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

[Avenger]

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

I think I understand. Basically it appears to hinge on the opposing phases balancing against each other. So what you're saying is even if the shorepower was taken from 3 phase that's only 120° apart instead of 180° the neutral would still only see the difference in flow. However shouldn't we expect to see some imbalance from the fact that the phases aren't exactly opposed? In other words even at full load on both legs the white pipe would have to carry some flow since the red and black aren't flowing in exactly opposite directions.

 

[Pete]

Scrod,

Exactly right. If using a phase angle difference other than 180 degrees a different calculation applies. In the case of using two phases of a three phase supply, assuming that the third phase is not being used somewhere else, which is unlikely but could happen, then the flow in the neutral would be one hundred percent of a single phase if only one phase has current flow, down to roughly 1/3 of a single phase if both legs being used of the three phases are being loaded at the same current. The important point is regardless of the current flow in both legs, the current flow in the neutral will never exceed the breaker value for any of the three phase legs.

If anyone would like to observe the exact operation of the neutral in various configurations, just put a clamp on amp meter on the neutral and vary the phase currents. This will only work at the dock pedestal if your boat does not have an isolation transformer. And above all, make sure you or someone helping you knows the safety precautions for working with lethal electric power. Electricity can kill someone in a flash.

Pete

 

[rsmith]

theory is great but 90% of the time when a plug burns up it's the neutral prong that's cooked!

 

[Boatsb]

theory is great but 90% of the time when a plug burns up it's the neutral prong that's cooked!

No way. I said the same thing but was corrected. It can't happen. Read the previous posts.

 

[rsmith]

No way. I said the same thing but was corrected. It can't happen. Read the previous posts.

What's Scrody's byline???

 

[Boatsb]

What's Scrody's byline???

Scroddys not bi. That's just going to far mr smith. If thats your real name

 

[roundIII]

Don't mean to hijack this thread with this question.

1989 40' MY with factory 50 amp / 125/250 volt. Moving to slip with 2 30 amp shore power connections.

Is there a splitter available that will accomplish this and still provide us with the necessary amperage to run all our appliances?

I have one 30 amp female connection next to the 50 amp also available but I do not think this will provide the power needed.

Thanks,

Ron

 

[Boatsb]

The 2 30 amp cords will limit how much power you can get. You will be able to get only 30 amps in contrast to the 50 the other cord can supply but the issue is how much do you need?

 

[Pascal]

Don't mean to hijack this thread with this question.

1989 40' MY with factory 50 amp / 125/250 volt. Moving to slip with 2 30 amp shore power connections.

Is there a splitter available that will accomplish this and still provide us with the necessary amperage to run all our appliances?

I have one 30 amp female connection next to the 50 amp also available but I do not think this will provide the power needed.

Thanks,

Ron

You need a smart Y. It s basically two 30 amps male plugs tied to a single 125/250-50 female. There is. Circuit in the middle to only allow power to flow if both 30 plugs are connected and only if connected to 30amp outlets on opposing phase. It will obviously only supply 30amp per leg instead of 50 per leg.

You should be able two run 2, maybe 3 air cons, on one leg, and your house loads on the other as long as you manage the loads like turning off the water heater and charger while cooking, etc.

Less than ideal, acceptable for occasional transient use, but not practical at a home slip

 

[roundIII]

Thanks for the reply. I am only going to be at this slip for maximum 2 months, not live aboard except for a few days at a time every few weeks.

Would you advise not using this splitter and just using the one 30 amp outlet already on the boat?

Need enough amps to run both a/c's and water heater sometimes. Other than that I can monitor usage of other electrical.

Does either method put strain on the electrical system of the boat?

Thanks,

Ron

 

[Pascal]

The smart y will get you twice what a single 30amp cord would so you re better off with it. No issue with the boat whatsoever. The downside is that they are fairly expensive, around $400 I think...

 

[TROUBLE]

I always thought the only real difference with the "smart y" was that if you only had one 30amp cord plugged into it, the open end wasn't live. If you used a standard splitter and had both cords plugged in all the time wouldnt hat be a much more inexpensive alternative?

Art
Trouble

 

[roundIII]

Pascal:

Thanks for the information.

I may try it with just the 1 30 amp from shore to boat and see how that works before buying a splitter.

I assume that is why Hatteras put the 30 amp receptacle there in case of situations as this one.

Ron

 

[Colwyn D]

I do occasionally run with just the one 30A supply instead of the 125/250. It won't run a great deal, one AC unit and lights etc or water heater and battery charger but no AC. Two AC's generally trip the shore breaker. Obviously this is very general info but should give you some idea.

 

[Pete]

theory is great but 90% of the time when a plug burns up it's the neutral prong that's cooked!

