125/250-50A or 125-50A?

[Boatsb]

Scott,

We are all entitled to our personal beliefs, so I have no problem with whatever you chose to believe. My posting was intended to provide a fact based view of how center tap 240 volt AC behaves. If you believe that factually what I wrote is incorrect, please provide some FACTS to support your position.

I can not help with why you see many examples of white wire contact problems.

Pete

Its because shore power without transformers will use the neutral. 2 x 50 amp loads of 125 will tax the white wire between the boat and the supply. Many of the boats I see do not have transformers. That leaves 2 completely independant paths sharing a single neutral. Even if they are phased ( 180 degrees hopefully) it seems to be the weak point.


Oh yea by the way I have seen boats plugged into 2 125v circuits that are not phased running 2 seperate 125 volt cords from adapters. There the 2 circuits share the single neutral and there is no offset of the phase from the 2 lines. Nerer plan for the user to always have clean, propper and ample power. Some will plug anythin in they can to get power not realizing it may be wrong.

 

[Pete]

Here is an article I found on wikipedia on the subject. Just substitute the power pedestal for the kitchen outlet as you read. Hopefully this will help.

Single phase[edit]
Main article: Split-phase electric power
In split phase wiring, for example, a duplex receptacle in a kitchen, devices may be connected with a cable that has three conductors, in addition to ground. The three conductors are usually coloured red, black, and white. The white serves as a common neutral, while the red and black each feed, separately, the top and bottom hot sides of the receptacle. Typically such receptacles are supplied from two circuit breakers in which the handles of two poles are tied together for a common trip. If two large appliances are used at once, current passes through both and the neutral only carries the difference in current. The advantage is that only three wires are required to serve these loads, instead of four. If one kitchen appliance overloads the circuit, the other side of the duplex receptacle will be shut off as well. This is called a multiwire circuit. Common trip is always required if there are loads that use more than one phase simultaneously. The common trip prevents overloading of the shared neutral if one device draws more than rated current.

The entire article can be found at http://en.wikipedia.org/wiki/Ground_and_neutral

Pete

 

[Boatsb]

Here is an article I found on wikipedia on the subject. Just substitute the power pedestal for the kitchen outlet as you read. Hopefully this will help.

Single phase[edit]
Main article: Split-phase electric power
In split phase wiring, for example, a duplex receptacle in a kitchen, devices may be connected with a cable that has three conductors, in addition to ground. The three conductors are usually coloured red, black, and white. The white serves as a common neutral, while the red and black each feed, separately, the top and bottom hot sides of the receptacle. Typically such receptacles are supplied from two circuit breakers in which the handles of two poles are tied together for a common trip. If two large appliances are used at once, current passes through both and the neutral only carries the difference in current. The advantage is that only three wires are required to serve these loads, instead of four. If one kitchen appliance overloads the circuit, the other side of the duplex receptacle will be shut off as well. This is called a multiwire circuit. Common trip is always required if there are loads that use more than one phase simultaneously. The common trip prevents overloading of the shared neutral if one device draws more than rated current.

The entire article can be found at http://en.wikipedia.org/wiki/Ground_and_neutral

Pete

In a perfect world but not a many marinas. I plan for the ID TEN T factor. People will not always follow propper wiring or common sense. Y cables can allow people to do stupid things. Especially on boats that are not 250v

 

[Pete]

When a man challenges scientific facts with gibberish, that is a clear signal that the discussion must end, as no resolution is possible. Over and out.

Pete

 

[Boatsb]

Wow how wrong could I have been if the burned up cords I repaired never happened.

Living in theory is great. Watching someone plug a cord into an adapter that is home made and wired wrong is exciting as long as no one gets hurt. Still if it you think it cant happen spend some time cleaning up after the diy crowd. .

 

[Colwyn D]

Thanks for all the replies and help. Pete, your explanations have been very educational and certainly improved my understanding of the issues. I have to also see Boatsb's side as well considering this boat had a green grounding wire tied into the hot side and the grounding tang on the back of the shore power receptacle hanging free on the end of the wire so no ground at all!
I've decided to forego the free 6/3 cord and get a longer length of 6/4 as advised.

 

[Pete]

Let me state this as diplomatically as I can. Much of what Scott offered is present in Scott's World and only there as far as my I am concerned. Not to say there is no merit, just that before acting on those items he insisted as accurate while contradicting science, should be backed by some sound references before acceptance. I am always willing to learn when proper evidence is present.

On a more positive note, many members that have isolation transformer boats can make good use of the 6/3 cable and have a cable that is easier to handle. You may want to consider offering a swap or sale rather than letting it sit idle. I am already using 6/3 cable so I am not a candidate.

Pete

 

[Boatsb]

Let me state this as diplomatically as I can. Much of what Scott offered is present in Scott's World and only there as far as my I am concerned. Not to say there is no merit, just that before acting on those items he insisted as accurate while contradicting science, should be backed by some sound references before acceptance. I am always willing to learn when proper evidence is present.

