Power to the Pump.

[rrc14]

I've just re-read this fire pump article in EC&M magazine;( http://ecmweb.com/nec/electric_power_pump/ ) and still somewhat unclear on conductor sizing for the fire pump controller.

The article has an example where they are using the locked-rotor amps to size the conductors for the fire pump controller, is this a mistake. I can't find a reference in the 2005 or 2008 NEC where it states that the locked rotor amps should be used for this calculation. I understand the calculations for the fire pump motor calculation and the overcurrent protection sizing. Shouldn't the (full load amps X 1.25) be used to determine the conductor size for the fire pump controller instead of using the locked-rotor amps?

Has anyone else read this article and had the same question? What am I missing here?

Thanks for any responses.

Ricky--

 

[wirenut1980]

Only the OCPD needs to be sized for the locked rotor current, not the conductors. Plus the artical states that the source needs to be strong enough to start the motor successfully. Quoted from the article below:

If conductors supply fire pump motors or accessory equipment, size them no less than 125% of the sum of the motor full-load current as listed in Table 430.248 (or 430.250) plus 100% of the ampere rating of the accessory equipment [695.6(C)].

Conductors supplying a single fire pump motor must be sized per 430.22. This means the branch-circuit conductors to a single fire pump motor must have an ampere rating of not less than 125% of the fire pump motor full-load current (FLC) ? as listed in Table 430.248 or 430.250. You can run wiring from the fire pump controller to the fire pump motor only in raceway listed in 695.6(E), and you cannot use ground-fault protection for fire pumps [695.6(H)].

Overcurrent protection [695.6(D)]. Overcurrent protection devices (OCPDs) must be sized to carry the sum of the locked-rotor current of the fire pump and pressure maintenance pump motor(s) indefinitely, and 100% of the ampere rating of the fire pump's accessory equipment.

Why such big OCPDs? To protect fire pump branch-circuit and feeder conductors against short circuits ? not overloads. Again, the fire pump must run no matter what ? so Art. 695 expressly forbids automatic overload protection. Isn't this risky? Not really. By design, you have overload prevention built in. These circuits have a dedicated load and supply, with installation requirements that greatly minimize the chance of overload.

 

[rrc14]

Ok, that's exactly what I was thinking, but, the graphic for the Fire Pump Controller - Voltage Drop calculation was throwing me off, it shows and uses the locked-rotor current in the calculation. Must be a typo.



Thanks--

 

[haskindm]

Take a look at 695.6(C)(2) which refers you to 695.7. The conductors must be sized to prevent a voltage drop exceeding 15% "under motor starting conditions". When a motor is starting it will draw "locked rotor current" so you must use this value in determining voltage drop. You will also need to make sure that the voltage drop does not exceed 5% when the motor is drawing 115% of full load current. So in sizing conductors you will need to look at three things.
1. Article 430 conductor size
2. 15% voltage drop at locked rotor
3. 5% voltage drop at 115% of full load current.
The conductor must meet or exceed all these criteria.
Nothing is easy....

 

[haskindm]

Also remember, we are not very concerned with "protecting" the conductors for a fire pump against overload. If the fire pump is running, THE BUILDING IS ON FIRE! We want that pump to run, no matter what, until the fire is out or until the pump melts down!