MEP-003a in 120V setting connectivity question

[TommyG45]

Hello Everyone,

It's been some time since I have posted. I check in on you guys from time to time though.

I just reciently acquired two MEP-003s. Actually more generator than I really needed but they were local and they are green so I just couldn't resist.

Is it better to connect at 120V so I am loading the generator evenly across all the poles rather than go 120/240 and have to deal with manually loading each side of the generator? The only thing that runs 220 is my water heater, stove, and clothes dryers. None of these are currently set up as a split phase coming in off the utility so even in theory I could just run the two legs of the 220 (black wires) from say the water heater to the same 120 source coming off the generator. My thought would be to feed the 120V @ the full 104 amps from the generator to a transfer switch and not worry about load balancing.

Is anyone doing this, or are you running in the 120/240 mode?

Any suggestins on manual transfer switchs?

Andy

 

[steelandcanvas]

From what I've read, the 120 single phase selection provides less than optimum power, maybe around 80%. The Genset Gurus will chime in on this.

 

[Isaac-1]

In theory you will get less total KW out of the generator before soemthing smokes in the generator end when wired for single phase rather than 3 phase, but given how overbuilt these units are it is not something I would be too concerned about, this applies to both 120V only and 120/240V so the last comment is somewhat moot for your configuration.

Assume we are talking single phase for the rest of this post, uless mentioned otherwise.

As to the original question, if you do set it up in 120V only mode, you don't have to worry about overloading either output leg and letting the smoke out (you do of course have to worry about overloading the whole thing), however as generators get larger it generally becomes easier and easier to avoid overloads of a 120V leg with just a little bit of preplanning, mainly in making sure your intended major loads are balanced in your breaker box if doing a whole house transfer swtich, on in the sub transfer panel if not and not have all planned 120V loads on one leg.

Let me give an example with 2 generators on the small and large end of the spectrum that you may find for home backup use (yours falling about in the middle)

First a 3.5KW generator which outputs 30 amps in 120V mode or 15 amps on each leg in 120/240V mode
A typical hair dryer or vacuum cleaner pulls about 12 amps, meaning that running one of these on a 3.5KW generator in 120/240V mode and even without running anything else you are within 3 amps of the output rating, which of course means running anything else at the same time and you will likely be in overload. By comparison in 120V only mode with 30 amps available, a 12 amp hair dryer draw will leave 18 amps available before going into overload .

Now consider a 20KW generator which outputs 167 amps at 120V or 83 amps per leg in 120/240V, when connected 120V only that same 12 amp hair dryer leaves 155 amps of available power before overload, and in 120/240V it leaves 71 amps available.

Conversly this means that as generators get larger the relative size of any single 120V load (being limited to about 12 amps by code plugged into a standard 15 amp outlet) gets smaller and smaller, and less likely in itself to push a generator into overload on a single leg. In a typical house all larger loads (air conditioners, water heaters, well pumps, etc.) will by 240V and draw from both legs of the generator and therefore be balanced. So some simple planning like making sure all the window air conditioners are not on the same leg when running 120/240V, goes a long way

I have a 30KW generator installed at my elderly mother's large all electric farm house running in 120/240V mode, and gave no thought at all into trying to balance the 120V loads, since in her case the 240V loads (electric heat, air conditioner, stove, well pump, etc.) so massively outnumber the amp draw of the 120V loads which with the exception of hair dryers, window air conditioners, and the like seem to be drawing less and less power every year with the invention of CF light bulbs, flat panel TV's etc. (my new 40 inch LED edge lit flat panel tv draws less than 60 watts)

Ike

p.s. wiring cost will likely be higher in 120V only mode, since you will have to run larger conducters for the higher amp draw, even though you will have one less wire the larger conducter size will likely still cost you more.

 

[TommyG45]

Ike,

Makes total sense. Thank you. From a transfer switch prospective, I would like to do a whole house transfer switch. With the MEP-003, wired in 120/240 volt single phase, I would be looking at a 100 AMP transfer switch, 50 for each leg? Is that how the transfer switch is rated? Or would that be a 50 AMP transfer switch because each leg would be rated for 50 AMPS (I got to believe there is a standard here)

Sorry for basic questions, and appreciate your insight.

 

[Isaac-1]

Doing a whole house transfer switch it woud be rated at the size of your utility service entrance, often 150 or 200 amp, sometimes larger. The NEC (electrical code) is always changing the latest version has a provision requiring the use of relatively expensive load shedding automatic switches unless you have a generator that can carry your full service entrace load if your using an automatic switch, but still allows manual transfer switches with undersized generators and manual load managements. To me it is a stupid rule, the problem was not a safety issue, it was Generac and many others selling packages sets with 5-15KW generators and whole house automatic transfer switches, if the power went out while there was a heavy load (electric stove, heat, air conditioner, etc.) and the automatic switch flipped to generator power it could overload the generator, and either trip an output breaker or fry the generator, but then again I think if people want to be stupid about the way they install equipment they should have the right to.

As to the type of switch you need (switched or unswitched neutral), it is best if you consult your local inspector and see what they want. We kind of get into a gray area on the NEC when it comes to military surplus generators, as part of the code includes words like installed per the manufacturers instructions or must be UL listed. Search on transfer switches here and you will see lots of information, but it all comes down to what your local inspector will accept.

Ike

 

[TommyG45]

Thanks Ike!