Breakers for 120/240v 4kw Onan diesel generator

Estragon · September 2016

As I understand it, the generator can output ~15A at 240v by tapping the windings in series or ~30A at 120v by tapping the windings in parallel. In my case, it's wired for 120v. It still has the L1 and L2 hots which are wired to two 15A breakers right next to the generator, then on to a knife disconnect which is used to connect either the diesel or a portable honda, at which point the two hots are tied together. Two hot wires then go to a pair of 35A breakers then on to the cabin panel. All the wiring from generator to cabin is the same size (looks like ~6awg to me). All this was in place when I bought the cabin, before I installed the solar system, and although I've moved the generator since, I've left it wired as I found it.

I've run into a problem in that the 15A breakers open when running the generator to charge batteries at anything like full load on the generator. For now, I've set the inverter/chargers (two outback 3548) to limit charging to ~12A, but would prefer to run the generator closer to fully loaded. Prior to the solar system, the heaviest load was a 120v jet pump, which would be under 15A. It's only since trying to charge the generator that the breakers have flipped, but it could be the generator has rarely or never been close to full load.

I'm a bit confused by the pair of 15A breakers. It seems to me the external wire should carry 30A no problem, but there are no breakers on the generator itself, so I have to assume they're sized to protect smaller wire inside the genset? The spec sheet shows 120/240v and continuous (standby power) rating as 33.3/16.65A. The breakers are old, so may just be opening prematurely, or it could be something is wired wrong.

I'm confusing myself thinking about the phase of the output on the two hots. With split phase, they would be 180deg out of phase, 240v hot to hot, 120v hot to neutral and ~15A on each leg. Wired in parallel, I'm not sure what happens. Is it still 15A per leg, 120v hot to neutral, 0v hot to hot, which seems to me to limit the output to 50% of the 4KVA.

Any thoughts?

BB. · October 2016

Assuming I understand your setup and questions--My first question is do you have a Current Clamp Amp meter so you can measure the current through each of the 15 amp breakers (I assume connecting to L1 off each pair of windings of the big genset)?

If you do not have a meter, can you turn off one breaker or the other to ensure that both are supplying current to the loads (i.e., one winding circuit is open and the other is handling all of the output current).

If the breaker handles are tied together--It is possible it it just the one breaker over currenting, and the other has no current flow...

-Bill

mike95490 · October 2016

maybe replacing old nasty breakers is an easy (but pricey) first step. But if they are built into the genset, maybe not so easy,

Estragon · October 2016

Bill - good idea. I have an AC clamp on ammeter and will check current on each leg. Breakers are not tied together. Mike - breakers are external to the generator. Trying to figure out what to replace breakers with (two separate 15A or single 30A.

BB. · October 2016

Each winding should have its own breaker... If you have one 30 amp breaker, and an open winding connection--Then the other winding would have to output >30 amps to trip the breaker (and probably cook the winding).

I would suggest two 15 amp breakers that are tied together--If one trips, the other trips too and hopefully protects your building wiring.

Are both (or either) breakers tripping, or only one repeatably tripping?

-Bill

Estragon · October 2016

The meter showing current at ~12A on one hot, and about the same on the neutral on the generator side of the breaker box. I assume it's measuring a waveform on the return at zero voltage to ground? Measured at a nearby subpanel (after the knife switch where the hots are tied together), there's 120v from hot to neutral and zero volts neutral to ground.

I'm thinking maybe the previous owners had it wired for 120/240v and later rewired to 120v (loads would have been unbalanced between legs and nothing ran at 240v), but didn't change the breakers?

I have a schematic for the genset, but the 120v breakout shows one leg and (size unspecified) circuit breaker as "when used".

BB. · October 2016

What I was curious about is the output from the two windings in parallel--Are they sharing ~1/2 the current each, or is one carrying near 100% and the other near zero (at the 15 amp breakers, before they are tied together).

If a winding connection (or one winding itself) has failed--That leaves the other winding to carry the whole load (and trip its 15 amp breaker).

Nominally, a standard USA home breaker will not trip at 85% or less of rated current. And will trip >100% current (may take seconds/minutes/hours -- depending on how high the over current is).

-Bill

Estragon · October 2016

BB. said:

What I was curious about is the output from the two windings in parallel--Are they sharing ~1/2 the current each, or is one carrying near 100% and the other near zero (at the 15 amp breakers, before they are tied together).

If a winding connection (or one winding itself) has failed--That leaves the other winding to carry the whole load (and trip its 15 amp breaker).

Nominally, a standard USA home breaker will not trip at 85% or less of rated current. And will trip >100% current (may take seconds/minutes/hours -- depending on how high the over current is).

-Bill

I have the charging restricted to 12AAC on the inverters to avoid tripping the breakers. All that current is being carried by the one hot wire to the breakers. I'm not sure how to test the windings.

I opened up the control box to confirm it is actually wired for 120, not 120/240. I had a theory that you might be on to something with the failed winding and it was wired for 120/240 and when it failed the previous owners did the fastest and cheapest thing they could to get both sides of the cabin fusebox lit. Stuff like floating ground (fixed when I put in new breaker panels) in the cabin leads me to think such a thing. Turns out it does appear to be wired for 120. One of the "hot" wires is cut in the control box. The other is connected in a way that suggests parallel wiring. If I'm understanding how the parallel wiring works, it likely couldn't produce enough current to trip the breakers if a winding failed.

I opened up the nearby breaker box, and the live hot feeds both 15A breakers, and the two hots become live again after the breaker box. Probably done with two hots after the breakers because the genset used to be in a shed a few hundred feet from the cabin and a single conductor with the size of wire they had at hand probably = too high voltage drop.

This is confusing stuff, but your suggestions have helped me get my head around it. If I'm understanding what I'm seeing in the guts of the beast, I'm thinking my next step will be to set the inverter limits to equal amounts at somewhat less than 15A each but more than 15A total, say 12A each. Not sure how I had them set when the breakers flipped, but it's quite possible they weren't balanced. Also not sure if they do a master/slave thing for charging as they do for inverting. Nice sunny day today, so will have to wait till tomorrow to try.

Thanks for your help - will post results.

Estragon · October 2016

I set each inverter to limit charge to 13AAC each this morning. Trimetric confirmed about 55ADC going to 48v bank so roughly 25AAC. After about 20mins both the 15A breakers opened. I reduced the limit to 12AAC and charged for about 15mins until the breakers opened again. The inverters cut back on charging current if other loads are present, but I kept other loads limited to a couple of led lights just to be sure.

Looks like the next step is to replace the breakers.

BB. · October 2016

Have you measured the AC line current (DC current clamp meter) through each breaker/circuit?

Assuming the pair of 15 amp breakers are in parallel (and carrying ~1/2 the current each)--12 amps AC should be fine.

-Bill

Breakers for 120/240v 4kw Onan diesel generator

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