Sizing a Genny based on requirements.

[JDCAVE]

I will think about this but I’m still leaning towards the 5 kw. We made do with the 4.2 which was not the best in part because the voltage dropped as it was used. It just wasn’t a very good Genny, and was mostly targeted towards operation of the washer dryer.

We have solar panels, which on a good day will provide for the house loads about 200-250 amp hours/day at 12 VDC. However, the panels never bring the the charging to “Float.” Even when the 4.2 kw Genny was still running, in 2017, we only put 20 hours on it over 3.5 months at sea, typically only running it if we are at anchor for extended periods.

We have no water maker, and no air conditioning. We make pour-over coffee—both at home and on the boat. In the summer time we never trip the breaker on shore power. Load management has only been necessary in the winter. SOC on battery bank is seldom less than 80%. And we are not afraid to use either the vacuum, toaster or microwave to exercise the inverter. Only one at a time. I turn the inverter off unless I require it or we are traveling—always on while traveling to charge devices or fuel polish. The washer dryer and water heater are not on the inverter side of the panel.

I put a Charles isolation transformer on the one shore power leg. Putting on another shore inlet will require another ISO transformer. So upgrading would require another one of those and probably a full rewiring of a new panel.

I’m going to check the service from the Genny to the panel and confirm wire size today.

Edit: the other part of this, is ensuring the Genny is loaded sufficiently. The Magnum charger drops down to 40-50 amps and absorb charge fairly early on in the charging process.

Jim

 

[ghost]

I don’t think the number or size of the shore power cables tells you anything about the generator sizing. The generator will have its own feed into your power panel and shouldn’t use your shore power input. Really, it’s just about the size of the cabling from the gen to your transfer switch, you would ignore the shore power that will not even be in the circuit at the time the gen is in use. Most bus bars are rated to a higher rate, but that would be the other thing to verify directly as to whether you could take more than 30 amps into the panel leg.

I’d go with the larger for inconsequential cost, because in those first couple hours of charging, every amp counts and consuming power while charging can significantly slow charge rates if at max. There are other ways to buffer those momentary peak demands that won’t care if your generator has an extra 8 amps like my Victron Multiplus that will either momentarily invert the excess load or momentarily reduce the charge current or both. If you are right at the limits, this is probably an option worth exploring.

I also think the difference between ratings is probably more tangible on paper than in real life. Probably more energy expended in making the decision than what will actually be useful as the reality is peak loads are usually transient, but all depends on your boat.

The Northern lights is a great unit. I think mine was claimed at 5, don’t recall.

 

[caltexflanc]

Someday, either you or a future owner of the boat is going to regret you not paying the extra 500-1000 for the 6kw. I cyberly guarantee it.

 

[koliver]

I'm curious about your mechanic's concern about the wiring being inadequate for a larger generator. Did he determine the wire gauge before reaching that conclusion or did he just guess?

short answer: No idea.
This is a mechanic that I trust, as I have known him for a long time and have lots of experience with him.
How he concluded that wiring on board could be inadequate is one of those things where he has the experience to examine the areas of concern to him, without my needing to know what he knows or even what he has examined. Besides trusting my mechanic's opinion, I saved a little cash in staying with the smaller unit.

 

[JDCAVE]

The service leaving the Genny is Ancor 8 AWG 3 conductor Boat Cable.

Jim

 

[JDCAVE]

Someday, either you or a future owner of the boat is going to regret you not paying the extra 500-1000 for the 6kw. I cyberly guarantee it.

I could care less, quite frankly, what a future owner wants. It’s not as though he’s going to refuse to buy based on Genny sizing. Or offer me more money because it’s got a near new, “right size” Genny. Everything is a compromise when you buy a used boat, particularly one that is a 1985 KK42. If I sell in 5 years, it will be 41 years old.

Right now I’m concerned about what I need now. I’ve used the boat 3 seasons, put 1800 hours on the main, with some trips lasting 3.5 months. I have a very good idea about my needs. What I don’t want is a Genny that is over capacity and insufficiently loaded. I have enough maintenance issues as it is without taking on a water maker. Or air conditioning.

