AC wiring modifications for onboard genset

[Snotter]

I am getting ready to bring my 7.5 kw genset on board for mounting. Before the engine room gets too cluttered, I would like to make the changes to the AC wiring needed to tie into the generator. Perhaps some of you can share your thoughts on how I should do this.

My shore connection consists of two 30 amp cords that tie into a 50 amp pigtail for attaching to a 50 amp socket at the dock. I assume that the two 110 volt lines feeding into the boat are not in phase, preventing me from connecting green-to-green, white-to-white, and black-to-black wires to join the two lines when a shore connection is maintained. I also assume I need multiple switches to: disconect the two 110 shore lines, and to connect the two lines to a common line coming out of the generator.

I know they sell shore power-to-generator switches that allow the shore power to be cut off and switched to the generator, all in one motion. However, it is not clear to me if switches are made to disconnect 2 separate lines from shore and join them into a single line that is switched into the generator.

You guys (and gals too) have always been patient with me. Forgive me if I am wearing that patience thin.

 

[seadaddler]

I don't know electrical

But on my 2007 Hunter 36 I have I think 2 switch boxs to my Gen and shore power setup.
Nick

 

[Rich Stidger]

Snotter-

I have done exactly what you are trying to accomplish.

First your questions-

Your two 30A shore power connectors should each supply power to two independent circuits on your sailboat. That is to say that on your boat the hot and neutral wires on each circuit should not be connected to each other. The only common connection between the two shore power circuits is the green ground wire.

You don't say what type of 50A pigtail that you are using. There are two types. The first and probably most common is a three wire pigtail that is called 50A 125V and it has three wires on each of the three ends. In this pigtail, the hot (black) wires are all connected together, the neutral (white) wires are connected together, and the ground (green) wires are connected together. This pigtail plugs into a 50A 125V receptical on the dock. In this connection, the dock is supplying 125V at 50A and there is just one phase of this power.

The second type of pigtail is a 50A 125/250V and it has four wires on the 50A connector and three wires on each of the two 30A connectors. This pigtail supplies each of the two 30A connectors with 125V at 50A but the two circuits are different phases. The 50A connection at the dock is four wires which are black and red (both hot), white (neutral), and green (ground). The black and red are different phases (180 degrees out from each other). From either the black or red wire to white will be 125V, but from black to red will be 250V. If you are using this type of pigtail you must not connect the white to white, black to black inside on your boat because when you use this pigtail you will be shorting the black and red wires which are both hot and opposite phases. Translation- MAJOR FIRE & FLAMES!!

You do not need multiple switches to switch your existing shore power inputs and your generator. In fact if you do have multiple switches, you may run the risk of operating the switches out of sequence and cause a short circuit. Having just one switch is the easiest and most fool-proof method. You need just one switch. But make sure you are sitting down when you get the quote for the switch.

The switch you need is a four-pole double-throw, center-off, break-before-make switch.

I have attached a two page pdf document that shows my original shore input wiring and then the switch wiring for the genset. In the shore position, the power flows from the two input connectors to the circuits exactly as they were originally wired. In the center off position, no power is connected. In the generator position, the power from the generator is connected to the two load circuits and these circuits are connect in parallel at this switch.

 

[JoeWhite]

There should be a switch to your shore power for each of the sockets. You should be switching off before disconnecting the shore power. So use those to turn off the shore power. To connect to the generator - this is where you can add two new switches for each of the shore power connections. Make is simple so that it does not need servicing.It does not matter if they are in phase or not. The shore power is more than likely the same phase. You can test this by measuring the AC voltage between each of the pins. If there is not voltage - then they are in-phase.

 

[woodster]

Snotter-


You don't say what type of 50A pigtail that you are using. There are two types. The first and probably most common is a three wire pigtail that is called 50A 125V and it has three wires on each of the three ends. In this pigtail, the hot (white) wires are all connected together, the neutral (black) wires are connected together, and the ground (green) wires are connected together. This pigtail plugs into a 50A 125V receptical on the dock. In this connection, the dock is supplying 125V at 50A and there is just one phase of this power.

