Generator Amperage

[delectric123]

I have a used 45 kW 208/120v 3 ph. generator. With a 0.8 PF, whats the proper way to calculate the size fuses I need in the safety switch.

 

[templdl]

I have a used 45 kW 208/120v 3 ph. generator. With a 0.8 PF, whats the proper way to calculate the size fuses I need in the safety switch.

Are you trying to protect the generator or wire? Keep in mind the it is questionable whether or not the genset has the capacity to put out enough current for a long enough period of time or be able to develop enough fault current to blow a fuse as that which can be provided by a utility.

 

[kwired]

Are you trying to protect the generator or wire? Keep in mind the it is questionable whether or not the genset has the capacity to put out enough current for a long enough period of time or be able to develop enough fault current to blow a fuse as that which can be provided by a utility.

good point, it should however be able to deliver 45kW indefinitely. That comes out to right about 125 amps @ 208 volts. I would have no problem with 125 amp time delay fuses or circuit breaker, just not sure how long it will take to trip under fault conditions, but the longer it takes means the less fault current actually is.

 

[delectric123]

I also got about 125 amps. though was not sure if i used the correct formula to figure it out. could you tell me the calculations you used?

 

[kwired]

I also got about 125 amps. though was not sure if i used the correct formula to figure it out. could you tell me the calculations you used?

V x A x 1.73 = kW

This does not consider power factor. But I think the power factor on your generator means that it is rated for a 45 kW load with power factor no lower than .8 which would mean the KVA @ .8 PF would be higher than 45 but KW can not be higher than 45. I guess you theoretically could increase the OCPD for this. But why would you size a generator that tight to the load in the first place? If starting a motor across the line it may not be able to handle the inrush current anyway.

 

[delectric123]

The generator has a rating of 56.25 KVA. Should i use that for sizing the OCPD?

 

[kwired]

The generator has a rating of 56.25 KVA. Should i use that for sizing the OCPD?

I would say that is the max value you can size the load to. Then comes a question already brought up, are you protecting the conductors or the generator? Both do need protected and one device can protect both.

 

[delectric123]

the safety switch will be mounted on the generator. so it will also protect the conductors going to the breaker panel.


so 56250 / 1.732 / 208 = 156 amps. so i could use 150 amp fuses. but would have to downsize to 125 amp fuses to feed a 125 amp breaker panel due to the 125% rule.



right?

 

[iceworm]

I have a used 45 kW 208/120v 3 ph. generator. With a 0.8 PF, whats the proper way to calculate the size fuses I need in the safety switch.

Are you trying to protect the generator or wire? ...

The generator has a rating of 56.25 KVA. ...

Good morning - (Love that first cup of coffee watching the sun come up - clear, no wind, only -4F. And I'm off work for a few days. Going to be a good day)

Okay, back to earth.

First thing is to answer templdl's question. And, is there a CB already on the generator? If there is just four leads coming out of a generator connection box it is a little more complicated. However, if you are not loading it right to the edge of the driver capability, there is a lot of forgiveness

Here's a start: 445 does not give many clues. Most are design decisions - left up to you and the application. 45kw, 208V, 3ph is out of the homeowner range, so no telling what the on-board protective functions are. You will have to tell us. Yes, you will use the current calculated from the nameplate KVA rating

No CB on generator: 445 says size the conductors at 115%. You can size the first OCPD where you want. 445.12.A pretty well says that. With no more information that we have, I'd pick
((56.25/208)/1.73)) X 1.25 = (I don't have a calculator). Use time delay fuses - you really don't want these to blow easily.

This all depends on the existing generator protective functions. And, don't forget the driver limitations generally prevent overload.

Existing CB on generator: Now you are protecting the conductors ot the generator.

Probably could give you some better ideas if we knew more specifics.

I'm off to take the kid to school - You guys and girls have a good day.

ice

 

[delectric123]

thanks for the help, no the generator does not have a CB, just a 15 amp for control. it looks like about 20 years old

 

[iceworm]

the safety switch will be mounted on the generator. so it will also protect the conductors going to the breaker panel.

so 56250 / 1.732 / 208 = 156 amps. so i could use 150 amp fuses. but would have to downsize to 125 amp fuses to feed a 125 amp breaker panel due to the 125% rule.

... no the generator does not have a CB, just a 15 amp for control. it looks like about 20 years old

I'm back (having a second cup - temp is clear up to 2 above, could get to 15 above - heat wave:roll: Hope so - I've got some outide work to do today)

Hummm... I don't know what the "125% rule" is. You will have to tell me about that.

From what we have, here are some suggestions:
First, go find the generator book/specifications. Send the mfg an email - maybe you wil get lucky. This will tell you about the included generator protective functions/controls. Hopefully it has a voltage regulator and governor - if not, you're fubared. Probably does, you mentioned it had a 15A control fuse - with luck that feeds, ... ahhhh ..., controls.:roll: Nothing wrong with 20 years old, you just need to know what you have.

