Question On Twin 30 Amp Shore Power Service

[Terra Nova]

Originally Posted by Blue Stocking

...His 2 shore current sources are coming from the same supply.

Negative.

It is common for docks to be fed 220v, which is two, out of phase, 110v legs. His two 110v cables could be connected to out of phase legs. NEVER combine the neutrals.

[Cap Erict3]

Originally Posted by Greggegner

Ok let me hijack the thread just a little...I have a reverse polarity detector on my 30a panel. I can't use a GFI shore source during the winter (it trips as soon as I load it). Anybody know why...


Greg S/V Sweet Dreams
Pearson P385 out of Racine Wisconsin

Ur line and neutral r not carrying equal load. Trips in winter? Heater element would be a likely bet. Easy fix.

[Cap Erict3]

Originally Posted by boatpoker

ABYC requires they be separate. It is potentially lethal to do otherwise.
Don't even consider the Triad isolation transformer, it does not come close to meeting ABYC Standards. Real marine transformers such as the Charles units will cost you 2k each and you can achieve close to the same protection with a couple of $350 failsafe galvanic isolators.

Ya may want to look at NFPA 70.

[QuarkElec]

In some counties, the dock sockets outputs 230V not 220V.

[rwidman]

Originally Posted by mitiempo

True, but a change to 50 amp 125 volt is pretty inexpensive and easy to accomplish.

Nope. It's far easier and less expensive to add an inlet and a second 30 amp panel.

And I agree with one of the posts above suggesting that this be done by a licensed marine electrician. Anyone who would have to ask this question isn't competent to do the work safely.

[rwidman]

Originally Posted by Greggegner

Ok let me hijack the thread just a little...I have a reverse polarity detector on my 30a panel. I can't use a GFI shore source during the winter (it trips as soon as I load it). Anybody know why...


Greg S/V Sweet Dreams
Pearson P385 out of Racine Wisconsin

Yes. The reverse polarity circuit is a ground fault by design. The green or "OK" light is connected from the hot conductor to the ground conductor. It indicates that the ground is there but it is not compatible with a GFCI on the shorepower.

You are probably looking for a fix. The right way is to make sure there is a resistor in series with the green lamp that limits the current to below the threshold of the GFCI. The simple fix is to disconnect the green lamp from ground and either connect it to the neutral (in this case it will only be a power on lamp, not a safety device) or just leave it disconnected.

[mitiempo]

Originally Posted by rwidman

Nope. It's far easier and less expensive to add an inlet and a second 30 amp panel.

And I agree with one of the posts above suggesting that this be done by a licensed marine electrician. Anyone who would have to ask this question isn't competent to do the work safely.

On a parts basis you may be correct. But if you have to pay for labor to install the second inlet and to cut and site the new panel - as well as wire each circuit to it - I think the switch from 30 amp to 50 amp will cost less.

I agree best to have it done by a qualified marine electrician.

[exMaggieDrum]

Originally Posted by Cap Erict3

Ur line and neutral r not carrying equal load. Trips in winter? Heater element would be a likely bet. Easy fix.

You can check this out by first unplugging any heater that you use in the winter and see if it does not trip. Then plug it in and see if it trips.

Think about any devices you use only in the winter - like a dehumidifier.

If you only have one 30 amp service, the hot and the neutral should be equally loaded as the full AC hot current should be returned by the AC neutral. The only thing that would trip the GFCI then would be a leak to ground that is caused by the device in question. If so, it is faulty and should not be used.

You can get an out of balance load with 50 amp service since you have two hots and only one neutral. The loads (both intermittent and permanent) on 50 amp service should be balanced as much as possible to prevent this. For instance, a big heater or a big air conditioning load could do this. Still, the ground should not be carrying any of the load, and you still have a safety ground from shorepower, at least in the US.

[boatbod]

This thread brings up some interesting questions and exposes a whole bunch of misconceptions. Speaking the point of view of a currently certified marine electrician, I figured I'd try to hit a few of those points with (hopefully) simple explanations.

1. Any time you have *separate* sources of shore power, the current carrying conductors (i.e. both live and neutral) need to be separately protected with dual-pole circuit breakers, fitted with their own polarity indicators and bussed separately.

