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Old 05-06-2011, 10:12   #1
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30 amp to 50 amp splitters.

If your boat is equiped with a 50 amp 110/220 socket, and you put a splitter to two 30 amp, 110 cables, is it possible to run too much amperage through the neutral line?

If I understand this correctly, a 50 amp, 110/220 socket provides two 110 hot wires, that are 180 degrees out of phase. Thus if your using both 50 amp hot wires to their maximum capasity, both hot wires should canncel out, and almost no amperage should go down the neutral wire.

However, if your using two 30 amp plugs, both are in phase with each other. Thus if you use one wire to full capasity, of 30 amps, and the other wire to full capasity of 30 amps, the neutral lines would need to accept 60 amps of current, or a solid 10 more than the 50 amp neutral line and plug on the boat is speced out for.

It would seem like you would then need to add a fuse or breaker to the neutral wire on the boat to protect aginst that possibility.
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Old 06-06-2011, 16:44   #2
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Re: 30 amp to 50 amp splitters.

No. 220 is single phase. 2 110 lines and no neutral. Either side to neutral is 110.
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Old 06-06-2011, 17:52   #3
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Re: 30 amp to 50 amp splitters.

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Originally Posted by ViribusUnitis View Post
However, if your using two 30 amp plugs, both are in phase with each other. Thus if you use one wire to full capasity, of 30 amps, and the other wire to full capasity of 30 amps, the neutral lines would need to accept 60 amps of current, or a solid 10 more than the 50 amp neutral line and plug on the boat is speced out for..
If your boat is wired for 50amps, then that should be the max that your boat's equipment could ever pull from shore. How can that equipment suddenly be pulling 60amps? When maxed out, you would only be pulling 25amps from each 30amp line.
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Old 06-06-2011, 19:03   #4
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Re: 30 amp to 50 amp splitters.

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Originally Posted by ViribusUnitis View Post
If your boat is equiped with a 50 amp 110/220 socket, and you put a splitter to two 30 amp, 110 cables, is it possible to run too much amperage through the neutral line?

If I understand this correctly, a 50 amp, 110/220 socket provides two 110 hot wires, that are 180 degrees out of phase. Thus if your using both 50 amp hot wires to their maximum capasity, both hot wires should canncel out, and almost no amperage should go down the neutral wire.

However, if your using two 30 amp plugs, both are in phase with each other. Thus if you use one wire to full capasity, of 30 amps, and the other wire to full capasity of 30 amps, the neutral lines would need to accept 60 amps of current, or a solid 10 more than the 50 amp neutral line and plug on the boat is speced out for.
Just a minor terminology thing - 110/220V in the US is referred to as split-phase rather than 2 phase. You are correct about each leg being 180 deg out of phase with the other.

If you power it via a commercially available reverse-Y cable, it only works if the two 30A circuits are opposite legs of a split phase circuit. The reverse-Y cables detect this and only pass power through to the 50A side if the 30A legs are correctly opposing each other. This is the only way to get 220V out of it, and it's the only way to not overload the neutral. The 220V is imprtant because many boats have a few 220 appliances on board. For example, my AC and water heater are 220V.

Note that some 50A boats are 110V only. In this case it's real simple to use a 30A to 50A adapter to power the boat, of course limited to 30A.
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Old 06-06-2011, 19:10   #5
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Re: 30 amp to 50 amp splitters.

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If your boat is wired for 50amps, then that should be the max that your boat's equipment could ever pull from shore. How can that equipment suddenly be pulling 60amps? When maxed out, you would only be pulling 25amps from each 30amp line.
A 50A 120/240V boat can draw 50A from both of the two legs that make up the circuit. Powering it via two 30A 120V circuits has the potential to overload both since each leg can independently draw up to 50A.

He is correct that if you power a 50A 120/240V circuit (split phase) via two 30A 120V outlets that are on the same leg, the neutral wire current will be additive, not subtractive. Leg 1 can draw 30A as can Leg 2, but because the two legs are in phase, the neutral would be carrying 60A. When the legs are out of phase the neutral currents cancel each other out.
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Old 06-06-2011, 20:12   #6
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Re: 30 amp to 50 amp splitters.