Since the OCPD you are installing is the first device, the conductors from the gen to the OCPD must be 115% nameplate - 179A. Without any additional information, I'd size the OCDP at 200A. Again TD fuses.

Don't forget the gen OCPD does not have to be at/on the generator. I don't know if you are portable or fixed, residential, industrial, construction, commercial, or ATS to a utility. If fixed, other than residential, the gen cb can be about anywhere you want - I've never had an AHJ squawk about having the gen cb located inside another structure. I can't say about residential - I don't do them, well - except mine *, and there are no residential inspections here.

Everything after the gen OCPD is just standard 240, 310, 408 rules.

Feeder to the panel:
If the panel has a main (125A) and the installation allows an outside unlimited tap, size the conductors for 125A. If no panel main, there are a couple of options for using 125A conductors to the panel:
1. Keep the 200A fused disconnect, and as you suggested, fuse the gen at 125A.
2. Again if outside unlimited taps are available, keep the gen fuses at 200A and install a 125A backfed cb in the panel

Of course, all this depends on the picture in my head matching what you actually have. Could be, but hard to tell from my side of your keyboard.

There are some issues with grounding/bonding. More on that later if your interested.

Welcome to the world of being the "engineer of record". :blink: **

ice
* Okay - and my dad, and my buddy, and my brother, and ....

** Nothing wrong with DIY engineering. None of this is rocket science. Just have to understand risks.

 

[delectric123]

This generator is for emergency standby for a hog barn. what i meant by the 125% rule is, conductors feeding a breaker panel must be sized 125% for the continuos load. lets say, for a 125 amp breaker panel. 125 x 1.25 = 156.25, so the generator is perfectly sized for that. ( 56250\208\1.73 = 156)

 

[iceworm]

This generator is for emergency standby for a hog barn. what i meant by the 125% rule is, conductors feeding a breaker panel must be sized 125% for the continuos load. lets say, for a 125 amp breaker panel. 125 x 1.25 = 156.25, so the generator is perfectly sized for that. ( 56250\208\1.73 = 156)

Highly recommend: Don't call it an emergency - that brings in another whole list of regulations.

So, you are feeding an ATS with a connection to a utility? And the ATS feeds the panel? Does the ATS have OCP? This is system design - not just a generator.

As for the "125% rule", take a look at 408.36. The 125A panel will have to be protected at 125A - no more.

Later - got some errands to run. And it's up to 8 above - Yeah

ice

 

[delectric123]

whoops didn't mean to classify the setup as NEC Article 700:roll:
no ATS, just a double-throw unfused switch.

 

[augie47]

Despite being a southerner, I have had little experience with a hog farm, but I ask, what makes the load continuous ?

Keep in mind you are having to look at two specifics here, The OCP device may well be the only generator OCP so it would need to be sized to protect the generator, but it also protects the conductors on the load side, so barring application of any tap rules, you would need to size it to protect the load side conductors.

 

[delectric123]

Despite being a southerner, I have had little experience with a hog farm, but I ask, what makes the load continuous ?

it really isn't 125 amps continuous, i just used that for a good safety figure.

 

[templdl]

The generator has a rating of 56.25 KVA. Should i use that for sizing the OCPD?

Take a look at in another direction. Should you overload the generator what happens? Does the field collapse on the generator? What happens to the prime mover, I.e. the engine?
What I'm trying to get at is if there is any other protective device that can be used that would active ate a shunt trip on the breaker ?

 

[delectric123]

which breaker?

 

[templdl]

which breaker?

It appears that fuses are not likely to work to well if your intent is to use fuses. Fuses do limit your options and will the generator have the capability to generated enough power to blow a fuse or trip a breaker for that matter? Should you elect to use a breaker at least you can add a shunt trip which would have the capability of opening the breaker from and external oil pressure sensor, speed sensor or another sensor than would be in tune more directly with an overload.

 

[iceworm]

It appears that fuses are not likely to work to well if your intent is to use fuses. Fuses do limit your options and will the generator have the capability to generated enough power to blow a fuse or trip a breaker for that matter? Should you elect to use a breaker at least you can add a shunt trip which would have the capability of opening the breaker from and external oil pressure sensor, speed sensor or another sensor than would be in tune more directly with an overload.

Generally speaking, it isn't worth engineering a protective system. I would recommend going with the protections already designed and installed. Be good to see what the Book says.

For this size gen (all generaliztion - possibly right. Get the book):
1. Most trips won't be wired to a shunt trip CB. Most all will be connected to a driver shutdown.

2. It won't have a UF or UV trip.

3. The overload shutdown may well be driver limit. It just doesn't have enough HP to sustain an overload.

4. As for short circuit, the fuses just have to open before the gen catches fire. Doesn't have to protect the gen from zero damage.

5. It might have an OF or OV trip

del -
Did I mention get the book

ice