Two 30amp 125v shore power service to 50amp 125v boat input

[Le Tour]

I think I may know the answer but thought I would enlist the experience of the forum. I have two 30 amp 125v shore power circuits that I would like to use to power a 50 amp 125v input boat side. The 50 amp input is three wire; hot, return and ground. Each shore power is 3 wire 30 amp. The shore side is apparently 180 degrees out of phase providing 240v across the hot wires. There are adapters that can take the two 30 amp lines and convert to four wire 50 amp 125/250v service (2 hot, 1 return and ground). Is there a device that can take two 30 amp service lines and convert 50 amp 125v 3 wire service? The issue at hand is that I have several bulkhead mounted electrical heaters that when powered simultaneously (dockside in the winter) can draw up to 27 amps (120v ac). With 30 amps available this does leave much capacity for other AC loads, coffee pot etc. I can isolate the heater breakers on a separate circuit and install an additional 30 amp service input, but would prefer to leave breaker panel as is and up the shore power capacity to 50 amps. If I did put in a separate 30 amp service, could the heaters use the hot line from the additional service and the return and ground from the primary service? My guess is that the answers are No and Not Advisable. Thanks for your comments.

[osirissail]

Basically - NO you cannot "gang" together two 30 amp sources (same phase) and expect to get more than 30 amps.

Most shore power dock boxes are wired with 240VAC lines and these are then feed to a 50amp 120/240VAC receptacle. Likewise, each "leg" (120VAC) as also fed to 30amp 120VAC receptacles. It is common that when two boats need 120/240VAC feeds that one boat uses an adaptor that connects to one side via a 30 amp plug and the other plug connects to the other side of the dock box's receptacle. Now you have 120/240VAC available but limited to 30 amps per leg. But these boats are already wired with split AC busses (port and starboard or main and aux).

The simple solution for needing more than 30 amps to run your boat equipment is to split your boat's AC distribution panel into two halves, each half connected to its own 30 amp boatside shore power receptacle. In other words, you will end up with two shore power cords, one for normal 120VAC needs and the other for "additional" 120VAC needs like air conditioning, etc.

Many catamarans and small power yachts use this "dual" 120VAC 30 amp shore power cord setup. There is a switch on the panel that selects whether you use the dual 30 amp shore power sources or put them both together to be feed by a 50+ amp 120/240VAC genset on board.

"After-market" you can install a small AC-panel that is supplied from an additional 30amp 120VAC shore power receptacle then is wired to auxillary outlets that will power you temporary cabin electric heaters. As an option, a cross-over switch can be used to shift the 120VAC already installed wall receptacles from their normal source to the auxilliary 30amp 120VAC shore power source.

[Le Tour]

Osirissail, thanks for the clarifications. I will look into wiring the AC panel with two separate 30Amp circuits supplied by two separate cords.

[Stu Jackson]

Originally Posted by Le Tour

Osirissail, thanks for the clarifications. I will look into wiring the AC panel with two separate 30Amp circuits supplied by two separate cords.

You still need two separate AC panels, NOT two shorepower circuits into ONE existing panel.

[Le Tour]

Originally Posted by Stu Jackson

You still need two separate AC panels, NOT two shorepower circuits into ONE existing panel.

Thanks Stu, I understand the issues. I have an ability to isolate the panel containing the breakers for the in-wall electric heaters. Those breakers are already in a separate panel that is jumpered to the main panel. I have space to install a separate polarity fault detecting breaker to feed that panel. That breaker will be powered by wiring from the second shore power cable.

[Stu Jackson]

Super. Just wanted to make sure...

[mitiempo]

The neutrals as well as the hot wires have to be separate.

[Le Tour]

Originally Posted by mitiempo

The neutrals as well as the hot wires have to be separate.

Thanks, I plan on maintaining segregation of the neutral as well for the new circuit on the AC distribution buss. But, I plan on connecting the new ground to the existing ground at the input (shore power side) to the galvanic isolator. It looks like the boat ground wire at that point is 6awg. I need to read up on the isolator to see what kind of current it can carry in a short condition. I expect coupling the grounds together at that point is OK. Any comment s appreciated, thanks.

As an aside I downloaded Nigel Calder's book "Boat Owners Mechanical and Electrical ...". The book along with the comments on this forum are a great reference. Calder recommends using two separated 30 amp circuits vs. a single 50 amp (boat is wired for the single 50 amp), but the book did not explicitly comment on where to tie the grounds together. The down side of the two circuit approach is that the second circuit will not be coupled to the generator when switching from shore power to genset. I suppose I could also wire the new circuit to the genset side of the shore power / genset switch, but I have not yet thought through the implications of that.

[mitiempo]

The grounds are common. Make sure the galvanic isolator can handle the load.

Do not wire the new circuit to the genset side of the switch - that allows the possibility of genset and shorepower together.

Best solution I think is for each shorepower inlet to go to its own panel, with a switch for combining off of one shorepower inlet when necessary. This is the arrangement on many powerboats with multiple shorepower inlets. Both hot and neutral have to be switched.

[Le Tour]

Originally Posted by mitiempo

The grounds are common. Make sure the galvanic isolator can handle the load.

Do not wire the new circuit to the genset side of the switch - that allows the possibility of genset and shorepower together.

Best solution I think is for each shorepower inlet to go to its own panel, with a switch for combining off of one shorepower inlet when necessary. This is the arrangement on many powerboats with multiple shorepower inlets. Both hot and neutral have to be switched.

Mitiempo, thanks for your comments. I can see that the second circuit will require a second shore power / generator switch if I would like to power it from the genset without connecting the genset to shore power.

