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Old 05-08-2021, 09:35   #1
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One 50A cordset vs. two 30A cordsets

We have a newer Beneteau. It has two 30A shore power cables, one for the air conditioner and one for everything else (battery charger, water heater, outlets, etc.). The strange thing is that, although both cables have 30A male plugs shore-side, they terminate in 50A female EEL plugs boat-side. We guess that Beneteau did this to use one type of outlet on all of its models to reduce inventory and production costs. There are other examples of this.

Anyway, here's the question. We have both 50A and 30A power at our dock. To avoid running two cables ashore, could we use a standard 50A cordset and a Marinco Y-adaptor (Part# Y504-2-504) boat-side? I understand that this would deliver 50A instead of 60A (2 x 30A) so we'd have to be sure not to overload the system. Other than that caveat, are there other concerns?
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Old 05-08-2021, 09:43   #2
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Re: One 50A cordset vs. two 30A cordsets

You could do that with no issue. And you'd still have a full 60A, as the common 50A marine outlets are 50A 125/250V, so you have 2x 50A / 125V legs available.

Before you buy the adapter, confirm if your boat side outlets are 50A / 125V (3 wire) or 50A / 125/250V (4 wire), as the connectors are different.

If you need help figuring any of it out, feel free to PM me, as I'm local to you.
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Old 05-08-2021, 09:58   #3
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Re: One 50A cordset vs. two 30A cordsets

Should work, and as rslifkin says, with 50A 120/240 split phase you actually end up with 2x 50A 120V circuits, which easily beats 2x 30A circuits.

Potential problems I see;

1. You need to make sure that the two inlets use the opposite phases from the 50A connection, otherwise you are limiting yourself to 50A total. This is a boat-side thing, you want to make sure one 50A connector uses the “X” phase connector in the boat and the other uses the “Y” phase connector.
2. Is the wiring suited for 50A on the boat? Presumably it is since it is connected to a 50A plug (by code it would be required), but worth at least checking.
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Old 05-08-2021, 10:16   #4
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Re: One 50A cordset vs. two 30A cordsets

All great points, thank you.
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Old 05-08-2021, 15:14   #5
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Re: One 50A cordset vs. two 30A cordsets

The new piers in our marina have new ELCI outlets. Some folks with twin 30s have experienced a problem with them.
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Old 05-08-2021, 15:53   #6
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Re: One 50A cordset vs. two 30A cordsets

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Originally Posted by Dsanduril View Post
Should work, and as rslifkin says, with 50A 120/240 split phase you actually end up with 2x 50A 120V circuits, which easily beats 2x 30A circuits.

Potential problems I see;

1. You need to make sure that the two inlets use the opposite phases from the 50A connection, otherwise you are limiting yourself to 50A total. This is a boat-side thing, you want to make sure one 50A connector uses the “X” phase connector in the boat and the other uses the “Y” phase connector.
2. Is the wiring suited for 50A on the boat? Presumably it is since it is connected to a 50A plug (by code it would be required), but worth at least checking.

You are not going to end up with 2 50A circuits. The 120v is going to use one leg each of the 240 50a So you will get 25 amps on each leg. With two 30 amp 120 lines you are using 30 amps on each leg. This will give you two 120v 30 amp circuits. This most likely will split into 120v 30 amp and 120v x2 15 amps in the box.
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Old 05-08-2021, 16:26   #7
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Re: One 50A cordset vs. two 30A cordsets

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You are not going to end up with 2 50A circuits. The 120v is going to use one leg each of the 240 50a So you will get 25 amps on each leg. With two 30 amp 120 lines you are using 30 amps on each leg. This will give you two 120v 30 amp circuits. This most likely will split into 120v 30 amp and 120v x2 15 amps in the box.
A 50A 120/240V split-phase supply is 50A on each leg and can provide up to 12kW (nominal) vs. 3.6kW for a 30A 120V circuit. Take a look inside a power pedestal and you’ll see a single 30A breaker for that service, and a two-pole 50A breaker for a 50A service, not a two-pole 25A breaker. That means there’s 50A available on each leg. You can run 50A at 240V, or 2x 50A @ 120V.

Looking at it another way, why would Marinco use 6AWG wire in their 50A cordsets while using 10AWG in their 30A sets if there is actually only 25A per leg in a 50A system?

