Fire pump

[hhsting]

I apologize for my ignorance and always find people input in here helpful. I am stuck on the following problem if you can help then I did appreciate it:

I have POCO 2 sets of 4#350 kcmil service conductor into service rated fire pump controller with internal manufacturer provided breaker.

From the service rated controller I have 2 sets of 4#350 + 1#3/0 Gnd feeding three phase 480V 350 HP fire pump. Question how does one verify if #3/0 AWG Gnd size is correct or incorrect? The upstream breaker size is internal not yet provided and does one need this?

 

[tom baker]

See 250.122. You need a full size EGC (not ground) is each parallel set if run in two raceways. The breaker in the controller is only for short circuit current not overload protection. When you say upstream breaker, upstream from what?

 

[gadfly56]

See 250.122. You need a full size EGC (not ground) is each parallel set if run in two raceways. The breaker in the controller is only for short circuit current not overload protection. When you say upstream breaker, upstream from what?

He's talking about the OCPD in the pump controller.

 

[hhsting]

Please see attached sketch. I am talking about how to verify ground conductor size between service rated fire pump controller and the fire pump.

 

[gadfly56]

Please see attached sketch. I am talking about how to size equipment grounding conductor between service rated fire pump controller and the fire pump.

Same as you do for the conductors. 125% of the FLA for the pump. NOT the locked rotor current.

 

[hhsting]

Just to be clear I am talking about equipment grounding conductor size. Does NEC 2014 section 250.122 mention this?

 

[tom baker]

What is the title of section 250.122?

 

[hhsting]

The breaker in controller is most likely sized 600% FLA fire pump motor. I guess but size is unknown. The EGC would be based on that 600% breaker size or something different?

 

[gadfly56]

The breaker in controller is most likely sized 600% FLA fire pump motor. I guess but size is unknown. The EGC would be based on that 600% breaker size or something different?

As earlier, the EGC is sized based on the FLA of the pump, upped to 125%.

 

[dkidd]

I apologize for my ignorance and always find people input in here helpful. I am stuck on the following problem if you can help then I did appreciate it:

I have POCO 2 sets of 4#350 kcmil service conductor into service rated fire pump controller with internal manufacturer provided breaker.

From the service rated controller I have 2 sets of 4#350 + 1#3/0 Gnd feeding three phase 480V 350 HP fire pump. Question how does one verify if #3/0 AWG Gnd size is correct or incorrect? The upstream breaker size is internal not yet provided and does one need this?

There is a good discussion at

CQD: 11/28/2014

www.neca-neis.org

The ECG is sized based on the OCPD, and could be as large as the phase conductors. FLA of the pump has absolutely nothing to do with sizing the ECG.

 

[dkidd]

The breaker in controller is most likely sized 600% FLA fire pump motor. I guess but size is unknown. The EGC would be based on that 600% breaker size or something different?

A common misunderstanding is that “short-circuit protection only” means “instantaneous trip circuit breaker.” This drives an incorrect specification for equipment and results it an NEC compliance issue. Instantaneous-only devices are Recognized Components (Not Listed); they have no marked short-circuit rating and are not (by themselves) suitable for branch circuit protection.

How do you size a circuit breaker for a fire pump to comply with NEC requirements? | Schneider Electric USA

 

[dkidd]

I apologize for my ignorance and always find people input in here helpful. I am stuck on the following problem if you can help then I did appreciate it:

I have POCO 2 sets of 4#350 kcmil service conductor into service rated fire pump controller with internal manufacturer provided breaker.

From the service rated controller I have 2 sets of 4#350 + 1#3/0 Gnd feeding three phase 480V 350 HP fire pump. Question how does one verify if #3/0 AWG Gnd size is correct or incorrect? The upstream breaker size is internal not yet provided and does one need this?

The breaker in the controller is sized for the locked rotor current of the pump.
You can determine this from the nameplate FLA and design code letter.
The ECG is sized from Table 250.122.

See https://instrumentationforum.com/t/locked-rotor-current-of-a-motor/8290
for how to calculate the locked rotor current.

 

[gadfly56]

The breaker in the controller is sized for the locked rotor current of the pump.
You can determine this from the nameplate FLA and design code letter.
The ECG is sized from Table 250.122.

