Gen bkr size fire pump

[hhsting]

The breaker has to allow the motor to start. That is the only requirement. People are questioning the 150A breaker because that breaker feels like it would be too small to allow a 100hp motor to start, but you wouldn't know for sure without comparing the motor start curve to the breaker trip curve.

Unasked in this discussion so far is if the generator itself can start the 100hp motor.

Where is this requirement in NEC 2017? “The breaker has to allow the motor to start”

 

[augie47]

Where is this requirement in NEC 2017? “The breaker has to allow the motor to start”

695.4(B)(2)(b)

 

[hhsting]

I believe yes it should be required to carry locked rotor current indefinitely. Magnetic trip setting upon initial energizing still needs to be higher than locked rotor current.

Listed fire pump controller is easiest way to go, problem here is OP has a breaker on the genset that is maybe good for protecting the genset but probably doesn't comply with fire pump requirements. Does genset need to be listed/rated to supply the fire pump? IDK, guessing at very least maybe needs to be able to provide locked rotor current indefinitely.

Are u saying generator breaker has to be sized locked rotor current?

 

[david luchini]

Are u saying generator breaker has to be sized locked rotor current?

The generator breaker is not allowed to be sized per the locked rotor current in accordance with this section...

695.4(B)(2)(b)

 

[don_resqcapt19]

Are u saying generator breaker has to be sized locked rotor current?

Why not try reading the code section before asking another question? The code section clearly tells you what the maximum permitted size of the OCPD.

 

[hhsting]

Why not try reading the code section before asking another question? The code section clearly tells you what the maximum permitted size of the OCPD.

Ha! It was first point out by one of ur mods post#5 said he can have locked rotor current Gen breaker sizing. i responded post #8 with correct code section it cannot be done so then by my account I read alright. Then again it was point out by someone post #40 it can be done with locked rotor current gen breaker sizing. So then I thought I missed something. Sorry i thought i missed something somewhere even after reading the code

 

[hillbilly1]

A lot of engineers, though not required on a generator, call for the fire pump to be tapped ahead of the ocp at the generator. The generator controller will trip out and shut down the generator on overload anyway.

 

[kwired]

Where is this requirement in NEC 2017? “The breaker has to allow the motor to start”

Isn't really worded "to allow motor to start" but rather says it must be able to handle locked rotor current indefinitely.

Idea here is you don't care if motor burns out in a situation where it is called upon, they want it to give all it can and if it is going to trip it does so once it has been destroyed from failure to start.

 

[hhsting]

Isn't really worded "to allow motor to start" but rather says it must be able to handle locked rotor current indefinitely.

Idea here is you don't care if motor burns out in a situation where it is called upon, they want it to give all it can and if it is going to trip it does so once it has been destroyed from failure to start.

It isn’t worded for handle locked rotor current Not for fire pump generator breaker sizing. Where?


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[augie47]

I believe you will find the "locked rotor" requirement is for primary sources (695.3) and not for the standby generator.

 

[kwired]

It isn’t worded for handle locked rotor current Not for fire pump generator breaker sizing. Where?


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695.4 Continuity of power should apply to all source types. Subsection (B) is about overcurrent devices. (B)(2) basically gives you two choices. One is an assembly listed for fire pump controller service or the other is a device that will carry locked rotor current indefinitely.

 

[hhsting]

695.4 Continuity of power should apply to all source types. Subsection (B) is about overcurrent devices. (B)(2) basically gives you two choices. One is an assembly listed for fire pump controller service or the other is a device that will carry locked rotor current indefinitely.

Look at what 695.4(B)(2)(a) says its for individual sources. The two choices you mention above fall under 695.4(b)(2)(a). What is individual sources is in 695.3(A)(1)(2)(3).

In 695.3(A) do you see standby generator anywhere? I dont then thats not individual source. Therefore 695.4(B)(2)(a) doesn’t apply to generators. Agree or disagree?

Note all above code section in reference to NEC 2017

Someone please give like to hhsting

 

[Tulsa Electrician]

We all need one now and then.

 

[don_resqcapt19]

695.4 Continuity of power should apply to all source types. Subsection (B) is about overcurrent devices. (B)(2) basically gives you two choices. One is an assembly listed for fire pump controller service or the other is a device that will carry locked rotor current indefinitely.

The rule in 695.4(B)(2) says the OCPD shall comply with (a) or (b). (b) applies to a standby generator and does not permit the device to be set at locked rotor current. It sets the maximum as that permitted by 430.62, which sends you to 430.52 and sets the maximum at 250% if using a thermal magnetic breaker.

 

[JEFF MILLAR]

124 x 1.25 = 155 A. We do not want the breaker to trip. Therefore please clarify the correct answer.

 

[tom baker]

124 x250% = 310 max

 

[WA_Sparky]

Not sure if its worth mentioning or not but whenever I've messed with Life safety, those electronic trip breakers on the life safety portion, had to be coordinated with respect to their tripping characteristics down to 0.01 sec on the time current curves. Make sure you're putting in the correct settings, if the Engineer doesn't specify, get clarification cause they matter.

 

[kwired]

124 x 1.25 = 155 A. We do not want the breaker to trip. Therefore please clarify the correct answer.

125% is for conductor ampacity selection. If you don't want breaker to trip 125% can be too low to hold for startup of motors.

 

[kwired]

The rule in 695.4(B)(2) says the OCPD shall comply with (a) or (b). (b) applies to a standby generator and does not permit the device to be set at locked rotor current. It sets the maximum as that permitted by 430.62, which sends you to 430.52 and sets the maximum at 250% if using a thermal magnetic breaker.

I see that. Any idea why? Seems if you want to hold locked rotor current indefinitely in other situations you would want to here as well. Why is protecting the generator more important here than the fire pump motor? If either fails you have no fire pump operation period.

 

[don_resqcapt19]

I see that. Any idea why? Seems if you want to hold locked rotor current indefinitely in other situations you would want to here as well. Why is protecting the generator more important here than the fire pump motor? If either fails you have no fire pump operation period.

No idea of why...maybe because in some cases that same generator is supplying other life safety loads such as egress lighting and smoke control fans that are are also needed when there is a fire.