On a more positive note, many members that have isolation transformer boats can make good use of the 6/3 cable and have a cable that is easier to handle. You may want to consider offering a swap or sale rather than letting it sit idle. I am already using 6/3 cable so I am not a candidate.

Pete

Im glad to see you live in the perfect world but get a reality check. If you've never seen a boat without transformers fry a neutral you have not been around boats. Not every boat has a transformer or a properly wired and balanced electrical system.

 

[Avenger]

Pete, we were having this discussion in an earlier thread of mine where I was asking whether to stick with two 30A lines or move to 50A. It turned into a theoretical discussion of the question how can a 50A-250V line can carry two 50A 125V loads through a common neutral? It was getting too far off topic so we moved on, but my understanding was because the two phases were 180° opposed the neutral was only seeing one phase at a time so never had to carry 100A. Is that interpretation correct?

 

[Boatsb]

Pete, we were having this discussion in an earlier thread of mine where I was asking whether to stick with two 30A lines or move to 50A. It turned into a theoretical discussion of the question how can a 50A-250V line can carry two 50A 125V loads through a common neutral? It was getting too far off topic so we moved on, but my understanding was because the two phases were 180° opposed the neutral was only seeing one phase at a time so never had to carry 100A. Is that interpretation correct?

Derek that's true if they are phased. How often do you see a boat plugged into 2 pedestals because 1 line in the 250v is down. If you use 2 from the same phase for your 2 125 volt services a common neutral could carry more than 50 amps.

 

[whoover]

Just a short question, where were you able to put the glendenning on your 42. I have wanted to do it also but did not seem to find the space.

Thanks

 

[34Hatt]

Hey Scott

You could have saved lots of typing and time if you just said


Shit Happens

 

[Boatsb]

Or "never underestimate the stupidity of ..."

 

[Colwyn D]

Whoover, starboard aft corner of engine room (just outboard of transmission) with cable outlet in original, I think, position in cockpit. Was quite an easy install using 3" pvc piping and the rollers that Glendinning sell between the pipe sections.

 

[Pete]

Scrod,

Close but let me clarify. What the neutral sees is both phases at the same time. But, the phases are constantly 180 degrees out of phase reversing 120 times a second. I hesitate to over simplify the subject but think of water pipes as a slow motion analogy. In our case we would have three water pipes, two of which are flow control pipes, a black and a red. The third is the differential pipe, it is white. It is a sealed system, all water flow stays in the three pipes, and all pipes are the same size. In our example the pressure of the water is the equivalent of electrical voltage. The volume of water flowing is electrical current. Key to the operation is at all times any water flowing in the two flow control pipes is in the exact opposite direction of each other. If water is flowing only in the red pipe all water will flow through the white pipe. Same is true if only water is flowing in the black pipe. Now, the key condition, what if both the the black and red pipes have equal flow. The net is no water will move in the white pipe as the red pipe forward flow will be taken to supply the black pipe reverse flow. If the flow rates differ between the black and red pipe then the white pipe makes up the difference in the flow rates. And water flows in both directions in the white pipe depending on the excess or shortage of water at any instant. Do not know if that helps, but hope so.

Pete

 

[Pete]

Derek that's true if they are phased. How often do you see a boat plugged into 2 pedestals because 1 line in the 250v is down. If you use 2 from the same phase for your 2 125 volt services a common neutral could carry more than 50 amps.

Scott, a non problem.

Pete

 

[Boatsb]

How is it a "non problem"?

 

[SKYCHENEY]

Derek that's true if they are phased. How often do you see a boat plugged into 2 pedestals because 1 line in the 250v is down. If you use 2 from the same phase for your 2 125 volt services a common neutral could carry more than 50 amps.

The only way that I see this happening is with some sort of homemade reverse Y that doesn't seek out opposite phase inputs. Even then, the Y would be plugged into just 2-30amp circuits. If they are 180 degrees out of phase, then no problem. If they are the same phase, then it is still no problem as the maximum load on the neutral would only be 60amps.

 

[Boatsb]

What about 125v 50 amp plugs?


2 of them could be hooked up from the same phase allowing how many amps?

 

[SKYCHENEY]

How often do you ever see a 125v 50a receptical? I don't think I've seen one at all in the last 20 years. And even if you did, here are all of the things that would have to be true:

1) The owner makes up his own reverse Y with two 50a 125 plugs feeding one 50a 250v cord.
2) The boat does not have an isolation transformer.
3) They hook it up to two 50a 125v plugs that are phased the same
4) The boat is drawing enough 125v loads that it overloads the neutral. This will not include 250v loads because they will not run without two legs that are on opposing phases.

If all of the above are true, then, yes I believe you could overload the neutral. Is this scenario likely to occur? Probably not, and if it did, the owner would probably be buying some new plugs and cords.