Design Letter A, B, D, D and E 1

[britboy]

I'm sizing the cabling and overcurrent protection for a 480-volt 50HP Fire Pump, our pump supplier will not adequately explain the difference between the design letters, a,b,c,d and e. I need to know what size the OCP needs to be. NEC 1996 table 430-151B Locked Rotor amps, indicated that a 50HP motor at 480-volt design letter b,c,d LRA would be 363amps, or at design letter e, LRA would be 515 amps. Can anyone explain the differences - apologize for my lack of knowledge on this subject, I only design Fire Pump connections once or twice a year.

Thanks.

 

[dpc]

NEMA and NEC have, unfortunately, used letters for two different aspects of motor characteristics.

The NEMA **DESIGN** code letter refers to speed-torque characteristics of the motor. The other letter is what you are referring to above - namely the Locked Rotor Code Letter. This provides information on the starting kVA of of the motor as a function of motor hp.

These two letters are separate specification items, although probably somewhat related.

NEMA Design B is most commonly used. Design C provides higher starting torque such as for conveyors. Design D is use for frequently started motors with varying loads such as presses, etc.

There has been discussion of a Design E that would have high efficiency with very high starting current and very poor power factor. Not sure if this has really been adopted by NEMA yet. I don't know what Design A is, or was used for.

You can speed-torque curves for NEMA A,B,C, and D in the Standard Handbook (Fink and Beatty).

Hope that helps and I'm sure you'll get a lot more answers.

Good luck.

 

[electricpete]

I believe the design letter referred to in the original posting is in fact the NEMA design letter (torque-speed characteristic).

 

[electricpete]

...i.e. Table 1501B is referring to the NEMA design letter as a means of estimating locked rotor amps from horsepower and voltage for sole purpose of articles 430.110, 440.12, 455.8c. I don't think that those articles apply to setting the overcurrent protection. They apply to such things as selecting disconnecting means.

dpc is right that the Locked Rotor Code Letter will probably be more relevant in determining overcurrent protection. That is identified in table 430.7b.

All my references are from NEC 2002. I don't have 1996.

 

[britboy]

Thanks guys – I appreciate the assistance. I just reviewed NFPA 20, Article 6-4.1.2, that states “ ALL Fire Pump motors shall be of the Design Letter B type”. The type that you refer to as “the most commonly used motor”. As a side note, we do not use NEC 2002 nor 1999, but 1996, the Counties that our Permit drawings go through are still all using 1996.

When you refer to the “Locked Rotor Amps” being a function of the HP of the motor, are we saying that if the motor were standing still, that the in-rush current at start up would equal the LRA? Similarly, when the motor reaches the end of its life, (when more current is required to turn it), when it quits, is that stage also equal to LRA? The reason I ask, is that the info that I have traded with other Electrical Designers in the past refers to the saying “the Fire Pump MUST run until it dies”. Therefore, I associate LRA at the end of the life of the motor. If indeed the start up current is equal to LRA, that is why the OCP is sized the way it is, to allow for start up, and motor failure.

Again, thanks for the input, would like to hear your thoughts on the above statement(s), and apologize for my lack of motor knowledge – not one of my strong points!

BB.

 

[cuky2000]

“A letter code defines the locked-rotor kVA on a per-hp basis. Codes are defined in MG 1-10.37.2 by a series of letters from A to V. Generally, the farther the code letter from A, the higher the inrush current per hp. A replacement motor with a "higher" code may require different upstream electrical equipment, such as motor starters”

http://www.joliet-equipment.com/nema_code_ac_motors.htm

FIRE PUMP OPERATION MODE: Fire pump will operate to destruction if necessary in all cases. We will design for an indefinitely locked-rotor current of the fire pump. “ Per NEC 450-3(a) and (b), the primary rating or setting of the protective device shall be sufficient to carry the equivalent of the transformer secondary current sum of the locked-rotor currents of the fire pump motor(s) and associated fire pump accessory equipment indefinitely”.
See also NFPA 20 6-5 for requirement of NEMA Design Code Letter for different HP ratings.

LRCmin = K*HP*1000/(1.73*Vn).

_{Where:
LRC.min = Min Locked Current (Amp)
HP = Motor power rating (HP).
Vn = Motor nominal voltage (Volt). For 48V use 460 V.
K = NEMA Code letter factor}

For additional info in FIRE PUMP, see the enclose site.

http://nema.org/engineering/icsstandards/ics_14.pdf

 

[dpc]

As cuky2000 indicates, the fire pump motor control and supply circuits must be designed to allow the pump to operate at locked-rotor current indefinitely (to destruction).

You do want to provide short-circuit protection for your motor circuit, but no overload protection. The logic is that the motor is much less valuable than what (and who) it is trying to protect. The fire-pump motor controller (starter) must be U.L. approved specifically for use as a fire pump starter. If this is a new pump, I'd suggest purchasing the system as a package, complete with pump, motor and starter, from a reputable fire pump supplier.

Also, keep in mind that the NEC has specific requirements on the power supply to the fire pump motor. You cannot take the fire pump power from your normal power distribution system. For a simple radial system, the fire pump supply must be tapped off **ahead** of the main breaker or disconnecting means.

This means it cannot be fed from an ordinary MCC or your main switchboard.

Good luck. Fire pumps are a lot of fun.

 

[jbartos]

Suggestion to dpc (Electrical) Jul 26, 2002
NEMA and NEC have, unfortunately, used letters for two different aspects of motor characteristics.
///Please, notice that there are NEMA:
1. Motor Design Code Letters

http://www.mastercontrols.com/EngInfo/Articles/Barr/TA_RBarr.htm

2. Motor Code Letters for Locked-Rotor kVA/hp

http://www.joliet-equipment.com/nema_code_ac_motors.htm

NEMA uses both types of Motor Code Letters as well as NFPA does.\\

 

[dpc]

To JBartos,

Yes, I understand. That was the point of my first response.

 

[jbartos]

To the previous posting: I am sorry, no offense.