If we’re talking a 50A 120V single-phase connection, well, that’s a different beast and you only have 50A @ 120V available. But talking split-phase 120/240V there’s 50A available one each leg. Power-wise that is equivalent to 100A @ 120V although the largest single circuit you can have is 50A because of the split phase.
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Old 05-08-2021, 16:50   #8
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Re: One 50A cordset vs. two 30A cordsets

You will find the 50 amp cord is about $450 and up last I checked though, unless you can use a short one.. May be better buying 30 amp ones in the future. The adaptors are expensive also.
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Old 05-08-2021, 16:55   #9
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Re: One 50A cordset vs. two 30A cordsets

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A 50A 120/240V split-phase supply is 50A on each leg and can provide up to 12kW (nominal) vs. 3.6kW for a 30A 120V circuit. Take a look inside a power pedestal and you’ll see a single 30A breaker for that service, and a two-pole 50A breaker for a 50A service, not a two-pole 25A breaker. That means there’s 50A available on each leg. You can run 50A at 240V, or 2x 50A @ 120V.

Looking at it another way, why would Marinco use 6AWG wire in their 50A cordsets while using 10AWG in their 30A sets if there is actually only 25A per leg in a 50A system?

If we’re talking a 50A 120V single-phase connection, well, that’s a different beast and you only have 50A @ 120V available. But talking split-phase 120/240V there’s 50A available one each leg. Power-wise that is equivalent to 100A @ 120V although the largest single circuit you can have is 50A because of the split phase.

Not so a 50 amp double breaker is 25 on each leg. A 30 amp double breaker is 15 amps on each leg. A single breaker @ 30 amps is 120v @30 amps. A double breaker uses one leg form each side (120v and 120v) gives you 240v. a 30 amp cord uses 2-120v legs @ 15 amps each. In other words it splits the 240 into 2) 120v legs.
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Old 05-08-2021, 17:08   #10
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Re: One 50A cordset vs. two 30A cordsets

If you have ever looked inside a box you will see that every other space for breakers is on one leg. The next one following is to the other leg. That is because when you use a double breaker you use two legs. One breaker 120v one breaker 120v If you tie the two together you get 240. If you have a double breaker and it says 50 it is 25 and 25. You can take two single breakers of 15 and tie them together with a pin and have a 30 amp double breaker. But it is still 15 on each leg.
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Old 05-08-2021, 17:47   #11
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Re: One 50A cordset vs. two 30A cordsets

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.... You can take two single breakers of 15 and tie them together with a pin and have a 30 amp double breaker. But it is still 15 on each leg.
No, you have a two-pole 15A breaker. Nowhere would it be referred to as a 30A breaker. The most current that can flow in any leg is 15A, it is a 15A breaker. Tie three of them together in a three-phase system it is still a 15A breaker. In terms of total power available it is equivalent to a single-pole 30A breaker, but two poles at 15A each is not a 30A breaker.

Apologies to the OP for the derailment, but this is too important to leave it be.
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Old 05-08-2021, 18:01   #12
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Re: One 50A cordset vs. two 30A cordsets

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No, you have a two-pole 15A breaker. Nowhere would it be referred to as a 30A breaker. The most current that can flow in any leg is 15A, it is a 15A breaker. Tie three of them together in a three-phase system it is still a 15A breaker. In terms of total power available it is equivalent to a single-pole 30A breaker, but two poles at 15A each is not a 30A breaker.

Apologies to the OP for the derailment, but this is too important to leave it be.

I am sorry but I know electric currents, It is what I do. I'm the guy that wires up those boxes. A double pole breaker that says 30 amps is 15 amps per leg. If you take two single amp breakers and tie them together you have a 30 amp double breaker. Amps is current it is the amount of current going through the line. A 50amp double pole breaker is using two legs. That's 50 amps total amps through that breaker. 25 amps on each leg. You are trying to pull 100 amps through that breaker by saying that you get 50 amps per leg. Not going to happen.
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Old 05-08-2021, 18:49   #13
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Re: One 50A cordset vs. two 30A cordsets

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I am sorry but I know electric currents, It is what I do. I'm the guy that wires up those boxes. A double pole breaker that says 30 amps is 15 amps per leg. If you take two single amp breakers and tie them together you have a 30 amp double breaker. Amps is current it is the amount of current going through the line. A 50amp double pole breaker is using two legs. That's 50 amps total amps through that breaker. 25 amps on each leg. You are trying to pull 100 amps through that breaker by saying that you get 50 amps per leg. Not going to happen.

You are totally wrong. And need to find a new job.
Before you kill someone.
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Old 05-08-2021, 18:54   #14
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Re: One 50A cordset vs. two 30A cordsets

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You are totally wrong. And need to find a new job.
Before you kill someone.
I second that,,,,,,, and get us a list of marinas where you did the wiring!!
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Old 05-08-2021, 18:54   #15
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Re: One 50A cordset vs. two 30A cordsets

I’d consider having a marine electrician install a standard US 50 Amp 240v inlet and wire it correctly in the boat (which should be simple based on your description of the current configuration). Adapters introduce another failure point and safety worry into the system.

That will give you up to 50 amp 125v for the air conditioning leg and 50 amp 125v for the other AC leg (depending on the breaker sizes in the boat)

It shouldn’t cost much.
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