2014, 10:11 PM   #9
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OK, if the inverter can't handle 208V 3-phase, then a a 240V isolation transformer should do what you need. And like you say, it has a bunch of other benefits too.

On the primary side you would connect the L1 and L2 from the shore connection. The shore neutral can be left off. On the boat side, you get a new, isolated L1 and L2, and pick up the ship's neutral from the transformer center tap. Now you have L1 and L2 that are 180 deg out of phase relative to the neutral. All the mid to large Nordhharles are set up this way, Charles makes a number of different capacity isolation transformers that will do the job. They have good wiring examples as well, and show how to properly connect up the grounds on each side.

If you go this route, there are a few other things you can do with the transformer which are handy.

First, you can run 30A 120V into the transformer on the shore side, and get 240V split phase out the secondary side. It's limited to the 3600W or so of shore power that a 30A 120V shore cord can provide, but it can be very convenient when only 30A service is available.

Second, You can wire the transformer to take the 208V that you get from a 3-phase dock, and boost it to 240V on the boat side. Total power is limited by the 208V 50A shore side, but 240V is arguably better for most US appliances than 208V.
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Old 04-02-2014, 06:05 PM   #10
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Have you consulted with Magnum about this issue? They are usually very helpful
Regardless, isolation transformers are a great thing to have for many other reasons; I'm a little surprised the boat in your avatar (if that is the OP's boat) doesn't have them.
I highly recommend the Charles transformer line. I was fortunate to buy a boat with their IsoBoost transformers, which overcome all sorts of shore power issues. They are also very helpful to consult on the phone.

http://www.charlesindustries.com/main/ma_iso_boost.html
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