Double Pole Genset breaker

[ORIF]

I'm having an issue the breaker for my genset.

Generator is an 8KW NorthernLights running at 120V.

The breaker is mounted on the bulkhead immediately adjacent to the genset.

Breaker is a 2 pole 50 amp Square D (Home Depot style) breaker and receives the hot and neutral cables (6AWG) from the genset. Those cables go on to the house AC panel which is divided for 2 30 Amp shore cables.

The double pole genset breaker trips when it's hot and rarely if at all when it's cool.

I measure 40 amps going through the hot cable back at the generator with several loads running including 3 air conditioners.

When taking the breaker out on those occasions, the breaker itself was hot and on the most recent change out with a new breaker I noticed that it seemed a little loose on its clip-on attachment. I'm thinking the loose contact is probably causing some if not all of these issues including the breaker generating some of its own heat.

So, I'm replacing the breaker with a proper Blue Seas double pole marine breaker that's rated for 185° F.

My boat mechanic/electrician tells me that I should also be able to safely increase the breaker to 70amps. Again, the genset runs at 120V and it's hot and neutral go to this 2 pole breaker. He's saying this setup could put out 66 Amps.
The breaker I ordered from Blue Seas is a 60 amp (they go from 50 to 80 with no 70).

Any support or disagreement with the above mentioned 70 amp breaker size recommendation or how to determine what it should safely be?

Thanks

sorry, my pics usually post sideways and I can't seem to find the rotate button

 

[Choices]

That breaker trips on overload with a bimetal strip that bends as it heats. It is roughly a time delay setup. In addition it has a separate short circuit trip.

Once a breaker trips until it cools to an ambient temperature it will trip sooner cause it is preheated, to the point of failure. A hot engine room just shortens the time to trip.

A loose connection will cause resistance and to the breaker seem like an overload.

I have not done the calcs to see what the genset can deliver, but 66amps seems about right.

In your Northern lights genset control box should be two fuses. Those should give you an idea what genset can deliver. May even be a spare set in there. Look at the schematic and confirm the sizing and spares. Most are Japanese porcelain fuses and very expensive. Call Fuseco with the part number and get extras slot cheaper.

Residential breakers have a UL approved tolerance of like 10% above and below it's rated amps. So a 50amp could deliver 45, and be within spec WHEN NEW.

This assumes a constant load. A/C's kicking on, microwave, battery charger, refrigerator, all can spoke the amps about 6times normal. This again heats the breaker till it trips.

I bet if you turn off one A/C your problem goes away.

 

[Comodave]

If I understand you correctly you are feeding 2 30 amp services with the genset. So why would you need a 70 amp breaker? You can only use 60 anyway between the 2 30s. Also a quick google for amps on 6 gauge wire says 55 amps so I would not even go with the 60 amp breaker. That is probably why the original breaker is only 50.

 

[ORIF]

If I understand you correctly you are feeding 2 30 amp services with the genset. So why would you need a 70 amp breaker? You can only use 60 anyway between the 2 30s. Also a quick google for amps on 6 gauge wire says 55 amps so I would not even go with the 60 amp breaker. That is probably why the original breaker is only 50.

Good point and the breaker size for 6awg is all over the place when I attempt to look it up.
1st check I made cake up 80amps but now that you mention it I’m finding as low as 50 and just saw 120 on a Boat US site. Probably due to temperature and I need to pull out Calder’s book.
Cable run is about 20 feet.
When I calculate recomended wire size on the ABYC calculator it comes out 6awg on the 3% drop table for a cable carrying 60 amps

 

[rslifkin]

How much the 6awg can carry depends on the wire and its insulation ratings. In the house wiring world it would be good for anywhere from 55 - 75 amps depending on cable type, but 6 awg marine cable can be good for as much as 120 amps.

 

[caltexflanc]

60 amps makes sense to me. I've never seen a 70, but am sure they must exist, but I like erring on the side of caution. I definitely bet cash money that the heat issue was caused by a bad connection. Perhaps in tandem with maybe running close to the amp limit, but I'd have to take a meter to it.

Speaking of which "while you're in there" be a nice idea to install an amp meter on each leg if the boat doesn't have one, and frequency meter for the generator (low frequency, low voltage, bigger amp demand).