.

now i am confused ....here on land its always been black is hot and white is common is there a reg calling for the white to be hot and the black to be common when wireing marine 120vac......

regards

woody

 

[Rich Stidger]

now i am confused ....here on land its always been black is hot and white is common is there a reg calling for the white to be hot and the black to be common when wireing marine 120vac......

regards

woody

Woody-

Thank you for pointing out my error on the colors. I should have known better. Hopefully not many readers have seen my error yet. I went back and edited my post to correct everything.

 

[woodster]

Woody-

Thank you for pointing out my error on the colors. I should have known better. Hopefully not many readers have seen my error yet. I went back and edited my post to correct everything.

now i am going to leave my question up there or maybe not now i am more cornfused than before....

seriousley though i wasnt trying the one upman ship....there very well could have been a reg governing this action....and as usual i would have wired it the way i knew it to be and would have been very poed if i were wrong....

regards

woody

 

[tommays]

Well

On land all 4 wire cord like 12/4 SJ is Black/Red/White/Green and is used on 3 phase motors for the most part and you do use the white as a leg

However if you were doing a conduit connection and pulling wire it would be Black/Blue/Red/Green

Which is why you do have to be careful about land based practice taken to the water

 

[Snotter]

Rich, thanks so much for the post and attachment. I have the 3-wire pigtail. I am surprised to learn that they are in phase.

The two lines coming into the boat go to separate main-breaker fuse boxes--one feeding the 2 heatpumps and the other feeding everything else. The 2 green wires are in parallel and go through an isolator. I will put the 500$ switch between the main breaker boxes and the genset, making sure that the green wire is connected to the block of the genset (per Calder's admonishins).

I am pleased that others are interested in this thread. All are welcomed to hijack away.

 

[Bill Roosa]

Start with the end in mind

With 7.5 kw you can certainly run both AC circuits. Is that what you want to do though? I could make a money argument that if you put the air conditioning, battery charger, and water heater on one circuit you don't really need to energize the other alternating current circuit. It is just supplying the outlets and most of that stuff has 12 volt solutions already. i.e. you are not going to be plugging in 110 volt lights when you have built in 12 volts ones.....
Consider what your alternating current circuits are powering now and how and what you really want to run while at sea. Put those "sea circuits" on one circuit and save yourself some bucks in the transfer switch.

This assumes you do not have an inverter in the mix. That could power your outlets BTW but then you have two transfer switches to worry about. Shore power 1 to inverter and shore power 2 to genset.

You have to be mindful of the max amps that you can put on one circuit with any of these considerstions. If your airconditioning is sucking up 100% of the circuit capability then this begins to get complicated with the power management.

 

[woodster]

Rich, thanks so much for the post and attachment. I have the 3-wire pigtail. I am surprised to learn that they are in phase.

The two lines coming into the boat go to separate main-breaker fuse boxes--one feeding the 2 heatpumps and the other feeding everything else. The 2 green wires are in parallel and go through an isolator. I will put the 500$ switch between the main breaker boxes and the genset, making sure that the green wire is connected to the block of the genset (per Calder's admonishins).

I am pleased that others are interested in this thread. All are welcomed to hijack away.

snotter you may want to check and see that your power cord from the dock is 125 a c volts only either 30 amp or 50 amp.... in the event you have a 220 ac volts power cord coming to the boat ...you could fry some thing by hooking up the 220 volts to your 120 ac volt system...just a thought ...and btw you can get that switch a lot cheeper by shopping around.....

regards

woody

 

[Bill Roosa]

shore power connections and correction

I'm pretty sure you can't plug in a 220 volt plug into a 110 outlet and vica-verca. The whole electrical industry plug standards are set up to prevent just that. That is what those angled terminals are for, to keep you from plugging in unless the voltages are correct.

also I am not (in my previous post) suggesting that you do not need a transfer switch, you just don't need the four-pole double-throw, center-off, break-before-make switch Rich is suggesting for the full blown system.

 

[Snotter]

With 7.5 kw you can certainly run both AC circuits. Is that what you want to do though? I could make a money argument that if you put the air conditioning, battery charger, and water heater on one circuit you don't really need to energize the other alternating current circuit. It is just supplying the outlets and most of that stuff has 12 volt solutions already. i.e. you are not going to be plugging in 110 volt lights when you have built in 12 volts ones.....
Consider what your alternating current circuits are powering now and how and what you really want to run while at sea. Put those "sea circuits" on one circuit and save yourself some bucks in the transfer switch.

