MOTOR WIRE SIZE & AMPACITY(using 99' NEC)

Status
Not open for further replies.

jw441

Member
Re-posting/wording of 3-26-06 posting, "Motor Feeder Calcs":
Using 99' NEC, 90degree C THHN CU wire within 50' in RMC - find the wire size required to connect a continuous run, 3 phase, 200 hp synchronous motor into a 480 volt circuit.
My calculations: First, I actually came up with 3 different calcs(#4/0, 250 & 350). I'm missing/overlooking the determinant variable(s)/factor(s) that define one size over another. Prints called for 350s, we RFIed on #4/0(3 - #4/0, 1 - #4 GND in 2" RMC) and it was verbally accepted, but I'm still not convinced on the calc and/or how to word it. Second, a parallel feed(in 2 separate conduits) of the same size wire(as used for 200 hp motor), can be used to connect same at 400 hp... correct?
Any/all input, assistance is greatly appreciated.
Fraternally, jw441
I'd like to apologize for any/all previous mistakes in content and/or etiquette, and hope to change my involvement here, after my two boys are off in the Navy this August - I'm a single parent. No excuses, just humble thanks.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
jw,
First you can't use the 90?C for the wire ampacity, in general you can only use that for derating calculations. Second, we need some more information, we need to know what power factor the synchronous motor is going to be operated at. The conductor ampacity must be at least 125% of the full load current shown in Table 430.250, but there the table is only for unity power factor and must be ajdusted per the note if you are operating at a lower power factor.
Don
 

ramsy

Roger Ruhle dba NoFixNoPay
Location
LA basin, CA
Occupation
Service Electrician 2020 NEC
jw441,

If thats JW of IBEW 441 in Orange, then I'm sharing the same card right in your backyard. Wana borrow my 99NEC code book, PM me and I'll drop by.

The prints may have considered 430-22(a) when calling for 350s. Using Tbl.430-150, 200HP 3? @ 460vac is 240A, before factoring * 1.25 = 300A for continuous use. The 350s show 310A @ 75?c in conduit. If both motor terminals and gear terminations really are rated @ 90?c then 300s show max 320A @ 90?c.

Those tables show 400HP 3? @ 460vac is 477A, * 1.25 = 596A for continuous use. You could parallel 2 sets of 300s @ 90?c, or 3 sets of 3/0 @ 75?c.

I missed Don's point about power factor. Thats why he makes the big bucks. On a second look, I see the bottom of Tble 430-150 has PF multipliers for 90% and 80% PF; the figures shall be multiplied by 1.1 and 1.25 respectively. Any extra cash from this job goes in Don's tip jar.

Fraternally,
 

jw441

Member
don_resqcapt19 said:
jw,
First you can't use the 90?C for the wire ampacity, in general you can only use that for derating calculations. Second, we need some more information, we need to know what power factor the synchronous motor is going to be operated at. The conductor ampacity must be at least 125% of the full load current shown in Table 430.250, but there the table is only for unity power factor and must be ajdusted per the note if you are operating at a lower power factor.
Don

Do I use next column, 75degreesC?

200hp - 87.6 PF at FL
400hp - 89 PF at FL

3-26-06 posting has more nameplate info

thanks for your help
 

jw441

Member
ramsy said:
jw441,

If thats JW of IBEW 441 in Orange, then I'm sharing the same card right in your backyard. Wana borrow my 99NEC code book, PM me and I'll drop by.

The prints may have considered 430-22(a) when calling for 350s. Using Tbl.430-150, 200HP 3? @ 460vac is 240A, before factoring * 1.25 = 300A for continuous use. The 350s show 310A @ 75?c in conduit. If both motor terminals and gear terminations really are rated @ 90?c then 300s show max 320A @ 90?c.

Those tables show 400HP 3? @ 460vac is 477A, * 1.25 = 596A for continuous use. You could parallel 2 sets of 300s @ 90?c, or 3 sets of 3/0 @ 75?c.

I missed Don's point about power factor. Thats why he makes the big bucks. On a second look, I see the bottom of Tble 430-150 has PF multipliers for 90% and 80% PF; the figures shall be multiplied by 1.1 and 1.25 respectively. Any extra cash from this job goes in Don's tip jar.

Fraternally,

thanks for your help Roger
i'll take you up on the PM offer
gotta get going, the rain has killed my banked hrs
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
jw,
Are these really synchronous motors or are they induction motors?
Yes, you have to use the 75?C column for the ampacity. If you need to derate for more then 3 conductors in a raceway or for high ambient you can start with the 90?C column.
Don
 

jw441

Member
don_resqcapt19 said:
jw,
Are these really synchronous motors or are they induction motors?
Yes, you have to use the 75?C column for the ampacity. If you need to derate for more then 3 conductors in a raceway or for high ambient you can start with the 90?C column.
Don

Yes, they're vertical turbine pumps for cold DW circulation, with a synchronous speed of 1800 rpm.
What would constitute/justify "high ambient" to use the 90C column?
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
jw,
Yes, they're vertical turbine pumps for cold DW circulation, with a synchronous speed of 1800 rpm.
I still doubt that these are synchronous motors. A synchronous motor is not that common and if larger than 30 hp, it would normally require a DC exciter circuit and the motor would have slip rings to supply the exciter current to the rotor.
What would constitute/justify "high ambient" to use the 90C column?
You almost never get to use the actual 90?C ampacity. You use it when you have to derate the conductor ampacity because the temperature in the area where the conductors are installed is greater than 30?C. There are temperature correction factors shown at the bottom of Table 310.16. These correction factors reduce the permitted ampacity of the conductors when they are installed in high temperature areas. You also use the 90?C ampacity to adjust the conductor ampacity when you have more than 3 current carrying conductors in a raceway of cable per 310.15(B)(2).
In either case, if the corrected or adjusted ampacity is greater than that shown in the 75?C column, you must use the 75?C ampacity. If the corrected or adjusted ampacity is less than that of the 75?C column, you must use the adjusted or corrected ampacity. These calculated values are the maximum permitted ampacity of the conductors.
Don
 
Status
Not open for further replies.
Top