300 Hp Pump motor

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Next Generation

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I need some help checking my calculations. Thank you in advanced for your help.

I have a 300HP 480v 3 Phase pump motor I am needing to wire up.

Per NEC T-430.250 The FLA is 361amps

For this I will need a OCPD at 361 x 1.25 = 451.25 or 500a Per NEC 240.6

Conductor with 75 degree terminations will be a 700 mcm.

I plan on using G-GC cable.

Did I miss something here????
 
The 125% factor is used to size the conductors. The OCPD would be size according to T430.52. Just curious as to where you will find 700 kcmil conductors? :)
 
infinity said:
The 125% factor is used to size the conductors. The OCPD would be size according to T430.52. Just curious as to where you will find 700 kcmil conductors? :)
Click to expand...


Agreed, you would also have a tough time handling/bending cable that size. You will have to install parallel cable runs for your motor.
 
Next Generation said:
I need some help checking my calculations. Thank you in advanced for your help.

I have a 300HP 480v 3 Phase pump motor I am needing to wire up.

Per NEC T-430.250 The FLA is 361amps

For this I will need a OCPD at 361 x 1.25 = 451.25 or 500a Per NEC 240.6

Conductor with 75 degree terminations will be a 700 mcm.

I plan on using G-GC cable.

Did I miss something here????
Click to expand...

You missed a little>

The FLA is per 430.250 and is 361A.
The cable ampacity is 361 x 1.25 = 451A.

The CB (or other OCP) will be sized per 430.52. And it depends on what you are using:
Time Delay Fuses - 175%
Nontime Delay Fuses - 300%
Instantaneous CB - 800% - 1100%
Inverse time CB - 250%​
An inverse time CB will be 2.5 x 361 = 902, maybe next size up = 1000A.

You will also need overloads, generally 125% to 140% of the nameplate current.

Cable Ampacity Again:
As for Type G-GC cable most all I have seen is multiconductor. And 700kcmil will be single conductor, type G. (at least the ones I have seen)

Look at table 400.5.A.2, 75C section. 3c - 500 (column F) is rated for 470A. If you use single conductor, (column D), 1c - 350 is rated for 505A.

This is absolutely non-trivial design It is not a cookie-cutter design by the NEC. You may well wish to also add further motor protection such as:
Single phase protection
Thermal modeling
​
ice
 
Next Generation said:
... As for the 700 mcm I would obviously run parallel 4/0 cables
Click to expand...

I also am missing where the 700kcmil came from. Table 400.5.A.@, 75C, only shows 700kcmil in single conductor, rated at 785A.

(edit to add) If you are considering parallel 3c, type G-GC cabl: Per table 400.5.A.2, column F, 3/0 is rated for 241A, which gives 482A which is sufficient. BUT check the grounding/bonding conductor in the type G-GC. The 3/0 cable likely has too small a grounding conductor for parallel installation .

ice
 
Last edited:
Phil Corso said:
Iceworm...

Don't forget Run., Start, and Reacceleration V-drops! Especially if Y-D Starting is employed!

Regards, Phil Corso
Click to expand...

Uhhhh .... Phil - are you reading all the posts before responding? :? (that would be the confused me asking)

In any case, I promise I won't forget.

ice
 
Here we go again. I am sure this will not be my last:

FLA: 361
Cable ampacity: 361 x 1.25= 451a
OCPD: 600a adjustable trip CB?
Overload: Name plate = 333 x 1.25 (service factor is 1.15)= 416.25???
Cable is 3c G-GC 500
This Motor is connected to a Soft Start.

Thank you again for the assistance.

I am trying to move this to another location and it would have been nice if whoever connected it in the first place would have done it right. I want to make sure it gets done this way.
 
Next Generation said:
Here we go again. I am sure this will not be my last:

FLA: 361
Cable ampacity: 361 x 1.25= 451a
OCPD: 600a adjustable trip CB?
Overload: Name plate = 333 x 1.25 (service factor is 1.15)= 416.25???
Cable is 3c G-GC 500
This Motor is connected to a Soft Start.

Thank you again for the assistance.

I am trying to move this to another location and it would have been nice if whoever connected it in the first place would have done it right. I want to make sure it gets done this way.
Click to expand...

That is looking pretty good.

I don't know what to say about the Soft Start and the OCPD. Look at 430.52. The nec calls this CB: "Motor Branch-Circuit and Ground Fault Protection" It is not really an overcurrent device. Without the Soft Start, I would be looking at a 1000A CB. With the Soft Start - I don't know. Maybe look at the SS IOM manual. 600A seems small. And yes, it is okay to have a 1000A CB protecting 470A cable.(yep - truth)

Make sure the overloads can be set as high as 140% nameplate. Sometimes motors don't start on the 125% and NEC allows going to 140%.

For something this size, I tend to get the CB Time-Current log-log sheet and lay out a best guess motor starting curve and the overload curve.

Seriously consider a motor protection relay (electronic overloads) with additional, in particular:
Ground Fault,
Single phasing
Thermal modeling

This is an expensie motor. Bummer if you have to go for a rewind if the pump bearings start dragging and it doesn't trip, or someone kisses the donkey and goes for a fourth start in half an hour:sick:

ice
 
iceworm said:
You missed a little>


You will also need overloads, generally 125% to 140% of the nameplate current.
Click to expand...
I'm not going to disagree, but be careful with this. Most, but not all, overload mfrs have ALREADY factored the 125% into the sizing of their heater elements or dial calibration. So if you buy/select based on 125% of the FLA, you might end up being at 125% of 125% and fry the motor if there is an overload. RTFM and select the OL based on what your favorite mfr tell you to do. That is not an uncommon issue by the way, it's been keeping motor shops in business for years...

It is not a cookie-cutter design by the NEC. You may well wish to also add further motor protection such as:
Single phase protection
Thermal modeling
​
ice
Click to expand...
I could not agree more on the importance of this for a 500HP motor. Good solid state OL relays will do that, but RTFM.
 
Next Generation said:
Here we go again. I am sure this will not be my last:

FLA: 361
Cable ampacity: 361 x 1.25= 451a
OCPD: 600a adjustable trip CB?
Overload: Name plate = 333 x 1.25 (service factor is 1.15)= 416.25???
Cable is 3c G-GC 500
This Motor is connected to a Soft Start.

Thank you again for the assistance.

I am trying to move this to another location and it would have been nice if whoever connected it in the first place would have done it right. I want to make sure it gets done this way.
Click to expand...
Check the soft starter manual, it may dictate the OCPD that you must use ahead of it. Many are NOT listed to be protected by circuit breakers, you must use fuses.
 
And don't forget that Type G is a finely stranded conductor and will require terminations suitable for use with finely stranded conductors.
 
Jraef said:
... Most, but not all, overload mfrs have ALREADY factored the 125% into the sizing of their heater elements or dial calibration. ....
Click to expand...

My curent favorite for purposely convoluted manual interpretation is the AB E3+. Try and pick a setting that gives 140% nameplate. Ugh


ice
 
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