OK, so this is entirely possible, is a fuse or a breaker recomended on the neutral to safeguard it being overloaded?
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Old 07-06-2011, 05:10   #7
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Re: 30 amp to 50 amp splitters.

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OK, so this is entirely possible, is a fuse or a breaker recomended on the neutral to safeguard it being overloaded?
The best thing to do is to buy a 30A to 50A reverse-Y adapter. Marinco, Charles Marine, and I'm sure others make them. They are expensive, but ensure that the two 30A plugs remain isolated and only pass power through to the 50A side if the 30A plugs are powered and on separate legs. On the 50A side you either get nothing, or you get proper 110/220V power (limited to 30A). Without this, there are numerous scenarios that will result in exposed AC power, burned out appliances, inoperative appliances, and of course overloaded neutral lines. A dead body, burned out appliances, or a boat fire will all be more expensive than the reverse-Y adapter.

If you want to build a bootleg adapter, here are some considerations to help scare you off:

- First off, are you sure your 50A boat is 120/240V split phase, as opposed to 50A 120V only? It makes a big difference. In this thread we've been assuming it 120/240 split phase.

- Do you have any 240V appliances on the boat, or is everything 120V? This too makes a big difference. If you have 220 appliances then you need to connect the 30A plugs to L1 and L2 (the two legs of a split phase circuit) to get 240V. If properly connected to L1 and L2, you don't need to worry about overloading the neutral.

- If you don't have any 240V appliances, then your boat is essentially just two separate 50A 120V circuits with a common neutral. If you are sure this is the case then the only consideration is overloading the neutral. This can be avoided by always plugging the 30A plugs into separate legs, or by fusing the neutral. One approach you might consider is to use a single 30A plug and wire it to both L1 and L2 on the 50A side. This will limit power to 30A, will not overload the neutral, but will only work if you have no 240V appliances. It's otherwise safe and simple.

- Danger, Danger. If you have 240V appliances that are turned on and have a bootleg 30A 120V to 50A 120/240V cable, when you plug in one of the 30A plugs the 240V appliance will pass 120V through to the other 30A plug, creating plug prongs that are hot. This is very, very dangerous, and is a great example of why an automatic reverse-Y cable is worth the price.

In another thread someone mentioned that marinas are starting to prohibit the use of anything other than commercially available cabling. When you look at the complexity and hazards associated with mistakes, it's no wonder they have taken this position. If I ran a marina, or had my boat in a marina next to a boat with bootleg cabling, I'd want it prohibited too. I think only 1 in 10 people could make a cable up correctly, and only 1 in 10 of those would understand the corner cases that pose the hazards. The other 9 out of 10 would find those hazards the hard way.
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Old 09-06-2011, 08:43   #8
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Re: 30 amp to 50 amp splitters.

If your 50A/240VAC boat is wired correctly, then the shore power circuit breaker on the main panel is:
1. rated at least as 50A
2. could be rated at 60A if the designer had his act together
3. is three pole; L1, N, L2

Regardless, the neutral in a two 30A/120VAC into a 50A/240VAC system will be protected at the rating of the main shore power breaker.

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Old 09-06-2011, 09:02   #9
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Re: 30 amp to 50 amp splitters.

Assuming you are using a correctly wired adaptor, the neutral will only be carrying the difference in load between the two phases. The purpose of the neutral in split phase 120/240volt is to balance the voltage between the two phases, I.E. you have a 1500Watt hair dryer plugged into 1 phase, and a 100watt reading light into the other. The hairdryer is pulling 15 amps, the light 1 amp, if no neutral the light will have to pull the full current of the hairdryer, causing a voltage drop at the dryer causing it to see 8 volts, the light bulb will see a 212volt surge causing it to burn out. In this case a properly operating neutral will pull 14 amps, (the difference between the two legs), balancing the two phases and keeping both light bulb and hair dryer at 120volts.

So to answer the OP's question no the neutral will only see the difference in current for a maximum, (30 amp load in one phase and no load in the other), 30 amps. If both legs are at full power, 25 amps plus 25 amps, the neutral will see nothing.
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