Generator Circuit Breaker

[nkalghatgi]

I have a question regarding sizing the CB for my generator. The generator is rated at 50 KW, 480 V.
Here is how I did my calculations. Generator rated current will be 50000/480/1.732 = 60.14 A.
Considering 125% of the current, I get 75.17 A. I choose a CB rated 100 A. Am I doing this right?

 

[templdl]

I have a question regarding sizing the CB for my generator. The generator is rated at 50 KW, 480 V.
Here is how I did my calculations. Generator rated current will be 50000/480/1.732 = 60.14 A.
Considering 125% of the current, I get 75.17 A. I choose a CB rated 100 A. Am I doing this right?

Since beakers protect wire what size wire are you running from the generator? One would think that you would size the wire bases upon the output of the generator which you calculated at 60.14a. Then the breaker sized to protect the wire.
If you went directly to the 125% do you think think that your generator would be capable of providing 125% of its continuous rating for a long enough person of time to trip the breaker on overload? Probably not a chance.
Just size the wire based upon the rating of the generator and then size the breaker to protect the wire.

 

[nkalghatgi]

I am running a 4/0 from the generator to ATS. You are right, I need not go to 125% of the generator output current since generator will only supply current needed by the load. For simplicity we are running a 4/0 as we do not want to confuse the contractor. In this I can also have a 70A CB right, using the next size up rule. This will also protect the 4/0 wire.

 

[iceworm]

The guiding code section is Art 445.

Assumption: The generator does not have an installed CB. Which is odd. Most small generation comes with a CB installed on the skid. If it had come with a CB, then the mfg made all of these design decisions. Since it does not, you are the systems engineer of record - you get to make the design decisions.

Amazingly the NEC allows wide latitude is selection of Over Current Protection. Look at art. 445.12.A. Depending on the protections built into the engine/generator control, the CB may not have anything to do with generator protection.

Also look at art. 445.13. If the CB you are installing is the first CB, then the conductors from the gen to the CB are required to be 115% FLA. The first CB is not necessarily sized to protect these conductors.

So what are you left with? All design decisions - not code mandates.

Look at the generator/driver specs. Most of the time the driver is too small to allow the generator to be loaded up over 100%. If the gen is loaded over 100% the throttle goes wide open and the driver just slows down. If so, that could be considered overload protection by "inherent design".

Improtant Note: The generator load is not set by the CB, it is set by the design. If you don't want the gen overloaded, limit the connected load.

Here is an example of how I might choose.
Generator mission is starting/running a motor close to the starting capacity of the generator - say 25hp - 30hp, plus auxilary loads.
Gen Short circuit current is 300A - 400A.
Art 445.12.A Generator overload protection is by "inherent design".

Consider a typical TM CB trip curve is:
500% - 4sec to 12sec
400% - 6s to 20s
300% - 10s to 30s

For an 80A CB:
300% --> 240A Won't trip under statring current
For a fault, will trip between 4s and 30s
80A is > 125% gen FLA, so it won't trip with the gen at continuous full load.

For this application, I'd probably pick an 80A CB. Will a 100A work? Sure, but look at your trip times for available SSC. For the same range of gen curents the trip times are 6s to 50s.

Hope this helped.

ice

 

[nkalghatgi]

Ice,

Thank you so much. It really helped me to get more insight on how to size the CB.

 

[iceworm]

I am running a 4/0 from the generator to ATS. ...

... For simplicity we are running a 4/0 as we do not want to confuse the contractor.

.... In this I can also have a 70A CB right, using the next size up rule. This will also protect the 4/0 wire.

4/0 gen to ATS --- hummmm

Check the specs on your 70A - 100A CB. Typical terminal wire range is #14 - 1/0.

For a 100A CB I'd suggest #2. For an 80A CB, I'd use #4. These will fit the CB lugs.

If this confuses the contractor, I'd recommend either hold their hand, or get a different contractor.

If you insist on 4/0 into a 70A - 100A CB, the wire will get a haircut.

ice

 

[templdl]

The guiding code section is Art 445.

Assumption: The generator does not have an installed CB. Which is odd. Most small generation comes with a CB installed on the skid. If it had come with a CB, then the mfg made all of these design decisions. Since it does not, you are the systems engineer of record - you get to make the design decisions.

Amazingly the NEC allows wide latitude is selection of Over Current Protection. Look at art. 445.12.A. Depending on the protections built into the engine/generator control, the CB may not have anything to do with generator protection.

Also look at art. 445.13. If the CB you are installing is the first CB, then the conductors from the gen to the CB are required to be 115% FLA. The first CB is not necessarily sized to protect these conductors.

