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Old 02-08-2020, 14:32   #1
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Shore Power Confusion

I’m confused about shore power breakers, wire sizes, and amperage.

If I plug a y connector into a 50 amp 240 service, I can get 2 outputs at 120/30.
How are the 30 amp wires protected when the breaker is 50 amps?

If I plug a reverse y into (2) 120/30 services (assuming opposite phases) I can get 50 amp 240.
How can the 30 amp breakers provide 50 amps?

Y and reverse Y are sold by the big guys (Hubbell / Marinco) so I assume they must be safe, but how can this be?

I just can’t figure out the math.

Jeff
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Old 02-08-2020, 14:41   #2
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Re: Shore Power Confusion

Half right; in the US the 50 amp power is two legs 120v or so above ground so you have two separate legs of 50amp 120v power or one 50 amp circuit between L1 and L2. It is available at 50amp service but the boat side breaker is probably 30 amps so that will limit it.

A reverse Y (with built in safety relays) can combine the L1 from 30a outlet to another L1 (if on different phases). But you don't get 50 amps, it's still 30 amps at 240 volts.
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Old 02-08-2020, 15:10   #3
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Re: Shore Power Confusion

Big guys make 240/50 dock side convert to single 120/30 boat side too.

30 amp cord set would probably not be properly protected by 50 amp breaker on dock? Breakers are supposed to protect the wires down stream.

Are there different rules for “extension cords.”



https://www.marinco.com/en/121a
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Old 02-08-2020, 15:11   #4
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Re: Shore Power Confu

Quote:
Originally Posted by Jpernick View Post
How are the 30 amp wires protected when the breaker is 50 amps?
I just can’t figure out the math.

Jeff
They aren’t. The boat is protected by the 30 amp breakers in it, but the shorepower cable would require a draw of 50 amps in the 30 amp cable to trip the breaker.

But think about this for a minute, if you plug in a thin extension cord in your house and you have 20 amp breakers in your box, how is the thin extension cord protected?
Answer is it’s not unless it has its own breaker which I have never seen.

Your shore power cord is nothing more than an extension cord, why does it need protecting?

On edit to further clarify, the boats CB prevents more than 30 amps being drawn by the shorepower cord by the boat. The only way the cord itself could draw power on its own is by an internal short and it’s not protected from an internal short, but that’s a very unlikely event.
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Old 02-08-2020, 15:55   #5
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Re: Shore Power Confusion

Quote:
Originally Posted by Jpernick View Post
I’m confused about shore power breakers, wire sizes, and amperage.

If I plug a y connector into a 50 amp 240 service, I can get 2 outputs at 120/30.
How are the 30 amp wires protected when the breaker is 50 amps?


they are not. however this would be used on a dual 30a shore boat so both should enter the boat through 30a main breakers on the boat side so the boat can not draw more then 30a on each. but if the wires shorted you'd be pulling 50a, it's not entirlly safe but is commonly done.
I would use it for tempoary use at other docks only. at your own dock you should get the correct plugs at the slip

if the boat is wired wrong or has no breakers it would be possible to pull 50a into the boat via a 30a rated shore cord and wire.

last month I saw a 50a 240v boat using a 50 to 30 adapter at the dock, a 30a shore cord, and a 30 to 50a adpater at the boat, because his heavy 50a cord was left at this own slip. 50a breakers at both ends he was pulling 50a (witnessed) from the dock to the boat through the 30a cord.


If I plug a reverse y into (2) 120/30 services (assuming opposite phases) I can get 50 amp 240.
How can the 30 amp breakers provide 50 amps?

they can't, you'll get 30a 240v (assuming the 2 plugs are out of phase) this is totally safe as you have 30a on a 50a rated cord and boat

Y and reverse Y are sold by the big guys (Hubbell / Marinco) so I assume they must be safe, but how can this be?

I just can’t figure out the math.

Jeff
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Old 02-08-2020, 16:37   #6
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Re: Shore Power Confusion

Protection against overload can be located anywhere between the source and load (30 amp main breaker(s) on yacht). Protection against short circuits and ground faults in the shore power cord assy must be at the source (pedestal). A 50 amp breaker will typically protect a 30 amp cord(s) against short/ gnd (current in hundreds or thousands of amps just not for long).

A slight variation to normal practice but in most situations reasonably safe.

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