Feeder & C.B sizing for Motors

Status
Not open for further replies.
T

tarekbadr

Member
W have 4 motors each motor has its own control Panel, 3 phase, 400 V and its rating as follow:

1- Press: 2 Nos. each 250 HP
2- Vertical Dryer: 1 No. , 125 HP
3- Glazing : 1 No. , 75 HP

I did calculations to obtain the conductor & C.B for each motor as follow:

1- Press: I full load = 353 A , so the conductor must have 1.25*353= 441 A
cable length = 155 m then the correct size after applying all factors is 2(3C x 150 mm2 CU/XLPE/PVC) ;{300 MCM} with voltage drop = 2.12 %
C.B rating : 630 A is that correct or not ?? ( if we follow NEC 430.52, So it must be 353 x 2.5 =882 A, lead to 800 A)

2- Vertical dryer: I full load= 177 A , so the conductor must have 1.25*177= 221 A
cable length = 150 m then the correct size after applying all factors is (3C x 120 mm2 CU/XLPE/PVC) ;{4/0} with voltage drop = 2.53 %
C.B rating : 250 A is that correct or not ?? ( if we follow NEC 430.52, So it must be 177 x 2.5 =442 A, lead to 500 A)

3- Glazing Line: I full load= 106 A , so the conductor must have 1.25*106= 132 A
cable length = 175 m then the correct size after applying all factors is (3C x 70 mm2 CU/XLPE/PVC) ;{750 MCM} with voltage drop = 2.7 %
C.B rating : 160 A is that correct or not ?? ( if we follow NEC 430.52, So it must be 106 x 2.5 =265 A, lead to 250 A)

and for main feeder, current carrying capacity =1.25*353+ (353+177+106)=1077 A , cable size 3 (4C X300 mm2 CU/XLPE/PVC) ; { 3 ( 3C x 600 MCM)}

the main C.B = 1500 A is that correct or not ??

Please if the above calculations is correct or not , and if not please advise me

Regards for all
 
Bit difficult to verify using the NEC as 430.6 requires using the Tables for calculations and there is no 400 volt Table.
In an attempt to use your numbers, I did come up with a 1500 amp MCB.
 
I don't see anyway to make this kind of thing NEC compliant given the voltages involved.

Having said that, many foreign motors have ratings that allow them to run at 480V/60Hz in addition to their foreign rating. I would be looking at the data sheets closely if you need to be NEC compliant.
 
that all available data , voltage operation 400 V, 60 Hz , 1800 rpm and NEC 's tables not support motor with operating voltage 400V , but any way , we can calculate the full load current for each motor , i would like ask, the calculated cable size and C.B for each motor is correct or need modify? please advise
 
tarekbadr said:
W have 4 motors each motor has its own control Panel, 3 phase, 400 V and its rating as follow:

1- Press: 2 Nos. each 250 HP
2- Vertical Dryer: 1 No. , 125 HP
3- Glazing : 1 No. , 75 HP

I did calculations to obtain the conductor & C.B for each motor as follow:

1- Press: I full load = 353 A , so the conductor must have 1.25*353= 441 A
cable length = 155 m then the correct size after applying all factors is 2(3C x 150 mm2 CU/XLPE/PVC) ;{300 MCM} with voltage drop = 2.12 %
C.B rating : 630 A is that correct or not ?? ( if we follow NEC 430.52, So it must be 353 x 2.5 =882 A, lead to 800 A)

I don't see how you get 353A for the 250HP motor running at 400V? My chart from UL508a shows 403A for such a motor. that would lead to a required ampacity of 504 A, but 2 300 kcm would still handle that. The CB can be anything upto 250% of FLC, so the max would be 1000A.

2- Vertical dryer: I full load= 177 A , so the conductor must have 1.25*177= 221 A
cable length = 150 m then the correct size after applying all factors is (3C x 120 mm2 CU/XLPE/PVC) ;{4/0} with voltage drop = 2.53 %
C.B rating : 250 A is that correct or not ?? ( if we follow NEC 430.52, So it must be 177 x 2.5 =442 A, lead to 500 A)

A 125Hp motor at 400V is 208A according to my chart. That would lead to a required ampacity of 260A. 4/0 would not cover that. The maximum CB size would be 250% of the FLC rounded up to the next standard sized CB. That would 600A.

3- Glazing Line: I full load= 106 A , so the conductor must have 1.25*106= 132 A
cable length = 175 m then the correct size after applying all factors is (3C x 70 mm2 CU/XLPE/PVC) ;{750 MCM} with voltage drop = 2.7 %
C.B rating : 160 A is that correct or not ?? ( if we follow NEC 430.52, So it must be 106 x 2.5 =265 A, lead to 250 A)

A 75Hp 400V motor is 128A according to my chart. that would make for a required ampacity of 160A. That leads to a 2/0 wire and a
max 350A CB.

and for main feeder, current carrying capacity =1.25*353+ (353+177+106)=1077 A , cable size 3 (4C X300 mm2 CU/XLPE/PVC) ; { 3 ( 3C x 600 MCM)}

I get a required ampacity for the feeder of 1131A. 1500A will work fine as long as the conductors are protected properly.

the main C.B = 1500 A is that correct or not ??

Please if the above calculations is correct or not , and if not please advise me

Regards for all
Click to expand...

tarekbadr said:
that all available data , voltage operation 400 V, 60 Hz , 1800 rpm and NEC 's tables not support motor with operating voltage 400V , but any way , we can calculate the full load current for each motor , i would like ask, the calculated cable size and C.B for each motor is correct or need modify? please advise
Click to expand...

See comments above in red.

This is such an oddball application to try and fit into the NEC that I suspect an inspector would want to take a lot more than one look at it. I am not even sure that the NEC would allow 400V as the main.
 
Last edited:
tarekbadr said:
the formula I = HP * 746/ (1.732 * 400 *0.8 * Eff. ) , 0.8 is power factor , we consider 100% efficiency or we got the full load current from table available in the following link : http://myelectrical.com/notes/entryid/156/useful-motor-technical-information

as you said the max C.B is 350 A ( for 3rd motor) , how can we choose the correct rating
Click to expand...

the NEC only gives a max. it is up to the engineer to select what rating to use based on whatever design criteria he or she feels comfortable using his or her best professional judgment for the installation at hand.
 
Status
Not open for further replies.
Top