This is good information. A little head fake, switching the subject from overloading the neutral wire and what current actually flows on the neutral to a discussion of plugs and outlets. But that is OK it is done all the time in Washington.

With this failure mode we can quickly conclude that since the problem is at the connection, the wire size and the current flowing in the neutral wire is not the problem, else you would see wire insulation melting problems all along the neutral wire, since the resistance in the wire is consistent throughout its length and thus the heat generated by overload would be uniform along the wire. So we are not dealing with a neutral wire overload.

The neutral plug leg being the most susceptible connector is not a surprise. Looking at the configuration of the 125/250 volt 50 amp plug you could quickly understand why. The neutral connector has less surface area than the two hot leads. The neutral connector does not have a locking tab to hold it steady and in constant contact with the opposing female outlet receptacle. The slightest movement of the male/female contacts will cause arcing under load, creating an irregular surface of contact and reduce the contact area further. Less contact area means more resistance. Keep this theory in mind, sorry Scott for more theory. Power in watts = current squared times resistance. In a worst case situation, one leg at full load, second leg at no load you have 50 amps flowing in the neutral. 50 squared is 2500 which with a resistance of 0.004 ohms will generate 10 watts of heat, more than enough to fry a connection. So what does one do while we wait for a better 125/250 volt 50 amp outlet and matching plug design, that stabilizes the neutral connection? First and foremost, never ever plug your shore power cord into an on shore active outlet. Always visually check the shore outlet for any signs of heat damage, before attaching to it. If any damage is evident, do not use it. Ask the marina to replace it with a good outlet. Same is true for your shore power cord plug. Check all the prongs for damage, if you see any replace the plug.

And I suspect many of us will have to continue to go through this drill for a long time, because someone is making good money selling the replacement parts needed. Another consideration for the manufacturers is the size of the problem and if it is in self correcting mode as today's accepted standard is 250/125 volts 50 amp into an isolation transformer which does not utilize the neutral.

Pete

 

[Avenger]

If using a phase angle difference other than 180 degrees a different calculation applies. In the case of using two phases of a three phase supply, assuming that the third phase is not being used somewhere else, which is unlikely but could happen, then the flow in the neutral would be one hundred percent of a single phase if only one phase has current flow, down to roughly 1/3 of a single phase if both legs being used of the three phases are being loaded at the same current. The important point is regardless of the current flow in both legs, the current flow in the neutral will never exceed the breaker value for any of the three phase legs.

Thank you Pete. It's nice to have a clearer picture of this. But it brings up another question. In the case of my plan to rewire my two 30A 125V sockets to a single 50A 125/250V connection, since there are no 250V anything on my boat I should have plenty of capacity by splitting the two hot legs as long as I'm using phased power whether 180° or 120°. So basically increasing the capacity of each 125V circuit to 50A. But what happens when I go to a marina that doesn't have a 250V service? I'm most likely going to see 30A service at a dock for a 36ft boat. I figured I could use a Y-adapter and plug two 30A services into it so I can run my A/C and hot water heater without having to switch back and forth. If these two 30A services are on the same phase we don't get the opposite flows anymore. Isn't this a potential problem?

 

[Pete]

Scrod,

In a word the answer to your question is YES. If you were on the same phase and drawing more than 50 amps combined on your two panels, then the neutral becomes over loaded.

But, there is good news and bad news. You do not have to be concerned with this potential overload safety problem, as I do not know of any commercially available Y connectors that will do such a hookup other than the $400 smart reverse Y's that have there own protection built in to the unit. If your goal is to attach to two 30 amp 125 volt pedestal receptacles and convert this to a 50 amps 125/250 volt connector providing 30 amps on each leg, then you will need one of these smart Y's.

Perhaps another member has an idea, but the only one I know is a DIY build which I do not recommend for safety reasons.

Pete

 

[Avenger]

Thank you Pete. I thought that would be the case. It's not that I'm planning on doing that, but you get what you get when you go to some marinas, and the admiral doesn't want to sweat while she's using her hair-dryer.

So based on this it appears that it's still okay to split the two phases to a pair of two-pole breakers to feed the two systems instead of a single 3-pole, as long as I don't make my own Y connector. Or at least put a meter across the hot legs to make sure I'm on different phases.

 

[Pete]

Scrod,

An idea you might want to consider is a panel type 250 volt indicator light, that you wire across the black and red wires. If you are on opposite phases the light will be on. If the light is off you are on the same phase and can try to correct that or limit your load. As usual if doing this, put a half amp fuse in line with one of the indicator leads just for safety.

Pete

 

[Avenger]

I like that. Simple, functional and to the point. Saves the trouble of probing plugs with a meter. Thank you very much.