See https://instrumentationforum.com/t/locked-rotor-current-of-a-motor/8290
for how to calculate the locked rotor current.

No, it isn't. Only the permitted additional disconnect, fused or unfused, upstream of the controller is sized for the locked rotor current. See NFPA 20 2013 part 10.4.3.3 for the full information on the circuit breaker. Some characteristics are: 115% of FLA indefinitely, 300% of FLA for 30 minutes, across-the-line start, instantaneous short circuit protection (setting not less than 20x FLA).

You should assume that the controller is a black box and just supply whatever the manufacturer says it needs and don't extrapolate from any of the observable internal components.

 

[dkidd]

9.2.3.4 Where the overcurrent protection permitted by 9.2.3 is installed, the overcurrent protection device shall be rated to carry indefinitely the sum of the locked rotor current of the largest pump motor and the full-load current of all of the other pump motors and accessory equipment.

10.4.3.3.1 The circuit breaker shall have the following electrical characteristics:

(1) A continuous current rating not less than 115 percent of the rated full-load current of the motor

(2) Overcurrent-sensing elements of the nonthermal type

(3) Instantaneous short-circuit overcurrent protection

(4) *An adequate interrupting rating to provide the suitability rating of the controller discussed in 10.1.2.2

(5) Capability of allowing normal and emergency starting and running of the motor without tripping (see 10.5.3.2)

(6) An instantaneous trip setting of not more than 20 times the full-load current

 

[gadfly56]

9.2.3.4 Where the overcurrent protection permitted by 9.2.3 is installed, the overcurrent protection device shall be rated to carry indefinitely the sum of the locked rotor current of the largest pump motor and the full-load current of all of the other pump motors and accessory equipment.

10.4.3.3.1 The circuit breaker shall have the following electrical characteristics:

(1) A continuous current rating not less than 115 percent of the rated full-load current of the motor

(2) Overcurrent-sensing elements of the nonthermal type

(3) Instantaneous short-circuit overcurrent protection

(4) *An adequate interrupting rating to provide the suitability rating of the controller discussed in 10.1.2.2

(5) Capability of allowing normal and emergency starting and running of the motor without tripping (see 10.5.3.2)

(6) An instantaneous trip setting of not more than 20 times the full-load current

That's two SEPARATE devices. The 9.2.3.4 device is NOT in the controller cabinet.

 

[infinity]

So what's the correct answer there seems to be two different opinions, use the FLA method or the size of the OCPD?

 

[hhsting]

No, it isn't. Only the permitted additional disconnect, fused or unfused, upstream of the controller is sized for the locked rotor current. See NFPA 20 2013 part 10.4.3.3 for the full information on the circuit breaker. Some characteristics are: 115% of FLA indefinitely, 300% of FLA for 30 minutes, across-the-line start, instantaneous short circuit protection (setting not less than 20x FLA).

You should assume that the controller is a black box and just supply whatever the manufacturer says it needs and don't extrapolate from any of the observable internal components.

One is for manufacturer of fire controller within cabinet while other is if you are providing a breaker outside. Their is a figure I saw which clearly indicates this NFPA 20 long time don’t remember now. Two sets of rule.


115% of FLA I think is the breaker frame size no?

So the question is if there are 3 different trip points one at 300%x FLA it trips, 600% FLA trips and other is at 20x FLA trips what should the equipment grounding conductor size be based on. NEC 2014 section 250.122 does not specify?

 

[gadfly56]

One is for manufacturer of fire controller within cabinet while other is if you are providing a breaker outside. Their is a figure I saw which clearly indicates this NFPA 20 long time don’t remember now. Two sets of rule.


115% of FLA I think is the breaker frame size no?

So the question is if there are 3 different trip points one at 300%x FLA it trips, 600% FLA trips and other is at 20x FLA trips what should the equipment grounding conductor size be based on. NEC 2014 section 250.122 does not specify?

See 695.6 (B) for the various conductor sizes on the load side of the disconnect (if present), part (A) for the supply conductors. Size the EGC on each side accordingly.

 

[hhsting]

Here what I don’t get it if NFPA 20 has 10.4.3 circuit breaker inside fire pump controller then why does it have 10.4.4 another overcurrent protection locked rotor overcurrent protection? What’s the difference? You have to provide settings for both?