MauiPunter

One Generator for two 30A circuits

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I have another electrical question.  When my boat had the generator installed by the last owner eons ago, the boat only had a single 30A shore powered AC circuit which this 5.5kw generator provided power for when disconnected.  My boat now has two 30A AC circuits and wondered, in order to provide enough power to both circuits do I need a generator that can output 60A minimum?  Would this be something around an 8kW generator?

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Since KW = A x V / 1000, yes a 8KW genset would give you full power.

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Unless you're going to fully load both circuits simultaneously, no.

Work out your maximum demand, the most that you're ever likely to use at one time and add some for a fudge factor.

Diesels don't like loafing, so loading it up is good for it.

Depending on the type of load, you may have to read up on power factor if you have a lot of inductive load (motors, coils) but if you have predominantly resistive loads (cooking or heating elements, incandescent lighting), then power (W) = current (Amps) X Voltage.

Microwaves (inductive) use more than the rated output power.  A 1500 watt microwave may use 3000 watts input.

Your generator should have overload protection so that the worst you will do if you overload it is to trip the circuit.

Cheers

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Size your generator based on total estimated usage, not based on maximum rated distribution. Also, read the generator's documentation carefully as there is no standard for AC voltage. Your 5500W generator could be 50A @ 110V or it could be 44A at 125V. This is probably selectable. Finally, the documentation will give you both a peak sustainable load and a maximum continuous load; until you understand which is 5500W you can't really figure any of this out.

The same holds true of your loads - a 50W TV or a 1500W microwave are both going to draw that power irrespective of the line voltage. However, a 1500W hair dryer, curling iron or kettle will draw less power as the voltage drops. Regardless of all that, 5500W is a lot of power and I would doubt that living within that envelope would present much problem unless you really need to substantial loads simultaneously.

Irrespective of the size of the generator, this setup seems a bit questionable. Based on what you've written ...

It sounds as if your boat has 2x 30A shore power cords. I'm guessing that these feed completely separate AC distribution systems on board and that they are fully independent.

It also sounds as if you have a single generator that presently only feeds one of the 30A AC distribution systems and you're considering changes to allow that single generator to now feed both 30A AC distribution systems. This requires some switching solution that parallels the two 30A AC distribution systems when the generator is running.

If the two 30A distribution system are combined when the generator is off and you connect the two shore power cords into sources that are different phases, your boat becomes the short between those two phases and the fireworks will be dramatic.

There are change-over switches designed to solve this problem, but they aren't for the faint of heart or the limited of distribution panel space - they are big, expensive, ugly things.

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So, I talked with my boat neighbor and he said the simplest way to do it would be having the generator output 240v 50A or so, and then you can split that into two 120V 30A circuits much like how you can split 50A shore power outlet into two 30A plugs.    He said a smaller genset could be used in combination with a charger/inverter to handle startup loads, which you guys mentioned above.  I am definitely not going to attempt any of this on my own just trying to do some homework on this.

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Your neighbor is mistaken. You can split shore power because if the 240V is delivered as three phase power you can reference each to the neutral to get 3x 120V feeds. This is exactly why you need to be careful about combining your AC distribution when using the generator because you'll potentially short two of the shore power phases together.

A 240V generator will be single phase and that cannot be split in two unless you use a relatively large transformer.

Do you really have more than 5500W of AC load on the boat? That's a lot of air conditioning, washer and dryer size loads. Is this really a problem that needs solving?

 

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22 minutes ago, Moonduster said:

Your neighbor is mistaken. You can split shore power because if the 240V is delivered as three phase power you can reference each to the neutral to get 3x 120V feeds. This is exactly why you need to be careful about combining your AC distribution when using the generator because you'll potentially short two of the shore power phases together.

A 240V generator will be single phase and that cannot be split in two unless you use a relatively large transformer.

Do you really have more than 5500W of AC load on the boat? That's a lot of air conditioning, washer and dryer size loads. Is this really a problem that needs solving?

 

No, I think the 5.5kW should be big enough I am just unsure if I can combine my two 30A circuits in parallel to it.  

AC BUS1 has:
Battery Charger
Outlets 1
Outlets 2
Freezer
Microwave
Water Heater

AC BUS2 has:
HVAC 1
HVAC2
HVAC Pump
Water Maker
Laundry


I want a solution that a single Genset can power both of these.

