80%

[ceb58]

On another forum some one posted this situation for a generator install. I have searched the code book and cannot find whether it is code or if the inspector is making up thing as he goes. Situation: 20 Kw gen on NG. The derating for the unit on NG would bring it down to 18 Kw. The inspector was telling them they were just inside the 80% of the switched load. The generator would work out to 75 amps and it has a 100amp breaker. Their calculated load was 50 amps. By this inspectors logic the cutoff would have been 60 amps. Now, is he pulling this 80% out of the air? Dose he have it confused with something else? Or is it something I could not find?

 

[mistermudd]

The 80% is an assumed power factor for the generator. So you take a 20kW generator divide that by an 80% power factor end up with a kVA output 20kW/.8 = 25kVA. Understand though, this is close to the max output when the generator is brand new and it much safer to use the kW rating for sizing the generator. So now we have an output of 25kVA, however you didn't mention the number of phases and voltage, so I'll use 480 vlolt 3 phase as an example. 25kVA * 1.73/480=90 amps, the ouput would be about 90 amps.
At 208 volt 3 phase woud be more like 207 amps.

 

[brian john]

The 80% is an assumed power factor for the generator.

I would bet money this is not the case, the inspector is thinking 80% of the CB rating at full load?

 

[kwired]

It doesn't matter what the main breaker is.

What does matter is the calculated load being transferred, and whether or not this is an automatic transfer or a manual transfer.

Automatic transfer means the transfer is going to happen unsupervised and therefore the transferred load better be below the setting of the generator overcurrent protection or it is probably going to be tripping. You can power a panel protected with a 100 amp breaker with a 5 kw generator if you wish, as long as the connected load while running on the generator is not more than the generator rating.

Manual transfer means an operator is there to make the transfer and to select what load is connected to the generator. If there is too much load and something trips, then someone is there to reset and make changes to connected load.

Either way the generator should have overcurrent protection that is intended to protect the generator from overloading.

 

[ceb58]

I would bet money this is not the case, the inspector is thinking 80% of the CB rating at full load?

Not sure what he is thinking. It is a 20 Kw 120/240 SP resid. generator. The breaker is a 100 amp so 80% would be 80 amp. The calculated load on the dwelling is 50 amps. Being NG the generator is derated to 18 Kw or 75 amps. So by his math 60 amps is the limit. What code section dose he have it confused with? Due to the fuel type and the inspectors logic you have taken a 20 Kw generator down to a 14.4 Kw generator

 

[infinity]

Not sure what he is thinking. . What code section dose he have it confused with?

You'll need to ask him for a code reference.

 

[kwired]

Not sure what he is thinking. It is a 20 Kw 120/240 SP resid. generator. The breaker is a 100 amp so 80% would be 80 amp. The calculated load on the dwelling is 50 amps. Being NG the generator is derated to 18 Kw or 75 amps. So by his math 60 amps is the limit. What code section dose he have it confused with? Due to the fuel type and the inspectors logic you have taken a 20 Kw generator down to a 14.4 Kw generator

None of that matters, if generator is 18 kW then all that matters is that 18 kW or less is what gets automatically transferred. Manual transfer can have unlimited load capable of being connected to it, and the user needs to manually select loads. The generator still needs overcurrent protection but it really has nothing to do with what size overcurrent protection is on the "normal" supply side of the transfer switch. If you desire same generator could be connected to back up a panel with a 400 amp main supplying it, it just can not automatically transfer more than the 18 kW of load so load shedding methods would need to be used if there is more than 18 kW of connected load.

 

[Smart $]

None of that matters, if generator is 18 kW then all that matters is that 18 kW or less is what gets automatically transferred. Manual transfer can have unlimited load capable of being connected to it, and the user needs to manually select loads. The generator still needs overcurrent protection but it really has nothing to do with what size overcurrent protection is on the "normal" supply side of the transfer switch. If you desire same generator could be connected to back up a panel with a 400 amp main supplying it, it just can not automatically transfer more than the 18 kW of load so load shedding methods would need to be used if there is more than 18 kW of connected load.