[morningstar10]

I am planning on installing the twin 30 amp system with a generator, two panels, second panel feeds loads that shouldn't come from the inverter like A/C and water heater. Does the switch combining the panels for the generator feed pose a danger if accidentally left on while connected to shorepower? That would backfeed the generator,right? Is there a way to lock out that possibility?

[Le Tour]

Morningstar10,
The short answer is "Yes, you can feed two AC panels from the generator without back feeding shore power... if the system is correctly wired." As you have already suggested you cannot have any connection between your generator and shore power. Such a connection risks electrocution of a marina worker if they happened to be working on the marina wiring while a generator is operating. If you have two Shore Power AC sources (AC1 and AC2) they each need their own breaker within 10 feet of the shore power connectors and both shore power circuits need to maintain separation of Hot and Neutrals as you have planned. For your AC2 electrical panel you could add an AC Source Selector (AC1 or AC2) switch between the second AC panel and breakers. (This switch is typically a rotary switch that has two sets of input terminals and one set of output terminals. The switch will not connect both input sources together) So, if you went to a Marina and only had access to one shore power connector, you could power your second AC panel (water heater etc) using the AC1 source. Of course, if your electrical loads are high you will need to balance power use in order to reduce the possibility of tripping the primary (AC1) breaker. Your primary AC panel should also have a source selector between Shore Power and Generator. Selecting Generator on such a switch should disconnect your boat from Shore Power. If you select Generator for your primary panel, and then select AC1 for your second AC panel, you could power both AC panels from your generator. You might want to add an AC power meter to your second AC panel so you can monitor the power use on that panel as well. Blue Sea corporation makes some nice breakers, switches and AC power monitors that display WATTS, AMPS, VOLTAGE, LINE FREQ etc. Obviously when dealing with AC power there are many liabilities including damage to electronics, fire, and electrocution etc. If you wire things incorrectly or don't comply with ABYC standards and your boat burns, your insurance may not pay for it. Likewise, if someone gets electrocuted ... So, use care, plan on spending some money on quality parts, read up on the ABYC standards and get some help if you are uncertain. Hope this helps.

[morningstar10]

I think I get it. You're saying use 30 amp circuit breakers on the primary panel with a selector for generator or shorepower, then feed the second panel from the first panel's breaker through a switch that also has a choice of sources, generator or AC1. If both panels were on generator, you'd have generator power to both panels and all the A/C loads. If the first panel were on shorepower, AC2 would either have shorepower or nothing, depending on the switch between the breakers. I have a Blue Seas panel 8466 that will be fine for AC1. I'm thinking that instead of a rotary switch, I could use a Blue Seas 8499 for AC2. What do you think?

[Le Tour]

Originally Posted by morningstar10

I think I get it. You're saying use 30 amp circuit breakers on the primary panel with a selector for generator or shorepower, then feed the second panel from the first panel's breaker through a switch that also has a choice of sources, generator or AC1. If both panels were on generator, you'd have generator power to both panels and all the A/C loads. If the first panel were on shorepower, AC2 would either have shorepower or nothing, depending on the switch between the breakers. I have a Blue Seas panel 8466 that will be fine for AC1. I'm thinking that instead of a rotary switch, I could use a Blue Seas 8499 for AC2. What do you think?

Morningstar,

What I have suggested is close, but a little different.

Primary AC Panel -
Two inputs, AC1 and Generator go into a Source Selector Switch "1" (SS1). The output of Source Selector "1" feeds the Primary AC Panel.

Secondary AC Panel -
This panel also has Source Selector Switch "2" (SS2). The inputs are AC2 and the Output of SS1. The output of SS2 feeds the Secondary AC panel.

So, the output of SS1 is wired to feed the Primary AC Panel, but it also has a set of wires feeding one of the inputs of SS2. The other input is AC2 and the output feeds the Secondary Panel.

In standard shore power use with two shore power cords SS1 for the Primary AC Panel is set to AC1. SS2 for the Secondary AC Panel is set to AC2. If you are at anchor and have the Generator operating, SS1 is set to Generator to feed loads on your Primary AC panel. If you need AC power to a device on the Secondary AC Panel you switch SS2 from AC2 to the Output of SS1. Likewise, if you are at a marina with access to only one Shore Power cable, plug it into the AC1 receptical, set SS1 to AC1 and set SS2 to Output of SS1.

There are other ways to hook things up, but this approach provides good flexibility. If you don't already have a copy of Nigel Calder's "Boat Owners Mechanical and Electrical Manual" I would recommend downloading it for reference. The book provides excellent tables for wire sizing given wire run lengths and conditions, breaker sizing and positions, wire types, connector types, ABYC standards etc.

The 8499 panel looks good as it provides the isolated source selection, source circuit breakers and several device breakers all prewired in one panel.

[morningstar10]

I think that's what I was visualizing. Two 30 amp inlets. The first inlet would go to the first 8466 panel 30A breaker with the output from the generator on the other 50A breaker to be selected on the panel by choosing between the breakers. The output of the both breakers on the 8466 would feed AC1 loads and the first breaker of 8499. The other breaker on 8499 would come from the second shorepower inlet. Both breakers on AC2 would feed AC2 loads.

Shore power to 2 inlets: AC1 to shorepower, AC2 to shorepower, 30 amp inputs feed the AC1 and AC2 loads. 60A total available.

Shore power to one inlet: AC1 to shorepower, AC2 to crossover, 30 amp from first inlet feeds both AC1 and AC2. 30A total available for both panels

Generator: AC1 to generator, AC2 to crossover, 50A total available for both panels. Is this right?

The only problem I have with this setup is what would happen if I accidentally left the second panel on the crossover breaker and plugged in shorepower cords to both inlets that were out of phase. I hope the breakers would trip and protect the system but can I count on that?
Or am I still unclear on your plan? Sure wish I had a picture of this. Calder's book has a lot of information but if this setup is in there I can't find it.