How to size overcurrent protection for generartor?

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augie47

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State Electrical Inspector (Retired)
Other than any OCP the generator manufacturer might request your OCP would serve as the protection for the conductors from the gererator to the load and size accordingly.
The generator 480v output if I did my math correctly is 150 amps but the OCP device you install will also dependant on what the generator is supplying (motors, fire pumps, etc)
 

Cold Fusion

Senior Member
Location
way north
Hello

I am looking at the following generator 3 phase 60Hz 480V, 125KVA:

http://www.marathonelectric.com/MGPS/details.jsp?item=362CSL1606&voltage=240Y/480Y&hertz=60&phase=3

According to NEC 445.12 generator needs to have overcurrent protecting device. How to size such device? What NEC articles should I use?

Thank you,

PaulBr.
You picked it out - 445.12 Whoops the NEC doesn't give you a number - oh-oh. It's a design issue. And the NEC is not very good at giving design criteria.

But they do give some clues. I'll use 100kw, .8 pf, 150FLA as an example - that fits the cut sheet fairly well.

The conductors from the gen to first OCP are given by 445.13 to be at least 115% fla.

My inclination is to protect these conductors per 240.4. So if the OCP is in the same structure as the gen:
1.15 x 150 =173, so conductors are 2/0, next size up cb, 200A cb.

Now if the OCP is located far away from the gen, then I tend to go with the outside tap rule, 240.B.5 and size the conductors up to the CB - no next size up. That would be 3/0, 200A. However, in this case, I would look for cts/pts at the gen to feed a gen protective relay. But 100kw is pretty small for anything very sophisticated

Like I said, this is not a code issue. It's a design issue.

cf

Heavily edited cause I made a mess out of the first try
 
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paulbr

Member
Another question. If I decide to use fuses as overcurrent protection devices, what class fuses should I use? Is class J OK?

PaulBr
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
It goes back to the type load you may have and the design of your system in terms of available fault current and selective coordination.
Again, a design issue.
Type J fuese have great current limiting values but are fast acting.
You may find yourself loosing your main fuse on a fault down line from another fuse.
If it's critical, it takes an engineering study.
 
You picked it out - 445.12 Whoops the NEC doesn't give you a number - oh-oh. It's a design issue. And the NEC is not very good at giving design criteria.

But they do give some clues. I'll use 100kw, .8 pf, 150FLA as an example - that fits the cut sheet fairly well.

The conductors from the gen to first OCP are given by 445.13 to be at least 115% fla.

My inclination is to protect these conductors per 240.4. So if the OCP is in the same structure as the gen:
1.15 x 150 =173, so conductors are 2/0, next size up cb, 200A cb.

Now if the OCP is located far away from the gen, then I tend to go with the outside tap rule, 240.B.5 and size the conductors up to the CB - no next size up. That would be 3/0, 200A. However, in this case, I would look for cts/pts at the gen to feed a gen protective relay. But 100kw is pretty small for anything very sophisticated

Like I said, this is not a code issue. It's a design issue.

cf

Heavily edited cause I made a mess out of the first try

Isn't next size OCP 175amp??
 

Cold Fusion

Senior Member
Location
way north
... Is class J OK? ...
I wouldn't. As Gus said, class J are fast - semiconductor fuses. And the current limiting doesn't matter - gen sets are pretty high impedance and generally will not produce much short circuit. The 100kw you list I'd guess between 400A - 1000A.

I'd use standard stuff - class K maybe? Fuses wouldn't be my first choice, just because of the dangers of single-phasing

If the time to trip is a concern, you will have to get the generator damage curve from the manufacturer and pick a fuse that opens inside of the damage curve. Or, as mentioned earlier, put in a generator protective relay and either shunt trip a CB, or shut down the engine - or both.

All of this depends on the application. If it is critical/high profile, add all the protection the job can afford. If not, don't worry about it.

One thing to keep in mind is the generator OCP does not particularly protect the generator - and the NEC recognizes that. Look at the wording of 445.12.A, "... protective means suitable for conditions of use." That tells me the NEC expects me to design the system so it is protected as far as my customer needs it to be.

Luckily for us most generator systems won't easily burn up from overload. The drivers (engines) are usually sized to where the the throttle is wide open at gen full load. If the gen load is increasedvery much, the driver runs out of horsepower and slows down - and the gen trips off-line.

Let us look at the example I gave: 100kw, 480V, 3ph, .8pf, 150FLA, OCP 200A

Now put a resistive load (pf = 1.0) on the gen and push the load up to just under 200A. The circuit breaker is not going to trip. The load on the gen is 199A x 480V x 1.732 = 165kw. So the driver mechanical output to the gen is 1.65X the design spec. So the spec driver output is 134hp. But at max overload, the driver must put out 221hp. The mfg may size the driver for a 15% overload, say 150hp. But generally not much more that that.

So where am I going with this: (for small systems <300kw)
If the customer buys a generator package, it comes with a generic protective system (minimum protective relaying and a CB) that generally won't let the customer overload the unit. And the driver is small enough that the customer can't seriously overload the gen for long time periods. And the package might even do what the customer wants.

If the customer purchases just the gen/driver, then it is up to the engineer to design the protective relaying. And that may well go beyond selecting an OCP and conductors.

cf
 

paulbr

Member
Thank you for the good analysis. :)
Application is mobile AC unit. Generator is part of this unit (same mobile platform). This AC unit contains 3 compressors, 2 condenser fans, blower motor feeded by VFD, and low voltage controls. This unit will operate from utility power if available, and from generator when utility is not available. There is double throw transfer switch for changing power source. This is not highly critical application.

PaulBr.
 

paulbr

Member
Another question related to application described above:
View attachment 4701

Will circuit breaker located in the control panel serve as overcurrent protection for generator or I need to place another circuit breaker upstream of transfer switch? Again generator and control panel are on same platform and wire length will be at most 10 ft.
Do you need to protect transfer switch? Is it device that NEC requires to be protected (which article)?
Transfer switch is double throw three position switch, which allows to switch power from generator to line supply.

Thank you.
 

Cold Fusion

Senior Member
Location
way north
...Will circuit breaker located in the control panel serve as overcurrent protection for generator or I need to place another circuit breaker upstream of transfer switch? ...
I don't know. It is pretty hard to design this from my side of your monitor. Too many thing I don't know.

Here are a few things to consider:
Is the normal feeder a service or a plant feeder?
If it is a service, where is the utility N-G bond?
Where is the gen N-G bond?
Is the transfer sw 3W or 4W?
Is the transfer switch service rated? Does it need to be?
Is the normal feeder cord 4W or 5W?

You are designing a package unit - you have a whole bunch of stuff to worry about besides just the gen CB. You are the integrator for this package.

cf
 
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