# PF in Generator Breaker Selection

W

#### WS32119

I'm trying to understand how this plays into breaker selection. If my generator is 40kw at 208 volt, it is rated 139 amps at .8 PF.
If the PF is 1, what is the amp rating - 158 amps?

Thanks for any input.

W

#### WS32119

Further to this, the generator can produce 300% of rated current in a dead short/ground fault scenario. Should the breaker be a thermal magnetic type ...? How does this 300% rating affect breaker selection? My assumption is that the trip times and curves need to be shorter.

Thanks again

>I'm trying to understand how this plays into breaker
>selection. If my generator is 40kw at 208 volt, it is rated
>139 amps at .8 PF.
>If the PF is 1, what is the amp rating - 158 amps?
>
>Thanks for any input.

B

#### Barbican

Hi buddy.

>I'm trying to understand how this plays into breaker
>selection. If my generator is 40kw at 208 volt, it is rated
>139 amps at .8 PF.
>If the PF is 1, what is the amp rating - 158 amps?

The answer to your question is no... The current won't change from 139A, instead the KW will raise if the PF is 1 and the KVAR will be Zero. think of it this way PF is a percentage of how much power is doing real work, say 0.8 which means 80% is doing useful work while 20% is being somehow wasted.

KVA Defines the total apparent power, while KW defines the total active power and KVAR defines the total reactive power.

#### PhilCorso

WS32119... typically, CB selection is based on 1.25 x kVA rating! Its kVA rating, 40kW at 0.8 pf, is kW/pf equals to 50kVA!

Thus, the generator's maximum current rating is 139A. How did you arrive at 158A?

Regards,
Phil Corso

Barbican... you can only increase kW above rated if Genset Mfg says you can.

B

#### barindra75

Don't go with KW or KVA rating. actually, KW & KVA ratio is the power factor which is a characteristic of load not your generator.

You must check the gen rated current written on its nameplate, which is 139A in your case. You should not cross this current.

If your PF is 1 till you can go upto 139A which will give you sqrt(3)*208x139 KW or KVA= 50.076 KW or KVA.

breaker selection done on two parameters

1. thermal current level i.e. max continuous current level. In your case it should be 1.25*139 Amp.

2. fault current level i.e. max current in short time interval. a typycal rating may be 10,000A with 1 sec.

In your case, gen can generate 300% full load current fault i.e. 3*139A=417A. you can choose 1.5*417Amp with 1sec. This short time interval is also specified in your gen fault level. select according to that, slightly higher for the breaker.

B

#### Barbican

Hi Phil

> Barbican... you can only increase kW above rated if Genset Mfg says you can.

What would the problem be if i am running a restive load (PF-1) and lets say my generator is rated for 500kva and the load is now 500KW?
(no reactive power)

W

#### WS32119

Thank you. So the way I now understand the PF is at unity the generator will put out its full output amps and whatever the diesel is rated at. At a lagging PF (or leading) the generator diesel will reach its full output BUT the amps will not be at 139, rather something less. Is this correct?

Most industrial breaker are rated something like 65k AIC. The way I understand this is, that is what fault current the breaker can function up to (before blowing apart) So what happens in a case like the generator breaker that can only develop 417 amps if I use a 65k AIC rated breaker? I assume the short circuit sensor will still work...but Im not 100% sure.

Thanks to everyone for your input, I appreciate it very much.

#### PhilCorso

Barbican...

your example works as long as the prime-mover has equal capacity!

Phil

B

#### Barbican

> your example works as long as the prime-mover has equal capacity!

certainly