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1 hour ago, MauiPunter said:

No, I think the 5.5kW should be big enough I am just unsure if I can combine my two 30A circuits in parallel to it.  

AC BUS1 has:
Battery Charger
Outlets 1
Outlets 2
Freezer
Microwave
Water Heater

AC BUS2 has:
HVAC 1
HVAC2
HVAC Pump
Water Maker
Laundry


I want a solution that a single Genset can power both of these.

as described each of these loads is a single phase "110V" and you have two separate circuits operating independently and in parallel. 

(Was BUS2 added for the HVAC, Water Maker and laundry loads at a later date?) 

If that's so... You "could" combine the two BUS's with disconnect breakers to the same power source and run off one larger power cord and one genset. 

That you have two independent shore power cords, would indicate suggest this was  instead of combining into a single "50A" circuit, as very often marinas have 30A circuits, though "Y" adapters are common. 

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4 hours ago, LionessRacing said:

as described each of these loads is a single phase "110V" and you have two separate circuits operating independently and in parallel. 

(Was BUS2 added for the HVAC, Water Maker and laundry loads at a later date?) 

If that's so... You "could" combine the two BUS's with disconnect breakers to the same power source and run off one larger power cord and one genset. 

That you have two independent shore power cords, would indicate suggest this was  instead of combining into a single "50A" circuit, as very often marinas have 30A circuits, though "Y" adapters are common. 

Yes, BUS2 added by me when I bought the boat, and yes they operate independently and in parallel.

Correct, it was easier to add a second 30A circuit rather than upgrading to 50A circuit.   

Currently we run off of two 30A power cables that are hooked into 50A outlets with an adapter  (basically the second hot wire is ignored in these).  We also have a Y adapter for when we sail to other ports that is a 50A plug to two 30A plugs.   So, we have the flexibility to plug into (2) 50A, or (2) 30A, or (1) 50A and split with Y cable.

 

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Check out the Victron Quattro and Multiplus line that are boost inverters with on board chargers that can be ganged.  If you have a sizeable house bank they can rob peter to pay paul for some loads that once the loads are off, let the generator  max out the charging circuit and replace what was used without requiring you to do any switching.  Also works on shore power.  https://www.victronenergy.com/blog/2018/04/20/multiplus-quattro-inverter-chargers-improved-current-limits/

Rather than a disconnect breaker or Change over switch, these guys interface between the generator/shore power connector and the distribution panels and do the voodoo of both.     

They can give you another 8-16amps a piece off your house bank, automatically, if you have a draw that is marginal, and can be ganged together to give a greater output and charging capacity.  It seems like most folks size them to fully tax the generator as battery chargers first and foremost, and try to size the inverter half to fully utilize on their own each of the distribution panels they are attached to.   

You may also want to look into soft start kit for your A/C if already present, as the momentary loads can be reduced, they've got some new black magic type stuff.  Microair makes one that isn't a soft start capacitors/hard start capacitor but motor controllers that ramps things up slowly for the lowest amp draw. 

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3 hours ago, Zach said:

Check out the Victron Quattro and Multiplus line that are boost inverters with on board chargers that can be ganged.  If you have a sizeable house bank they can rob peter to pay paul for some loads that once the loads are off, let the generator  max out the charging circuit and replace what was used without requiring you to do any switching.  Also works on shore power.  https://www.victronenergy.com/blog/2018/04/20/multiplus-quattro-inverter-chargers-improved-current-limits/

Rather than a disconnect breaker or Change over switch, these guys interface between the generator/shore power connector and the distribution panels and do the voodoo of both.     

They can give you another 8-16amps a piece off your house bank, automatically, if you have a draw that is marginal, and can be ganged together to give a greater output and charging capacity.  It seems like most folks size them to fully tax the generator as battery chargers first and foremost, and try to size the inverter half to fully utilize on their own each of the distribution panels they are attached to.   

You may also want to look into soft start kit for your A/C if already present, as the momentary loads can be reduced, they've got some new black magic type stuff.  Microair makes one that isn't a soft start capacitors/hard start capacitor but motor controllers that ramps things up slowly for the lowest amp draw. 

I currently have four 4D 100Ah lead acid batteries in my house bank.  I am planning on also upgrading the house bank to more modern battery technology.  This does seems like a nice solution to bridge the gap. 

