Generator. Which receptacle to feed power to panel?

Amphicar770December 8, 2012

Hi All,
I just picked up a Master MGY500C generator. This is an older, low hour unit powered by a genuine Yanmar diesel. 5,000W rated / 6,500 surge.

First off, I do plan to have an electrician install everything. My questions are more around understanding the capabilities and limitations of my generator. I do have a cutler interlock panel cover and plan to have an electrician wire it up for me. However, I did have a question as I notice something that seems odd about this model.

It has a "Full Power Selector Switch" which the manual indicates sends either full power to the 120v receptacles or full power to the 120/240v receptacle with wattage capacity to the 120v receptacles.

Here is the odd part. The 120/240 volt receptacle is only a 20 amp twist lock (NEMA L14-20P) while the other 120V receptacle is 30 amp (NEMA L5-30P). Why would the 240 receptacle be lower amps? Normally I thought the 240V connector would be the higher AMP one. The generator spec is 41.7 amp at 120 volt which makes the max 20 amp connector seem even odder to me?? Most important, I thought the 240V connector is what you would usually feed into the panel? In this case should I be using the 120V?? Am i getting the generators full capacity if i use the 240? What am I losing by doing so?

Electrical Specifications
Rated Wattage* 5000
Rated Amperage
120 Volt 41.7
240 Volt 20.8
Receptacle Specifications
120-Volt, 15-Amp Duplex Yes
120-Volt, 15-Amp GFCI Yes
120-Volt, 30-Amp Twist-Lock Yes
120/240-Volt, 20-Amp Twist-lock Yes
120-Volt Full-Power Switch Yes

This post was edited by Amphicar770 on Sun, Dec 9, 12 at 8:22

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" Normally I thought the 240V connector would be the higher AMP one."

I'm not sure why you would think this. It's all about the math. 240V is the SAME thing as 120V in functionality, then the amperage has to be taken into account. Or did you just assume a 240V receptacle would be higher amperage?

You must use the L14-30 receptacle on the generator to feed your panel. You'll get ~21A per leg from it.
Be VERY careful as to what you run at the same time. That is a pretty small genset to feed the whole house, especially with a main breaker interlock.

    Bookmark   December 9, 2012 at 6:44AM
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Thanks. That makes sense. I did notice a typo in my earlier message. The 240v is only a L14-20p while the 120 is a 30 amp connector. That is what had me wondering if it was carrying full load. Why the lower amps on the 240 connector?

Fortunately I do not have too much need for power and utility service is very reliable where I live. Most of my lighting has been switched over to LED. Likewise, I have gas heat, stove, etc. for me it is mostly about keeping the lights and refrigerator on and the forced air for furnace. I do not expect this unit to be able to power the central AC.

I had initially been looking at bigger units but got too good a deal on this to pass up. Almost wound up with a large natural gas genset but then discovered I would need to upgrade my gas service and it was all getting to be way too much cost and hassle for a few hours use every few years.

    Bookmark   December 9, 2012 at 8:19AM
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Ron Natalie

As Petey points out you're not getting "higher amperage" PERIOD. What you are seeing is somewhat misleading PUFFERY in the specs. No electrical person outside the marketing department would have written those specs.

With a manual interlock, you're free to try a small generator. I'd take some colored markers or tape and mark the circuits you want to keep active when you're on the generator so when you have a power failure it's as easy as flipping off all the unmarked circuits, starting the generator and throwing the interlock to the generator side. You don't need to be futzing around in the dark trying to figure out what you have to shut off while you're getting the thing running.

    Bookmark   December 9, 2012 at 10:08AM
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"Rated Amperage
120 Volt 41.7
240 Volt 20.8 "

These actually are about the same,

120 v * 41.7 A = 5004 V-A,

240 V * 20.8 A = 4992 V-A

You are being misled by the puffery and confusion over the receptacle capacity that is limiting the available power.

Even them the most you cam really expect is about 80% of the 5,000 W including starting surges).

Just ignore the "surge" BS since t is exaclt yhat, BS.

    Bookmark   December 9, 2012 at 10:57AM
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Thanks all, very informative. This is also why I leave most electrical work to the pros.

I am considering going with a generlink transfer unit right at the meter.

I know I could get away with the smaller unit but this gives me room to grow for future. Any experience with these?

Thanks again.

Here is a link that might be useful: Generlink

    Bookmark   December 9, 2012 at 3:04PM
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I'm going to take a SWAG at this.

Normally on a generator like this, there are two phases or coils wound in the generator. Each is 120v by itself but they are 180* 'out of phase', or exact opposites. This lets you 'tie' them together to make 240v.

In your case, the wiring of these coils is rated for a bit more than 20 amps. Tied together for 240v it is still rated for a bit more than 20 amps.

My SWAG is that this 'full power switch' reverses one of the coils and lets both of them be in parallel, allowing +40 amps at 120v at that 30-amp socket. Hence you would never want to operate this switch when putting 240v into your house.

Around here, those Generlink things are sold and installed by the power company. I think one of the advantages is that it will automatically switch back to the line when power is restored.

    Bookmark   December 12, 2012 at 3:38PM
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