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Old 04-08-2021, 17:58   #1
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Shore power for dummies

Been doing some googling and canít really grasp the differences in shore power. Terms like phase and legs arenít really clicking with me.

Bottom line, why are some boats 30a and some 50a, and whatís the difference? For an off grid boat running off batteries, does it matter if the boat is 30a or 50a?
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Old 04-08-2021, 18:55   #2
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Re: Shore power for dummies

Iím really basic terms the amperage reflects how much power you can get from the circuit. A 50A circuit can provide 50/30 = 1.67x the amount of power that you can get from a 30A circuit - at the same voltage.

If you are US-based 30A is usually 120V single phase - that is you can draw 30A from a single 120V circuit. 50A is usually 240V split phase - there are actually two 50A x 120V circuits. So, if you are using 120V there is actually 100A (or about 3.3x available power compared with a 30A circuit). You can also use this as 50A@240V or some combination of 120V and 240V that adds up to no more than 50A on either leg of the split phase.

If all you run is a battery charger, your vacuum, and maybe a small heater or a single, small air conditioner then a single-phase, 30A circuit is generally sufficient - and the most commonly available. For a bigger boat, or if you want to run lots of heat or AC then 50A becomes necessary.

Without discounting for load factors, etc. a 30A circuit can provide 3600W, a 50A circuit (because the voltage is doubled in the standard shore power configuration) can provide 12000W.

Does that help at all?
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Old 04-08-2021, 19:01   #3
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Re: Shore power for dummies

It does, thank you. I converted to 50a from 30a and don’t know why, especially since I don’t plan on using shore power. But because I converted, I’m being advised to upgrade my inverter from 3000w to 5000w, to take advantage of the 50a shore power circuit at pass through. But if I don’t care about shore power, why should I care about 50a pass through?
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Old 04-08-2021, 20:06   #4
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Re: Shore power for dummies

Basically,
  1. you list out all the things you expect to run off of AC,
  2. then you remove the 1500w hair dryer and the 1200w popcorn-maker, the waffle machine and any other silly or extravagant AC loads that you can live without
  3. then beside each of the remaining items, you write down the rated wattage of each,
  4. then you make a guess at which of those AC loads would reasonably be on at the same time, add up those wattages, divide by 110, and you will be left with a number which is the AC current they would require...
  5. and if that number is still scary-big, go back to 2 and edit some more.
... from this you get an idea of whether you need a 30A or a 50A shorepower connection. Air conditioning is a big load, and a reason why some go to 50A.

It's even more important to look hard at what AC appliances you want to run from the inverter. You need to be pretty aggressive with this if you want your battery bank to last longer between charges.

If the AC stuff you plan to run at the same time from inverter is less than say 2400W continuous, then a 3000w inverter should be fine.
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Old 04-08-2021, 20:29   #5
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Re: Shore power for dummies

Lake Effect pretty much covered it. You need to make a list of things you want to run on shorepower, figure out how much power they use, and then make a decision.

Personally, for us, we have 2400W of inverter capacity and haven’t bumped up against the top yet, but we do run an energy “light” boat - no generator, solar provides > 95% of battery charging. If you have a generator and want shore power to handle everything it does then that needs to go into the equation. If you don’t have a generator then how much AC stuff do you really have?
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Old 04-08-2021, 22:46   #6
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Re: Shore power for dummies

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Originally Posted by Networker View Post
It does, thank you. I converted to 50a from 30a and donít know why, especially since I donít plan on using shore power. But because I converted, Iím being advised to upgrade my inverter from 3000w to 5000w, to take advantage of the 50a shore power circuit at pass through. But if I donít care about shore power, why should I care about 50a pass through?
Gernerlly you donít pass the whole boat through the inverter anyways... A 3000w from 50a shore plug is fine. Just put in a 30a inverter breaker. The hot water tank etc will be before inverter. And using the other 20a.