Interconnecting or attempting to "share" the neutrals is dangerous because you may - due to dockside wiring problems etc - inadvertently connect one shore cord to a reversed polarity outlet and the other to a regular outlet. Under such conditions you would effectively have a dead short from live to neutral occurring at the place where the neutrals were joined. Under the best case circumstances a breaker would pop and be a marginal inconvenience. Worse case your boat burns down or you die from electrocution.

2. If you have separate shore power inlets you need to protect both the grounding (green) wires from stray current using suitably sized galvanic isolators. It is permissible to share a single isolator and interconnect the grounding wires, but under such circumstances you must make sure the isolator is rated to carry the full fault current of both shore inlets simultaneously. i.e. you need a 60A isolator for twin 30A inlets.

3. A single 50A 120/240 shore connection has only one neutral wire. The reason this is allowable is because there is only a single dockside connector, and there is an implicit assumption that it is wired correctly and protected with a suitably sized triple-pole breaker. Equipment damage may occur if that assumption proves to be incorrect - a lost neutral is one of the most destructive failures you can experience with 120/240 service aboard a boat or ashore.

4. There should never be any connection made between the vessel's neutral bus(es) and the grounding system. This is a fundamental difference between shore based Natl Elec Code (NEC) and marine electrical systems. Bonds are required at sources of power (inverters, gensets) but these have to be switched in and out based on which equipment is the source of power at any given time. For this reason, shore-genset selector switches need to have sufficient poles to switch all live and neutral conductors at the same time. You'll need a 2 pole switch for single 30A service, 3 poles for a single 50A 120/240 service and 4 poles for twin 30A service. The neutral-ground bonds are made at the equipment providing the source of power, so when they are deselected the bond goes away too.

5. There should be a single interconnection between the vessel's shore power grounding (green) bus and the DC negative bus. Try to keep DC current out of the grounding bus as it can cause stray current corrosion. For that reason, always run both positive and negative wires from the breaker panel to DC appliances and resist the temptation to use the engine block as a convenient negative bus distribution point.

I could go on, but I think I've hit the essentials.

[tanglewood]

Nicely summarized - thanks

[Emmalina]

What the hell does anybody want more than 30A for anyway ? Cruise ship !

[smac999]

Originally Posted by Emmalina

What the hell does anybody want more than 30A for anyway ? Cruise ship !

heating / air conditioning / proper sized battery chargers. could each draw 30a

[boatpoker]

Originally Posted by smac999

heating / air conditioning / proper sized battery chargers. could each draw 30a

My boat runs quite happily all winter (in water, under shrinkwrap in Mississauga with an Electric furnace (Flagship Marine) and a 40amp (properly sized) battery charger

Except for this winter as we are currently in Cape May enroute to Bahamas.

[botanybay]

Originally Posted by Emmalina

What the hell does anybody want more than 30A for anyway ? Cruise ship !

Well, in my case the following:
2 - 15000 BTU air conditioners - ~2000 watts each
1 - watt hot water heater - ~2000 watts (new is 1200 watts)
1 - washer/dryer - ~1800 watts in dryer mode
1 - microwave - ~1200 watts
1 - holding plate compressor - ~1000 watts
1 - blow dryer for GF - ~1800 watts
1 - Charger in Inverter - ~4500 watts

Now these don't all run at once but combinations do. One can argue that with care you can turn on and off things to manage the load. Would you do that in your shoreside home?

I routinely exceed 3600 watts (the size of the isolation transformer) luckily the inverter can support the load. However, during the winter if both air conditioners come on I am loosing ground, someone takes a shower and even with a 7000 watt isolation transformer I am going to be in deficit.

This is my primary residence so that is somewhat different than a vessel which is rarely used.

Now, the previous 35' vessel, I did install the second 30A panel when I was living in MD. The reasons were twofold

1) If the dock power went off I did not want the inverter to try to carry the resistive loads if it was accidently left in automatic invert mode.