I may still go with a 6 kw, but I am concerned with insufficient loading, and service to the panel.

Jim

 

[cafesport]

The 5/6kw aka 673L4E is bigger and heavier than the 4.5/5kw aka 673LD3G. The 6 kw delivers 5 kw at 50 hz while the 5 kw delivers 4.5kw @ 50 hz. The smaller generator should be fine for the uses you mention. These things are little monsters. I can run two Watermakers and a battery charger in absorption mode or one Watermaker ac in two cabins and a float charge all night without a problem.

 

[rslifkin]

The service leaving the Genny is Ancor 8 AWG 3 conductor Boat Cable.

Jim

Based on that, I'd plan for the 5kw. The 6kw would be pushing it ampacity wise for 8 AWG.

 

[JDCAVE]

I’ve used the boat 3 seasons...

Jim

Actually 8 seasons. Not sure how this ended up as 3...

Jim

 

[DennisB1]

If you look at a lot of AC panels on a boat -- most often there are more circuits that if all used would be greater than the 50 or 60 amp capability of the generator. The critical wiring size is from the gen set to the distribution panel. The other wires are already protected.

So you load shed to control output. I have the same magnum 2800. It takes most of 25 amps from the gen set when charging the batteries at start. A hot water heater takes about 15. They typically both come on in the morning generator run. It nice to have some extra for a Coffee pot, maybe a hair dryer or even an electric heater if the water temp is cold. That all consumes the available on the 4 to 6 kw. Look carefully at what you want to run at the same time. I am happy with trying to keep 30% load or more on my NL 8kw unit. It runs at 170 F range. I use synthetic oil which helps keep engine internals clean. Had no problems with 100's hours run time.

 

[ksanders]

OK, I have been in the generator end business for over two decades, and have sold thousands of Marathon Generator ends.

Go with the larger generator end. Always go with the larger generator end.

The reason is that heat, over time is what kills generator ends (any brand). A larger generator end will produce less heat for a given load.

This is no secret in the generator end business. We actually specify generator ends based on heat.

Great example..

Lets say a customer needs a X KW generator end on a certain frame size (diamater)

We can meet that application with several generator ends based on the tolerance for heat.

Again...Always go with a oversized generator end. Less heat = longer life.

 

[JDCAVE]

Very good. Thanks Kevin.

Jim

 

[caltexflanc]

Thanks Kevin from me too. I was about to deploy one of those head banging emojis.

 

[JDCAVE]

An update. I’m taking the Northern Lights Zoom course, taught by Bob Senter. I asked what was the difference in the generator heads In the M673L3G and the M673LD3G and he said they both have the identical engine and the identical generator heads. He said the 6kw “punches way above its weight”. His exact words.

So I presume they accomplish the additional 1kw electronically some how.

Jim

 

[Lucky Chucky]

FF is spot on. I would listen to him. Bob Sertner @ Northern Lights just had this same question put to him yesterday on a zoom class call and he said for $200 more, go with 6K. And I trust Bob. FYI, I got a 8k and am happy with it.

~Lucky Chucky

 

[JDCAVE]

FF is spot on. I would listen to him. Bob Sertner @ Northern Lights just had this same question put to him yesterday on a zoom class call and he said for $200 more, go with 6K. And I trust Bob. FYI, I got a 8k and am happy with it.

~Lucky Chucky

Yes, that was me! It’s not $200 more. It’s $500. Plus exchange duty and taxes.

So we are basically a DC boat. I had trouble getting enough load on a 4.2 KW Genny. And while FF is spot on, I don’t have any of those induction loads installed in my boat.

The unit was purchased several days ago. I have already exceeded my budget and what I can realistically afford. Same engine. Same generator head. And if the 6 kw was punching way above its weight, the 5 kw should also be doing the same.

Jim

 

[rslifkin]

Keep in mind, with the 2 units having the same engine, a 2kw load, for example, is the same for either unit. So the 6kw isn't any more of an under-loading concern than the 5kw.