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Watts = Amps x Volts

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Hat52MY

Hat52MY

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What is considered "Full Load" & "Half Load" on a generator? This question hit me the other day when I was thinking I have a 20kW generator but the panel only shows me generator output in Amps. Doing some quick calculations via excel, the chart below resulted. So which is correct for understanding the load on the generator? In addition, what should be the minimum load be (Wattage or Amperage) to prevent any glazing of the cylinders? Additional thoughts?

Equipment: Onan MDL4, 20kW

Amperage Perspective Watts = Amps x Volts
Quarter Load @ 15 Amps 3,600 = 15 240
Half Load @ 30 Amps 7,200 = 30 240
Three Qtr Load @ 45 Amps 10,800 = 45 240
Full Load @ 60 Amps 14,400 = 60 240

kW Perspective
Quarter Load @ 5kW 4,800 = 20 240
Half Load @ 10kW 9,840 = 41 240
Three Qtr Load @ 15kW 14,880 = 62 240
Full Load @ 20kW 19,920 = 83 240
 
I don't understand your "amperage perspective". Go with the kw perpective. I have the same generator, has about 4300 hours on it, purrs right along regardless of load and doesn't smoke or burn oil. I do an oil analysis every change and everything comes back fine. We try to keep at least a 25% load on her (20 amps) but it doesn't seem to care much, and has run long intervals with less.
 
"Go with the kw"

Agreed - that's all that really matters - how many watts are being used. The amperage changes with the voltage but watts is watts! :)
 
Because he doesn't have a "Wattmeter".

Voltage isn't supposed to change unless your system is over loaded or you have a bad regulator. What I don't understand is how these calculations were achieved. If you have a 20KW generator operating 240V your full load amperage is about 83. How did you come up with 60?
 
The "amperage perspective" chart is incorrect....for example your "full load" figure of 6o amps times 240 volts is 14,400 watts which is 14.4 KW....but you have a 20 KW generator.

The other chart is correct:

Full load amperage is about 83 amps as SCROD posted: 20 KW (20,000 watts) divided by 240 volts is your full load amperage.....and half load current is about 41+ amps.

We've discussed the necessary "no glazing" load on several occasions without resolution.....it might even vary by manufacturer and rated RPM....Best to call the manufacturer and see what they recommend. It seems like as long as a genny is not running unloaded for days on end, cylinder glazing is not typically an issue.
 
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an excel spreadsheet? I prefer the KISS method!!

seriously... on a 20kw genny, if you can keep it around 30% minimum load you shoudl be fine.

so, looking at AMP meters, try pulling about 50 amps at 110v. so if you have some 240v stuff on board, figure about 20amps on the 120v and another 15amps on the 240v side.

if you are in an area where you dont' need to use the AC, you'll never be able to pull a big enough load. this is why an inverter is a nice thing to have: when you dont' need AC, you can run off the inverter then when you need to recharge the inverter bank it will help pull a load off the genny.
 
Agree on the inverter. As soon as it gets cool enough to turn off the A/C units we shut off the generator, close its sea cock and do not open until the weather heats back up. The 5000 watt inverter has paid for itself.

This is whats perplexing (to me anyway), Watts = Amps x Volts. So if I calculate it based on the 60 Amp breakers on the generator side it comes to 14,400kW (14,400kW = 60amp x 240v). If I calculate from the specs on the generator side of 20,000kW, I should be able to pull a Full Load of 83 Amps (19,920kW = 83 Amps x 240volts). But at 83 Amps, the breakers would hopefully of tripped. So which is correct? I'm not sure I have ever seen the Amp meter read over 40 amps with the A/C units, water heater & fridges running). Do you have to divide the amps by two for each leg....so it would be approx 42 Amps equal a Full load???
 
No no no no no. 40 amps on each 120 volt leg = 40 amps on your 240 system. Highly advise you get a copy of Nigel Calder's "Boat Owner's Mechanical and Electrical Manual" . By the way my MDL4 20KW has one 80 amp breaker, then sub breakers for each panel. Something has caused you serious confusion.
 
Good point, I intended to bring home the Nigel Calder book (great reference manual) to review and study up but left it on the boat. Electrical is the one area where I rely on a good electrician because the mistakes can have devastating effects. Words to live by.... know your limits and ask lots of questions. Thanks for the help.
 
whats watts wat? what about torque?

Don't confuse yourself.
 
it should be remembered when looking at generators that they are rated in kw not kva as a transformer. the kw rating takes into account the powerplant running the generator and its horsepower etc. a transformer kva rating uses utility power which is basically unlimited for its formula. given a typical gen set the kva rating on the generating side will be greater than the kw rating of the whole unit
 
kW is real power, kVA is apparent power.

kVA includes reactive power (laymen "power factor"...vAR's). This matters for inductive loads like AC compressors.

You could max out your generator before you ever hit the rated kW rating in certain instances.

For those that strive to be nerds, look up power triangle and vAR
 
krush said:
For those that strive to be nerds, look up power triangle and vAR
Click to expand...

Hey Krush - No one can match your level of nerdishism. Not that that's a bad thing...
 
I was always told the power triangle was my mother, my wife, and my mother-in-law.

I've been lied to?
 
z28jimi said:
I was always told the power triangle was my mother, my wife, and my mother-in-law.

I've been lied to?
Click to expand...

Well, I guess both triangles will kill one quickly if not respected!
 
The primary difference between KW and KVA is the power factor.To expand on it a bit more, the kW rating is essentially the resulting power output a generator can supply based on the horsepower of an engine. kW is figured by the horsepower rating of the engine times .746. For example if you have a 500 horsepower engine it has a kW rating of 373. The kilovolt-amperes (kVa) are the generator end capacity. Generator sets are usually shown with both ratings that can sometimes be confusing.
 
OK, was at the boat yesterday and I think I found the error in my ways regarding my confusion on this topic. They say the mind is the first to go or possibly I just need to make an appointment for the eye doctor, but thanks to "GJH" I went back and stepped through the breakers. Indeed it is an 80amp (not 60 amp) breaker so the math now works. And since the math works, the load scenarios (quarter, half, three-quarter & full load) now make sense looking at the Amp meter in relation to the kW being utilized.

Much appreciated and thank you GJH! Now, excuse me while I make my eye doctors appointment....
 
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