That is what matters, but a finer point must be considered. Motor and and motor-operated loads must be capable of starting on an automatic transfer system. As the calculated load approaches the generator's rated output, the chance a motor may not start increases, unless there is some type of load management involved. That said, I don't know of any truly "standard" method of determining the generator's capacity for such.

I don't believe the preceding is what the inspector is considering.

 

[kwired]

That is what matters, but a finer point must be considered. Motor and and motor-operated loads must be capable of starting on an automatic transfer system. As the calculated load approaches the generator's rated output, the chance a motor may not start increases, unless there is some type of load management involved. That said, I don't know of any truly "standard" method of determining the generator's capacity for such.

I don't believe the preceding is what the inspector is considering.

I agree with you. There is no 80% rule that guarantees a particular motor will start. A motor that will not start across the line without causing problems may very well work fine with soft start controller or other reduced current starting methods.

 

[hurk27]

My thoughts are coming from this PDF from Generac on the 17-20kw models (page 3 gives the alternator spects), They rate there generator @20,000 watts and it has a 1 PF so there is no loss from that as a matter of fact a quick look up of several generators I could not find one with a lower then 1 PF???

I don't see anything in the NEC that requires us to size a generator any less then 100% of its rated output, the only requirement is for conductors which are required to be sized @125% of the rated output and I think that is only for 700 and 701 installs not 702, but I could be wrong on this.
a 20kw generator has an output of 83.3 continuous amps, this output is reduced to 75 amp continuous when used on natural gas not because of the alternator but because of the ability of the engine to produce the required horse power, most generator manufactures use the same alternator for both NG and LP, the selection of LP or NG is field changeable for most manufactures so even if you are running it on NG you still have a 20kw rated alternator, but we must follow the requirements to only ATS into what the generator is rated for or there will be problems as the #one problem would be the warranty if it was found that the generator was being over loaded , yes the Nexus has a black box that would tell on you when they send a person out to administer the warranty just like the new cars.

Almost all generators do have a surge amp rating that allows for the start up of motors such as air conditioning systems and such but this is only for short periods of time and while a 20kw can handle in some cases up to 3 such A/C systems it would not start all 3 up at the same time, I have installed time delays on A/C condensers to over come this problem and it worked great and saved purchasing the Nexus smart control transfer system that basically does the same load shedding there are other options to load shedding that can be done with current relays to prevent two heavy loads from running at the same time.

But basically if the generator is rated for 20kw or 18 kw in the case of NG then that is what we must size the load for, then up size the conductors for the 125% requirement if it does apply to 702 installs?

Now I would find it a good thing to leave a little head room and maybe factoring in a 20% head room might be a good idea but I can't see this as enforceable?

 

[ceb58]

But basically if the generator is rated for 20kw or 18 kw in the case of NG then that is what we must size the load for, then up size the conductors for the 125% requirement if it does apply to 702 installs?

Now I would find it a good thing to leave a little head room and maybe factoring in a 20% head room might be a good idea but I can't see this as enforceable?

I love this site and the people on it, for the most part. You can ask a question and it will go in 100 different directions :happyyes:. Wayne, your answer is what I was looking for. I could find nothing in CODE about 80% but was just confirming that there was no such code. I do agree in a real world situation it is not a good idea to max load a generator. As far as the 125% size for conductors you may be thinking about 445.13 (2008) for the ampacity of conductors being not less that 115%. If the generator has built in OCP then this code has no bearing.

 

[hurk27]

I love this site and the people on it, for the most part. You can ask a question and it will go in 100 different directions :happyyes:. Wayne, your answer is what I was looking for. I could find nothing in CODE about 80% but was just confirming that there was no such code. I do agree in a real world situation it is not a good idea to max load a generator. As far as the 125% size for conductors you may be thinking about 445.13 (2008) for the ampacity of conductors being not less that 115%. If the generator has built in OCP then this code has no bearing.