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Thinking about all this a bit, I recommend that you rewire your AC distribution so that your boat has a single 50A 110V (115, 120, 125) entry and main breaker. This is well matched to your generator, is inexpensive, simplifies your wiring, eliminates all the safety and logistical problems associated with the dual entries and reduces your entire problem to just a single concern, which is what to do when faced with a dock having only 30A outlets.

The there are three solutions when faced with that problem:

  • Use a Y setup to draw power from two shore side receptacles, having first made sure they're both the same phase
  • Get by with "only" 30A of shore power while you're on the dock
  • Run your generator on the dock when you need more than 30A

I recommend against any kind of inverter setup given the size of your house battery, especially if you're thinking that the inverter can augment the generator output.

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14 minutes ago, Moonduster said:

Thinking about all this a bit, I recommend that you rewire your AC distribution so that your boat has a single 50A 110V (115, 120, 125) entry and main breaker. This is well matched to your generator, is inexpensive, simplifies your wiring, eliminates all the safety and logistical problems associated with the dual entries and reduces your entire problem to just a single concern, which is what to do when faced with a dock having only 30A outlets.

The there are three solutions when faced with that problem:

  • Use a Y setup to draw power from two shore side receptacles, having first made sure they're both the same phase
  • Get by with "only" 30A of shore power while you're on the dock
  • Run your generator on the dock when you need more than 30A

I recommend against any kind of inverter setup given the size of your house battery, especially if you're thinking that the inverter can augment the generator output.

Ok.  That definitely would simplify things.   


 

 

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To have two separate AC sources (generator + shore power), you need to phase synchronize one source to the other. Generally speaking, forget about it. You certainly can't do this with your generator as the diesel spinning the generator would have to run at exactly 1500 RPM and that's not going to happen in the face of varying loads. And even then, it would need to have synchronization capability which is only a feature with new inverter-based generators.

Your choices for getting more than 30A at the dock are:

  1. Find a 50A shore side receptacle
  2. Find sufficient shore-side receptacles on the same phase that sum to the current you want (2x 30A, 2x 20A, 3x 15A, etc)
  3. Unplug from the dock and fire up the generator

I suggest spending an evening with all the manuals for all your AC equipment and create a spreadsheet that has turn-on and typical power values for each. Then, I'd spend some time on board measuring the actual running power demand of each load. 50A is a lot of power at 120VAC - I really believe you're over thinking a problem that most likely either:

  • Doesn't exist; or
  • Can be easily managed by not using the microwave and doing laundry (for example) at the same time.

 

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I agree.  After converting to a single 50A, circuit that solves 95% of the issues and I dont have to wonder how to combine anything.

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^^^ Moon is on the money.

The dual shore power inlet arrangement 2x 30A 110v or 2 X 15A 240v) was used by a number of prominant builders to cater for varying marina AC outlet capacities/number per boat. For example house power with or without Air Conditioning etc. However it was in an era pre-pass through inverters and generators etc appearing in smaller sailboats so a dated arrangement.

Anyone contemplating a conversion to a single shore power inlet and or catering for multiple power sources, Rotary Switches with multiple stages like those of Kraus & Naimer cost a packet but a necessity for setting up any failsafe  arrangement. This includes doing things like isolating inverter circuit from things like Hot Water, AC Chargers etc. Any arrangement involving the human brain remembering say multiple switching and sequences to save money on conversion avoid like the plague.

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You can (could?) get a simple sliding cover to go over a bank of 2 or more circuit breakers which stops the simultaneous use of 2 breakers. Useful in these situations.

Had one on my previous boat which stopped the use of the genset and shore AC at the same time.

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If I decide to take the route of combining my two 30A@120v circuits into a single 50A@120v, what is involved in that conversion?  Does the wire size between shore power and distribution panel need to be upgraded?  What size?  I figure I need to upgrade my AC MAIN breaker on my panel from 30A to 50A.   What other issues should I be aware of in this conversion?

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   Briefly:  new input socket, new breakers very close to input, larger wire from input to panel, new main breakers, and a close examination of your breaker panel and it's main wiring gauges, splitting the AC loads into two evenly loaded sections.

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1 minute ago, longy said:

And don't forget a galvanic isolator

Have one of those already.  :)

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They are rated to the amperage of the AC circuit (don't know why, but they are) so yer existing unit would have to be replaced

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11 minutes ago, longy said:

They are rated to the amperage of the AC circuit (don't know why, but they are) so yer existing unit would have to be replaced

Ahh, gotcha.   Thanks!

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