2) Resistive loads (hot water heater, oil radiator heater for cabin, cube heater for cabin) coupled with regular loads of the boat tended to burn up the shore power cable connection at the dock. If loaded for extended periods at near rating (lets say 25A) I found that the less than perfect shore power connections would heat up. After the first 3 winters aboard in MD I added the second shore power and never burned up a cable again.

It is essential to keep the two neutrals completely independent as well as the hots (obviously), in my case I used a pair of galvanic isolators (one for each 30A service) which were then each independently connected to ships ground (and the ground bus of each panel)

For this "resistive service" panel I used #10 wire throughout, not just from the shore power connection to the panel but to each point of service (hot water heater, a plug in the forward cabin, two plugs in the main cabin. Each point of service was supported by a single 15A breaker, the 30A main breaker on this panel (and the other panel) were double pole.

In my case, the second panel was physically separated from the main panel which resulted in two independent systems and was very useful when maintaining the primary electrical system I had drop lamps and heat available if necessary.

One of the other significant advantages of the dual 30A service is that the shore power cables are far more manageable. A single 125/50 or 250/50 shore power cable is rather heavy due to the wire size required for the 50A service. In addition, I can generally by 8 or more 30A shore power cables on sale for the price of a 50A cable which I have never seen go on sale.

[exMaggieDrum]

Originally Posted by botanybay

Well, in my case the following:
2 - 15000 BTU air conditioners - ~2000 watts each
1 - watt hot water heater - ~2000 watts (new is 1200 watts)
1 - washer/dryer - ~1800 watts in dryer mode
1 - microwave - ~1200 watts
1 - holding plate compressor - ~1000 watts
1 - blow dryer for GF - ~1800 watts
1 - Charger in Inverter - ~4500 watts

Now these don't all run at once but combinations do. One can argue that with care you can turn on and off things to manage the load. Would you do that in your shoreside home?

I routinely exceed 3600 watts (the size of the isolation transformer) luckily the inverter can support the load. However, during the winter if both air conditioners come on I am loosing ground, someone takes a shower and even with a 7000 watt isolation transformer I am going to be in deficit.

This is my primary residence so that is somewhat different than a vessel which is rarely used.

Now, the previous 35' vessel, I did install the second 30A panel when I was living in MD. The reasons were twofold

1) If the dock power went off I did not want the inverter to try to carry the resistive loads if it was accidently left in automatic invert mode.

2) Resistive loads (hot water heater, oil radiator heater for cabin, cube heater for cabin) coupled with regular loads of the boat tended to burn up the shore power cable connection at the dock. If loaded for extended periods at near rating (lets say 25A) I found that the less than perfect shore power connections would heat up. After the first 3 winters aboard in MD I added the second shore power and never burned up a cable again.

It is essential to keep the two neutrals completely independent as well as the hots (obviously), in my case I used a pair of galvanic isolators (one for each 30A service) which were then each independently connected to ships ground (and the ground bus of each panel)

For this "resistive service" panel I used #10 wire throughout, not just from the shore power connection to the panel but to each point of service (hot water heater, a plug in the forward cabin, two plugs in the main cabin. Each point of service was supported by a single 15A breaker, the 30A main breaker on this panel (and the other panel) were double pole.

In my case, the second panel was physically separated from the main panel which resulted in two independent systems and was very useful when maintaining the primary electrical system I had drop lamps and heat available if necessary.

One of the other significant advantages of the dual 30A service is that the shore power cables are far more manageable. A single 125/50 or 250/50 shore power cable is rather heavy due to the wire size required for the 50A service. In addition, I can generally by 8 or more 30A shore power cables on sale for the price of a 50A cable which I have never seen go on sale.

My home's electrical panel has 220v service, which is typical. My furnace and clothes dryer are 220v so it's required in any case, but I think most all service panels in "modern" homes have 220 service. The panel splits the the two hots to the two sides of the panel. The 110v circuit breakers on one side are powered by one hot and the ones on the other side are powered by the other hot. The big 220v breakers for the furnace and dryer used both hots. So the 110v circuits are roughly required to share the two hots provided roughly the same number of breakers are on each side. It's important in a home as well as on a boat to balance the loads as well as possible.