This derating is more of a manufacture design then it is any type of code requirement, as NG produces 1520 BTU's less heat per FT?/hr then propane so an engine running at higher RPMs would require a much higher NG fuel flow which would not be available in a residential NG gas service considering that most utilities only allow a final NG gas pressure of 5" of water column for a dwelling for safety reasons, while commercial/industrial 14"wc is more common place and sometimes can relive the derating of the generator on NG if the regulator is changed for a higher fuel flow rating.

There are some generators that do not need to be derated for running on natural gas over propane, most are the quiet series or commercial/industrial with 4 pole alternators that run at 1800 RPMs instead of the 3600 RPMs that lower cost 2 pole generators run at, these will have larger engines and since they are running at lower RPM's can utilize the NG fuel better and have a higher fuel flow regulator.

Here is one such generator again look at page 3 of this PDF: http://www.generac.com/SpecSheets/0187710SBY_QuietSource_22kW.pdf

I do see that the 3-phase versions are rated with a power factor of .8 but the single phase version of this generator is still a PF1

The above generator in the PDF has a GM 2.4, it is nothing but an industrial version of the quad-4 engine they used in many GM cars and light trucks but they are all cast iron blocks and heads, and instead of fuel injection they have a down draft carb made for NG/propane, some of the 45kw thru 80kw 4-pole generators will use the industrial version of the 4.3 GM v6 vortex engine again using carburetor aspirated NG/propane carb, I would love to stick one in astro van instead of the aluminum 4.3 that comes in the astro with the spider injection system that so many had all the problems with the popits sticking.

And even some larger 4-pole go with the industrial 5.7 GM vortex I think the 150kw does, while Generac has also used some foreign motors like Toyota and Kamoto over the years most today are GM made.

 

[conmgt]

I would love to stick one in astro van instead of the aluminum 4.3 that comes in the astro with the spider injection system that so many had all the problems with the popits sticking.

The 4.3 in my '02 Astro is 100% cast iron. I know, I had to replace the intake manifold gaskets last year thanks to DexCool extended life coolant...ate right through the stock gaskets on tens of thousands of engines. The only aluminum V6 Vortec's made were designed strictly for racing. I wish my engine was aluminum. Then I might be able to hit 20mpg on the highway.
Now back to our regularly scheduled program.

 

[shockerz]

gen loaded

gen loaded

i have seen a few 20 kw gen supplying whole 200 amp services with no load shedding. i guess its up to the home owner to know what to run

It doesn't matter what the main breaker is.

What does matter is the calculated load being transferred, and whether or not this is an automatic transfer or a manual transfer.

Automatic transfer means the transfer is going to happen unsupervised and therefore the transferred load better be below the setting of the generator overcurrent protection or it is probably going to be tripping. You can power a panel protected with a 100 amp breaker with a 5 kw generator if you wish, as long as the connected load while running on the generator is not more than the generator rating.

Manual transfer means an operator is there to make the transfer and to select what load is connected to the generator. If there is too much load and something trips, then someone is there to reset and make changes to connected load.

Either way the generator should have overcurrent protection that is intended to protect the generator from overloading.

 

[kwired]

i have seen a few 20 kw gen supplying whole 200 amp services with no load shedding. i guess its up to the home owner to know what to run

That is exactly what is done with manual transfer and selection of loads.

ATS with connected load over generator rating - likely to open an overcurrent device.

Just because there is a 200 amp service does not mean it can't have less than 200 amps of actual connected load.

 

[Smart $]

...

Just because there is a 200 amp service does not mean it can't have less than 200 amps of actual connected load.

Also does not mean the normal demand is always at the level of calculated or connected load.

I have effectively used an 8kW gennie to power a 100A resi service with no load shedding.

 

[kwired]

Also does not mean the normal demand is always at the level of calculated or connected load.

I have effectively used an 8kW gennie to power a 100A resi service with no load shedding.

Correct.

I do have a 8kW generator and can power my home with a 200 amp panel. I do need to watch loading but generally only need to be most concerned with water heater, heat pump, and electric aux heat. I can run those items (except for aux heat) but maybe not at same time if other loads are running and do need to be a little conservative with other loads if they are running.