This assumes you do not have an inverter in the mix. That could power your outlets BTW but then you have two transfer switches to worry about. Shore power 1 to inverter and shore power 2 to genset.

You have to be mindful of the max amps that you can put on one circuit with any of these considerstions. If your airconditioning is sucking up 100% of the circuit capability then this begins to get complicated with the power management.

I live aboard and work a full-time job. I love my toaster oven and electric coffee maker. My led lights aren't enough to live under and my refrig, though 12 v, is power hungery. Moreover, I prefer my electric heaters, on mild nights, over the heatpumps--much quieter. To each his own--my boat, my habits. I don't want to change my habits just to get away from the dock for a night or two. When traveling, I like the feel of home when I am below.

 

[woodster]

also I am not (in my previous post) suggesting that you do not need a transfer switch, you just don't need the four-pole double-throw, center-off, break-before-make switch Rich is suggesting for the full blown system.

i agree with you bill ...the barrel switch does not need to ever break the ground/green hence the three pole switch and you can find them on ebay
for $100.00 and up in buy it now in most cases

regards

woody

 

[Snotter]

I'm pretty sure you can't plug in a 220 volt plug into a 110 outlet and vica-verca. The whole electrical industry plug standards are set up to prevent just that. That is what those angled terminals are for, to keep you from plugging in unless the voltages are correct.

also I am not (in my previous post) suggesting that you do not need a transfer switch, you just don't need the four-pole double-throw, center-off, break-before-make switch Rich is suggesting for the full blown system.

I am pretty good at scavenging--though not the best. I will be watching ebay; however, I like the idea of just one switch to turn--it may be worth the extra money to me. Please, though, don't take that to mean I am not interested in learning different ways to skin a cat.

 

[Bill Roosa]

Transfer switches

I agree with woodster but Rich's analysis is correct to switch two alternating circuits from one source to another you need a four pole switch. two hots and two neurtals.
You NEVER switch the ground in any setup you care to discuss. It ALWAYS connects directly to the source's ground wire with out any switch

 

[Bill Roosa]

There is also the slider CB switch concept. If you add an additional CB to the sea power side that connects the two hots together but separates them when you are on shore power that will work also. It is a cheaper solution but some have said they don't like it from a longevity viewpoint.

 

[Snotter]

I agree with woodster but Rich's analysis is correct to switch two alternating circuits from one source to another you need a four pole switch. two hots and two neurtals.
You NEVER switch the ground in any setup you care to discuss. It ALWAYS connects directly to the source's ground wire with out any switch

I have to re-read Calder's section about the differences in galvanic isolator connections for shore sources and generator sources. It is not fresh in my mind and I don't have access to his book right now.

 

[Snotter]

The switch you need is a four-pole double-throw, center-off, break-before-make switch.

Rich, do you have a link you can post for the switch?

 

[Rich Stidger]

i agree with you bill ...the barrel switch does not need to ever break the ground/green hence the three pole switch and you can find them on ebay for $100.00 and up in buy it now in most cases

The need for the four pole switch is because you need to switch the hot and neutral on both shore power inputs. The green shore power ground is not switched. In fact on my boat the green shore power wires are not connected at all- that is because a ground fault on shore or on another boat that is plugged in could cause a stray current to flow into my ground and then to my underwater metal causing extreme electrolysis. This practice is controversial so you may have a different opinion for this connection.

If the neutral is not switched, it would be possible to plug in both shore power cords to dock sockets that are not of the same phase. If the neutrals are not kept isolated when on shore power, you have a short between the phases on your boat.

Using a four pole switch, all power-carrying lines are switched and the connections with shore power are made exactly per original design and when on ship's power (genset) you need to be able to parallel the hot and neutral on both circuits so that everything can be energized.

If you separate the most critical AC loads and use one circuit to power those, that would allow you to use a simpler switch for just that one circuit. However when you go to the shore power mode, that circuit would likely be overloaded and would exceed the 30A capacity. That is exactly the reason that the A/C-heat pumps are all on one circuit and everything else is on the other one.