Rehash: 120 volt or 240 volt for 12.5 KW generator

[belfert]

I got my 12.5 KW generator installed a few months ago and it was wired for 240 volt.  I had always intended to set it up at 120 volt at a later date.  Thoughts on 240 volt vs 120 volt?  In my mind 120 volt gives me more flexibility so I don't end up with one leg overloaded and tripping a breaker.  120 volt is 104 amps which requires much larger wire. 

I have it wired right now with 2 A/C units on one leg and the other leg with an A/C unit, 30 amps for inverter circuits, and water heater.  Every circuit inside the bus except A/C and water heater goes through a separate inverter panel.  I have no plans at this time to ever use anything is 240 volt.

A friend of mine said I shouldn't have unbalanced legs, but that is almost impossible with a bunch of 120 volt stuff.

[richard5933]

Nearly every one I've seen, both DIY and commercial motorhomes, have the generator wired as 240v with two 50-amp legs of 120v, not one. If you ever plug into a pedestal, that's exactly what you'll have. Same balancing problems will exist there, so you might as well get things balanced as well as you can. At least in my opinion.

[sledhead]

I always try to make both legs close but some times it just does not work . I use the 2 ac meters to see what is going on


dave

[6805eagleguy]

As far as I know, I have the same exact genset as you do, I ordered it as a 120v genset and I have 2 120v legs. Curious how you would get one leg of 120v

[belfert]

My previous Powertech generator also had two 120 volt legs when in 120 volt mode, but Powertech had you connect the two legs together.  I talked with Justin at Wrico who suggested combining the two 50 amp legs into a single 100 amp leg in a junction box outside the generator.  What I hope to avoid is to have say 10 amps remaining on each leg and need 15 more amps and overload a leg, or have one leg running a bunch of stuff and the other leg with minimal load.

I am pretty sure I will just leave it on 240 volt for this year and see how things go.  It certainly will provide at least as much power as the 8KW even if the legs are not balanced.  It is already wired 240 volt so nothing would have change right now.

List of what I might have running at the same time:

1. Four 15,000 BTU air conditioners (one in enclosed trailer)
2. Water heater with 700 watt element
3. Microwave
4. Refrigerator
5. 30 cup coffee pot
7. 700 watt food warmer
8. 15 cubic foot chest freezer
9. Battery charger in inverter
10. Three to four laptops
11. Various power tools outside plugged into bus

I had been running all this stuff off of my previous 8KW generator and realistically it isn't all drawing power at once.  Now, with my old generator I could only run one or two A/C units at a time.  I tripped the 35 amp generator breaker a few times only because the voltage was getting very low which increased the amp draw.

I went with a 12.5KW generator so hopefully I could run at least three A/C units at a time.  I ran three A/C units plus refrigerator, water heater, and occasionally the microwave on my drive from Las Vegas to Phoenix in May with no issues.

[thomasinnv]

If wrico says all you have to do to convert from 240 to 120 is connect the 2 legs together, the simple thing to do is just jumper L1 and L2 together at the generator input to the transfer switch. That way both legs are pulling equal amperage from the genny no matter how much amperage is being pulled from either leg of the main panel. With the jumper on the generator input side of the transfer switch, you still have 2 seperate legs when connected to 50 amp shore power.

My old generac was wired this way and worked flawlessly for years. You need to pay attention to the neutral leg though, a split phase 240 service will not require as large of a neutral as 2 single phase 120 lines. The split phase 240 legs cancel each other out so the neutral only needs to be large enough to carry full load of one leg. With 2 120 single phase lines you need neutral capacity to carry both legs at full amperage.

[belfert]

There is a bit more to a 120 volt conversion than just connecting the two legs together.  A few wires would have to be reconnected in the generator electrical box.

The two feeds don't need to be tied together, but you lose the advantage of 120 volts offering 100 amps of power otherwise.

[buswarrior]

You have noted the limitations of only having so much power left on either side, neither enough alone.

Your 12.5k generator is really two 6.25k generators when wired for 240. Where people really get into trouble is down at the 6-8k range, and wonder why they can't do anything without tripping breakers...
Ónce saw a 4k wired for 240... wouldn't even start a single old roof air without tripping... that one got rewired at a bus rally, owner was ecstatic to actually have some power, after dragging that thing around for years...

The percentages of imbalance in our small generators is often quite large, the vendor who took care of a mobile command unit i was involved with, that constantly had generator troubles, blamed our engineers for their foolish 120/240 design, duplicated twice, and powered by twin 8k gens... which were really 4k legs... hvac, telecommunications, kitchen fridge coffe maker... no wonder...

After that foolishness, i prefer them all tied into a single leg and wire it to support.

Happy coaching!
Buswarrior

[dtcerrato]

Our 6.5kw Onan is wired for 240V that's two legs of 120V @ 30A each. In our case it is strategic to have the 240V capability for the ponderosa well & central hvac units in blackouts during hurricanes & aftermath. In the bus our distribution panel of six branch circuits are split and balanced across the two 120V genny legs. We don't require 240V in the bus but need the capability on the home front & hasn't been an issue as long as you do your homework and do the balancing act...

[belfert]

Your 12.5k generator is really two 6.25k generators when wired for 240. Where people really get into trouble is down at the 6-8k range, and wonder why they can't do anything without tripping breakers...

Basically, I get two legs of 120 volt at 50 amps with the 240/120 volt configuration.  I will probably give it a try with 240 volt this year so I can spend time on other bus projects.

For my 8KW generator it was recommended to run it at 120 volt unless there was a need for 240 volt.  My previous generator came set up for 240 volt and I converted it to 120 volt.

[richard5933]

If you have two legs of 120v at 50 amps then you have as much as any campsite pedestal. Should be enough to run any rig out there if set up correctly.

[belfert]

My bus generator supports a lot more than just the bus itself when at our campsite.  I have two 30 amp 120 volt receptacles.  One is for a 24 foot enclosed trailer that has a chest freezer and a 15,000 BTU rooftop A/C unit.  The other 30 amp outlet supplies a whole electrical distribution system for our camp.  One of my friends sells food so she has a 700 watt food warmer and a coffee pot plugged in.  My other friends have various power tools and laptops plugged in.

I have never parked my bus at a campground with power.  I plug it in at home and I plugged it in at Gary Bennet's place plus my friends in Phoenix had a 50 amp receptacle at their house that I used for a night.

[Dave5Cs]

Interesting enough we are starting to see 100Amp services at some campsites.

[richard5933]

Interesting enough we are starting to see 100Amp services at some campsites.

What kind of receptacle are they using? Two 50-amp?

[Jim Blackwood]

I'm surprised that nobody has mentioned that phase matching is required any time you tie two 120v legs together. Do it wrong and you can burn your house down.

240v is nothing more than two 120v legs that are completely identical in all respects but one, and that one is all important. They both carry a sine wave power signal and that wave is exactly 180 degrees out of phase so that if you place one waveform above the other, the peaks go close together and then they go as far apart as possible. The 240 volts is where you measure them when they are far apart.

Now, if you put both of those on the same line by tying them together they will cancel each other out completely, give you zero volts out, pop your breakers and maybe burn down your generator. Something to keep in mind.

Jim