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Old 03-14-2019, 10:08 AM   #1
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Smaller 220v supply for shore power

All,

I have 220 power at my dock for a lift and would like to use that power to supply shore power to my trawler that is set up for a 50a 220 service.

However, the wiring I have on the dock won't support 50a, and will more likely only support 30a.

Is there anything I'm missing in setting up a 220v 30 plug for my shore power?

Sure I realize that I won't be able to run everything, and wouldn't have the need to. The main goal is to simply keep the batteries charged. Would like to have AC for those few summer nights when we occupy the boat overnight. AC specs for breaker are 20a to 14a for the 16btu unit, and 15a to 10a for the 12btu unit. Need to check the sizes of the breakers.

No need to ever run the grill, stove or high draw stuff, with the exception of rarely running the AC.


Thoughts?
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Old 03-14-2019, 10:34 AM   #2
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I don't see a big issue as far as overcurrent protection. Your dock breaker must be 30A, and needs to be verified, along with the wire size (10AWG). A 30A rated dock lift at 240V is quite large...

Is this going to feed a isolation transformer in the boat? Do you have a 3 or 4 pin shore power inlet?
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Old 03-14-2019, 11:56 AM   #3
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Diver Dave,

Yes, forgot to mention, My boat requires 220v as an input, which runs through a Charles 93-IXFMR12I-A Isolation Transformer, then into the boat.

If I could just put a simple 110v supply to charge the batteries, I'd probably do that. However, nice to have the extra power for the AC, a dehumidifier, and little things.

It's wired with #10 wire. The boat plug is a 230a 50, three prong. A bit confusing as I'm not sure where to tie in the ground.

However, I'll have a marine electrician overlooking this.

Just want to get my ducks in a row.
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Old 03-14-2019, 12:04 PM   #4
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That's a nice xfrmr. 235 lbs of it! As you already know, it will make 120 x 2 on the secondary.

The advantage of using the transformer, even for small 120V loads, is that it will tend to block most line surges, due to lightning. To be precise, it will do a great job eliminating common mode transients, and be a slight help with normal mode transients.
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Old 03-14-2019, 12:07 PM   #5
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" 230a 50, three prong"

Is this a home range or dryer plug?

Most of the home 220v is still 2 legs of 120v .

The missing prong is usually a ground . Black, White and Red wires , no green.

I have never seen a US boat that uses 220v for a device , most split it into 2 120v legs .

Do you have a 220V TV or light bulbs?
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Old 03-14-2019, 12:18 PM   #6
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You could wire a standard 50/250 receptacle like found at a marina. But put a breaker on it that limits current to what the existing feed wiring can handle. So put a 30A breaker in the receptacle feed. May not be able to run everything on the boat, but it will probably be a non-issue with a little load management.
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Old 03-14-2019, 12:31 PM   #7
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FF- Kind of loose terminology is common when discussing USA power: Some call it 110/220, some call it 120/240, dock power is labeled 125/250. As long as it is 60Hz USA power with the L1/L0/L2 arrangement, it's the same stuff. On boats if you meter it is common to see a low of 110 to a high of 125 and everything works fine.
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Old 03-14-2019, 12:36 PM   #8
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The hardware is rated at 250/125V. The service is 240/120 rated. 110/220 is "grandpa" terminology. House service has increased over the last 50 years, about 10 volts worth. Not really sure why.

ps; part of my life at work is writing industrial electrical stuff manuals. Have to know the diff between "rated" and "allowable range" when talking about mains power. My stuff usually has DC rated inputs as well, and they are not the same.

example:
Mains rated Voltage / frequency; (standard) 100 – 240 Vac, 50/60 Hz; OR 125 - 250 Vdc

Mains operating range EN60255-1; +/- 10% of rated ac V; +/- 5% frequency; +/- 20% of rated dc V.

(there is different meanings for the "-" and the "/" too!)
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Old 03-14-2019, 07:38 PM   #9
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Good replies, thx!

Mine is is labeled as 125/250, on the plug. But the transformer only takes the 250, not 125.

I have a three prong plug, and from my understanding most Mainships have the same. Still wondering how the ground fits in.

Heck the reverse Y matches.
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Old 03-14-2019, 07:44 PM   #10
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Ground is the metal ring around the prongs.
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Old 03-14-2019, 07:45 PM   #11
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Three prongs there, but the metal circling the the three prongs is ground
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Old 03-14-2019, 07:46 PM   #12
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Ground will likely go to either a galvanic isolator or to the transformer core/bonding point. Do u have a good view of the transformer input?
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Old 03-14-2019, 07:57 PM   #13
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Also, the N (neutral) prong not used for ur boat. The cable would likely have a neutral conductor, but ur transformer wont need it.
U will need to connect to L1, L2 and GND at ur dock receptacle.
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Old 03-14-2019, 08:33 PM   #14
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Stopped at a friend's dock in FL. Their guest shore power receptacle was the standard 125/250 volt 50 amp found on most power pedestals. The breaker was a 30 amp 240 volt as the wire feeding the dock was only 10 gauge. No big deal; met all electric codes; was plenty of power. Really no different than using a Y cord adapter and two 125 volt 30 amp receptacles on a power pedestal.

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Old 03-14-2019, 09:53 PM   #15
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Quote:
Originally Posted by Seevee View Post
All,

I have 220 power at my dock for a lift and would like to use that power to supply shore power to my trawler that is set up for a 50a 220 service.

However, the wiring I have on the dock won't support 50a, and will more likely only support 30a.

Is there anything I'm missing in setting up a 220v 30 plug for my shore power?

Sure I realize that I won't be able to run everything, and wouldn't have the need to. The main goal is to simply keep the batteries charged. Would like to have AC for those few summer nights when we occupy the boat overnight. AC specs for breaker are 20a to 14a for the 16btu unit, and 15a to 10a for the 12btu unit. Need to check the sizes of the breakers.

No need to ever run the grill, stove or high draw stuff, with the exception of rarely running the AC.


Thoughts?
If your boat (and the isolation transformer) were wired by Mainship for the US market, your setup will look like the ABYC E-11 diagram attached.

In St. Pete, your residential electric service will be "125/250 volt single phase" (no such thing as "220" anymore).

If your 250v boat lift was wired professionally by a licensed electrician in Florida (within the last 30 years or so), you will at least have a black wire and a red wire (plus the green ground). If there is ALSO a white wire present, then your dock is wired for both 125v (two 'legs') and 250v. When you look at the diagram attached, you will see that the white wire is not connected aboard the boat.

If your setup looks like the diagram attached, then you only need to make sure you connect red-to-red, black-to-black and green-to green.

Also...please remember, Power (watts) equals Volts x Amps. This means that 30 amps at 250v is like 60 amps at 125v. Either way you get 7.5 kVA (roughly 7,500 watts), which should be more than enough to meet your needs. You would be able (for example) to run at least one of your AC units, along with microwave, etc.

Voltage drop may be a problem if your current wiring to the dock is undersized. Depending on the length of your run from your panel to the dock, you may need to use larger conductors. For example, to keep your voltage drop under 3% at 30A, you would need to use 8/3 wiring for a 100ft run. Voltage drop is important for certain appliances (air conditioners and refrigerators for example), because under-voltage will cause them to overheat and fail.

You will need to discover the size of the wiring from your house to the dock, if it is less than 8 GA, you will want to upgrade it.
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Old 03-16-2019, 06:05 AM   #16
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River Guy,
Thanks for the good explanation. That makes it much more clear.
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