So what are you left with? All design decisions - not code mandates.

Look at the generator/driver specs. Most of the time the driver is too small to allow the generator to be loaded up over 100%. If the gen is loaded over 100% the throttle goes wide open and the driver just slows down. If so, that could be considered overload protection by "inherent design".

Improtant Note: The generator load is not set by the CB, it is set by the design. If you don't want the gen overloaded, limit the connected load.

Here is an example of how I might choose.
Generator mission is starting/running a motor close to the starting capacity of the generator - say 25hp - 30hp, plus auxilary loads.
Gen Short circuit current is 300A - 400A.
Art 445.12.A Generator overload protection is by "inherent design".

Consider a typical TM CB trip curve is:
500% - 4sec to 12sec
400% - 6s to 20s
300% - 10s to 30s

For an 80A CB:
300% --> 240A Won't trip under statring current
For a fault, will trip between 4s and 30s
80A is > 125% gen FLA, so it won't trip with the gen at continuous full load.

For this application, I'd probably pick an 80A CB. Will a 100A work? Sure, but look at your trip times for available SSC. For the same range of gen curents the trip times are 6s to 50s.

Hope this helped.

ice

Considering what you have posted can the generator produce enough current for a sufficient length of time to trip the breaker?
Taking this into consideration going with an 80at breaker will not going anything over a 70at breaker. I thing it's a mute point when the output capability of the generator is considered.

 

[iceworm]

Considering what you have posted can the generator produce enough current for a sufficient length of time to trip the breaker? ...

Maybe, maybe not. Are you thinking "overload" or Short Circuit? Either way, that's a problem with small generation using only a typical TM CB.

...Taking this into consideration going with an 80at breaker will not going anything over a 70at breaker. I thing it's a mute point when the output capability of the generator is considered.

Okay. That would be a definition of a design decision. The engineer of record gets to pick.

If I am the E-O-R, I like the CB being just over 125% FLA. Allows the gen to run 100% continuous and be pretty sure the CB won't trip. Typically small generation is not rated for 100% continuous. But the gen's "continuous" rating has a different definition than CB "NEC continuous".

If one wants the gen CB to not trip at 100% FLA, for more than 3 hours, the CB needs to be at least 125% of the gen FLA.

As for the gen not being rated for "continuous" just means that if you do run it more than 80% for long periods, the driver will wear out much quicker - typically the driver is the limit, not the alternator.

If you are the EOR and you like 70A, that's okay - untill you have to explain to the client the best you can guarentee is 40KW, 80% (NEC) continuous (more than 3 hours). If that meets spec, you're golden.

ice

 

[steve66]

I have a question regarding sizing the CB for my generator. The generator is rated at 50 KW, 480 V.
Here is how I did my calculations. Generator rated current will be 50000/480/1.732 = 60.14 A.
Considering 125% of the current, I get 75.17 A. I choose a CB rated 100 A. Am I doing this right?

I would start with the generator KVA, not the KW. The KVA is normally 125% higher. (ie. a 50 KW generator would have a 62.5 KVA rating.)

I would size the breaker for the KVA rating, and then size the wire for the breaker.

You may want to add another 125% for generator overload capacity to both the breaker and the wire, but its not required.

 

[iceworm]

I would start with the generator KVA, not the KW. ...

That is absolutely true. Good catch. Of course, one must use the generator nameplate - and not guess at the rated pf.

Can't say I agree with sizing the gen to first CB conductors at 100% FLA. NEC 445.13 mandates 115% minimum. And, practically speaking, there is no reason to upsize.

Can't say I agree with sizing the CB at 100% nameplate FLA. That is purposely limiting the continuous gen output to 80% (nominal). However, as noted that is a design decision.

Can't say I agree with up sizing the gen conductors is one chooses to up size the CB. The CB does not protect these conductors.

ice

 

[templdl]

Can't say I agree with up sizing the gen conductors is one chooses to up size the CB. The CB does not protect these conductors.
ice

Then it appears as though the CB is simply an expensive disconnect switch. Often more than not there is very little consideration give to the capability of the generator as compared to the trip curve of the breaker. How long and to what degree can a generator be overloaded an what happens to the generator when it does. Also' how much short circuit current can a genset provide?
Often it is thought that power as supplied from a genset is comparable to power as supplied from a utility which they can not be.

 

[kingpb]

The rating of a DEG is usually given as the Prime Rating. A typical DEG has a Standby Rating that will be 110% of the Prime Rating.
The most common power factor is 0.8.

So, 50kW at 0.8pf = 62.5KVA (Prime) the standby capability would be 62.5KVA x 1.1 = 68.75KVA

Therefore the maximum amps should be around 83A