FLC motor Drawn, motor question.

Status
Not open for further replies.

O'donisR

Member
Location
Miami
Hi, please I need help to understand this question. here is:

  1. What is the full load current drawn by a 5hp, 208V 3phase motor?
    Answer:
    table 430.250 (15.2Amps x 1.10 = 16.72A)
I don't know where the 1.10 came from, also table 430.250 for a 5ph/208V/3phase the F.L.C is 16.7 amps. I'm litter confused on this one. Please help. Thank you
 

texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
Hi, please I need help to understand this question. here is:
  1. What is the full load current drawn by a 5hp, 208V 3phase motor?
    Answer:
    table 430.250 (15.2Amps x 1.10 = 16.72A)
I don't know where the 1.10 came from, also table 430.250 for a 5ph/208V/3phase the F.L.C is 16.7 amps. I'm litter confused on this one. Please help. Thank you
Per Table 430.250 the correct answer is 16.7. It seems that the answer key is wrong if the only information given in the question is 5 HP, 208 volt, 3 phase. Service factor does not apply in sizing conductors for normal motors, rather the tables are used regardless of of SF and nameplate. Now if they are talking about overload sizing, that is a different story. But you indicate that info was not given.
 

BPoindexter

Inactive, Email Never Verified
Location
MT Vernon, WA
Actually SF does not dictate full load current either. As pointed out it is for overloading- how much can it take and how big can you size the overloads. Overloads are chosen based on the FLC given on the motor nameplate. Service Factors are 1.0 or 1.15 and I have not seen a 1.10 SF? 125% for 1.15 and motors less than 40C rise, 115% for all others (based on continuous duty >1HP). Not sure where the 110% comes from unless it is talking about minimum disconnecting means sizing?
 

charlie b

Moderator
Staff member
Location
Seattle, WA
Occupation
Electrical Engineer
Answer: table 430.250 (15.2Amps x 1.10 = 16.72A)
That table gives 15.2 as the current for a 230 volt motor, not a 208 volt motor. The value for a 208 volt motor is 16.2. Did you look at the table, or just at the answer key?
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
Hi, please I need help to understand this question. here is:

  1. What is the full load current drawn by a 5hp, 208V 3phase motor?
    Answer:
    table 430.250 (15.2Amps x 1.10 = 16.72A)
I don't know where the 1.10 came from, also table 430.250 for a 5ph/208V/3phase the F.L.C is 16.7 amps. I'm litter confused on this one. Please help. Thank you
I suspect that this was a test question that was first derived from an earlier NEC cycle, and not updated particularly well.

The 1987 NEC's Table 430-150 "Full Load Current - Three-Phase Alternating-Current Motors" had voltages of 115V, 230V, 460V, 575V, 2300V. The notes below the table instructed you that for 208V and 200V motors, increase the corresponding 230V motor full load current by 10% and 15%, repectively.

So for a 5HP motor, you'd get 15.2 FLA from the 230V column, and increase it by 10% to get the 208V FLA: 15.2A * 1.1 = 16.7A.

In 1990, Table 430-150 was updated to include columns for 200V and 208V, so you no longer had to apply the 10% or 15%. And of course, Table 430-150 (from '87 & '90) has become Table 430.250 in the more recent code cycles.

That's where the 1.10 came from.
 
Status
Not open for further replies.
Top