Why did you upgrade the shore inlet? Sounds like someone is selling you crap you donít need.
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Old 04-08-2021, 23:46   #7
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Re: Shore power for dummies

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Originally Posted by smac999 View Post

Why did you upgrade the shore inlet? Sounds like someone is selling you crap you don’t need.
It was an EU wired 16a / 220v boat, so it had to be done to convert it to NA 110v shore power.
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Old 05-08-2021, 05:38   #8
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Re: Shore power for dummies

I find Lake-Effect's electrical advice to be sound but the lack of love for waffle irons concerning...
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Old 05-08-2021, 09:23   #9
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Re: Shore power for dummies

Since you say you will never use shore power, then none of this matters to you. However, 'never' is an awful long time. To my knowledge, most marinas are 30a 110v service. Just because they may have two receptacles on your side of the post does not necessarily mean they are opposite phases. (220 together.) You can run 30a through a 50a cord if you can find an adapter, but not the other way around.

If you are running a generator, then this will matter to you. You will need to know what it can produce.

What we did on our boat was to add a dedicated 30a 110v inlet for our air conditioner. It was cheaper than converting to 50a 220v service since we already had two 30a ELCI main breakers and did not need to upgrade the wires to handle the higher amps. The office clerk did not know if our pedestals are two 110v of same phase or two 110v out of phase that could be connected by an double pigtail to make a single 50a 220v service. We did convert both inlets to SMARTPLUG however.
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Old 05-08-2021, 09:43   #10
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Re: Shore power for dummies

Quote:
Originally Posted by Networker View Post
It does, thank you. I converted to 50a from 30a and donít know why, especially since I donít plan on using shore power. But because I converted, Iím being advised to upgrade my inverter from 3000w to 5000w, to take advantage of the 50a shore power circuit at pass through. But if I donít care about shore power, why should I care about 50a pass through?
It worries me that you say you converted to 50amp from 30 amp and don't know why and have asked this question. I seriously hope you meant to say that you had a qualified marine electrician do this conversion. It's pretty complicated and involves replacing wires and the electrical panel.

That said, others have explained it, 50 amps is more power than 30 amps and allows the boat to run things like multiple air conditioners and electric stoves. Only when connected to shore power, of course.
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Old 05-08-2021, 09:47   #11
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Re: Shore power for dummies

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Originally Posted by OS2Dude View Post
........ To my knowledge, most marinas are 30a 110v service.
In my experience, most marinas on the eastern portion of the USA have both 30 amp and 50 amp service available although not necessarily at every slip.

I find that I often have to use a 50 to 30 adapter to plug my 30 amp boat into a 50 amp receptacle.
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Old 05-08-2021, 13:15   #12
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Re: Shore power for dummies

Lake Effect, as is usually the case, provides a good, concise answer. I would only add that the 30A shore power is not meant to be used at 30A continuously. The manufacturers of shore power cable recommend a continuous draw of 80% of 30A, or 24A. Marine How To recommends no more than 70% or 21A of continuous draw. The difference is to account for spikes in usage.

If this was mentioned already I missed it and apologize for any duplication.
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Old 05-08-2021, 13:34   #13
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Re: Shore power for dummies

We have a boat originally set up for 230 50 amp. The previous owner added an isolation transformer on the 230 primary with two secondaries, 115. Some marinas charge extra for 50 amp connection so I added a 30 amp single phase shore connection and switch gear to isolate the two at the transformer (or a short can happen). The 230 is preferred and the 115 is not isolated. Having both options, we can use shore power on both sides of the pond. Also, in the marinas without 50 amp, we ca still run the charger. We have a 30-30 to 50 combiner that takes two opposite phased 30s to make 50. It trips if the legs are not properly phased.

After all of that, we live on the hook 99.9%. Generator is 12.5 kw. Lots of solar.
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Old 05-08-2021, 13:36   #14
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Re: Shore power for dummies

We have a boat originally set up for 230 50 amp. The previous owner added an isolation transformer on the 230 primary with two secondaries, 115. Some marinas charge extra for 50 amp connection so I added a 30 amp single phase shore connection and switch gear to isolate the two at the transformer (or a short can happen). The 230 is preferred and the 115 is not isolated. Having both options, we can use shore power on both sides of the pond. Also, in the marinas without 50 amp, we ca still run the charger. We have a 30-30 to 50 combiner that takes two opposite phased 30s to make 50. It trips if the legs are not properly phased.

After all of that, we live on the hook 99.9%. Generator is 12.5 kw. Lots of solar.
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Old 05-08-2021, 13:37   #15
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Re: Shore power for dummies

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