Onan MDKC 110v to 220v wiring question

[HeartWarrior]

I am in the process of updating the power on my boat from 125 to 125/250 capabilities. I have finished the shore power conversion with the installation of new input connections and the addition of new breakers, etc. The 220V will support the installation of a scuba compressor that I am installing this fall. My current generator is a 1990's vintage Onan MDKC that run perfectly. Although it is only a 4KW unit and I have to be selective what I run at the same time, it suits my purpose perfectly and do not have a burning desire to put in anything bigger. I have spent time reviewing the wiring both physically and from the manuals in hope that it would be easy to identify L2 of the generator which from my interpretation of the manuals and diagrams should already be in existence in the control box and I have not been that lucky. I'm really not that excited probing around with my meters with generator running to see what lights up! I have attached a few pages from the manual that has the electrical diagrams for the unit and was wondering if anyone can figure out how or what is L2 on the device so I can run a supply wire from it back to my panel to complete my upgrade. I have narrowed it down to thinking it should be connected to one of the 2 standoff insulators that are referenced on the drawing and referred to TB1 and TB2. I have also included a picture of the control box where you can see on the right side of the box, TB1 and TB2 as the red isolators. The wiring is still old school in that the red wire heading out the back of the control box is L1, and the black wire is the neutral. These will all be upgraded to match everything else on the box for AC wiring using the Black as L1, Red as L2, White as neutral, and green as ground.

Any help would be appreciated. Thanks,

 

[diver dave]

I am trying to do about the same. I have a 3 hp 15 Amp/240V Alkin that will be a marginal start with a 5.5kW genset.

 

[Mischief Managed]

You need to move a few wires to make it work, It is not currently producing 240 volts.


In 120V operation, it should be connected as follows:


H1 to T1
H2 to T3 to L1
H3 to T2 to L2 to Ground
H4 to T4


To convert to 120V/240V operation, you need to have it connected as follows:


H1 to T1
H2 to L1
H3 to L2
H4 to T4
T2 to ground
T3 to ground


This is all shown in figure 8.2, which has the 120V/240V shown as normal and the 120V reconnection diagram in the lower left corner.


Labels like "H2" are printed on the wires themselves, except for "L1" and "L2". "H" wires are typically thick and black rubber insulated, "T" wires are typically thin and brown fabric insulated.



Additionally, you will need to change the main AC circuit breaker (CB22) from a single pole, single throw, to a double pole, single throw so that both L1 and L2 are protected. If you can't find DPST a breaker that fits in the control box, you can bypass the internal breaker and install one in a junction box near the genset instead.



NOTE: Bear in mind that your boat main AC panel is setup for a single phase 120V input instead of two 120V inputs 180 degrees out of phase. If you leave it as-is, CB22 in the genset will trip immediately because L2 will still be connected to ground downstream of the genset. You must re-wire the panel so that it makes proper use of 240V. You also cannot simply use one leg of the 240V genset output to power the AC panel because it will only have half of the 4000 watt capacity when you re-wire per my instructions. You need to balance the AC loads across both 120V legs of the 240V output as closely as reasonably possible. I suspect you know this already, but I felt compelled to point it out regardless.

 

[tiltrider1]

An alternative to look into. Instead of rewiring your generator and panel, look into a 120v to 240v transformer just for your compressor.

 

[diver dave]

An alternative to look into. Instead of rewiring your generator and panel, look into a 120v to 240v transformer just for your compressor.

Good idea; and here is a candidate:

https://www.victronenergy.com/upload/documents/Datasheet-Autotransformer-32A-and-100A-EN.pdf

 

[HeartWarrior]

You need to move a few wires to make it work, It is not currently producing 240 volts.


In 120V operation, it should be connected as follows:


H1 to T1
H2 to T3 to L1
H3 to T2 to L2 to Ground
H4 to T4


To convert to 120V/240V operation, you need to have it connected as follows:


H1 to T1
H2 to L1
H3 to L2
H4 to T4
T2 to ground
T3 to ground


This is all shown in figure 8.2, which has the 120V/240V shown as normal and the 120V reconnection diagram in the lower left corner.


Labels like "H2" are printed on the wires themselves, except for "L1" and "L2". "H" wires are typically thick and black rubber insulated, "T" wires are typically thin and brown fabric insulated.



Additionally, you will need to change the main AC circuit breaker (CB22) from a single pole, single throw, to a double pole, single throw so that both L1 and L2 are protected. If you can't find DPST a breaker that fits in the control box, you can bypass the internal breaker and install one in a junction box near the genset instead.



NOTE: Bear in mind that your boat main AC panel is setup for a single phase 120V input instead of two 120V inputs 180 degrees out of phase. If you leave it as-is, CB22 in the genset will trip immediately because L2 will still be connected to ground downstream of the genset. You must re-wire the panel so that it makes proper use of 240V. You also cannot simply use one leg of the 240V genset output to power the AC panel because it will only have half of the 4000 watt capacity when you re-wire per my instructions. You need to balance the AC loads across both 120V legs of the 240V output as closely as reasonably possible. I suspect you know this already, but I felt compelled to point it out regardless.

Thanks so much! I installed a 3 pole (to include the neutral) already to support the upgrade to the shore power conversion to 125 / 250 so I just need to add L2 from the generator to the 0/1/2 source selector and I should be good to go. Appreciate the info and the quick reply!

 

[HeartWarrior]

Great idea on the transformer. Never even thought of it. Great backup plan.