cable sizing question (125HP vs. 75HP)

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tankfarms

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I was approached by my mechanical engineer to consider re-size of the cable of a motor as they?re thinking of getting a new motor for this pump and the new motor will be 125HP, whereas the old motor is 75HP. Below are the specs I found from the existing MCC design:

Motor Volt FLA Cable Cable Ampacity Upstream Breaker
125HP 460V 182 1-3/C #350kcmil 350 400AF/225AT
75HP 460V 109 1-3/C #4/0 260 150AF/150AT

The FLA is calculated FLA assuming 0.8PF and 0.8 Eff. Cable ampacity is based on NEC table 310.16 assuming temp. rating 90C.

Before I suggest to my mechanical engineer to go with a 3/C #350kcmil cable system (as the case for other 125HP machines on the same existing MCC), I want to understand if it is possible to utilize the existing #4/0 size cable as I feel its ampacity seems to be able to adequately accommodate the new 125HP motor. It?ll make a huge cost difference if we have to re-size and install new cables.

I?m not as familiar in the sizing of cables, so my questions here are: are we able to use the existing 75HP cable system to accommodate the new 125HP motor? Also, what was the basis of determining the size of the cables as you can see that for the existing 125HP motor, our #350kcmil provides some ampacity reserve. Pretty much I want to know what's the math/calculation behind the specs that as shown above.

Thank you.
 
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haskindm

Senior Member
Location
Maryland
Go to 430.250 to get the amperage of the motor. Looks like 460-volt 125 HP = 156 amps. Multiply by 125% for a single motor and your conductor ampacity must be a minimum of 195 amps. Go to 310.16 and USE THE 75 DEGREE COLUMN (see 110.14 (C) if you don't understand why the 75 degree column is used). It looks to me like 3/0 copper would be sufficient (200 amps), so your existing 4/0 is fine. Not sure how you came up with 350kcmil, especially if you are using the 90 degree column.
 
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skeshesh

Senior Member
Location
Los Angeles, Ca
How far is the machine from the source? Voltage drop should be considered but I don't see how you would get to 350 even assuming a considerable length.
 
G

G._S._Ohm

Senior Member
Location
DC area
tankfarms said:
the new motor will be 125HP, whereas the old motor is 75HP.
Pretty much I want to know what's the math/calculation behind the specs that as shown above.
Click to expand...
Some posters have said that their finished installation seemed to have cable/conduit surfaces that are too warm [above 50C probably].

Can you get a temp. reading of the cable surface and ambient temp. near the cable with the motor you have?

I think I can confirm the adequacy of the existing cabling by using a formula, and it should match what the tables say within a few percent.

With a lot of cost riding on the answer extra confirmation can't hurt.
 
T

tankfarms

Member
haskindm said:
Go to 430.250 to get the amperage of the motor. Looks like 460-volt 125 HP = 156 amps. Multiply by 125% for a single motor and your conductor ampacity must be a minimum of 195 amps. Go to 310.16 and USE THE 75 DEGREE COLUMN (see 110.14 (C) if you don't understand why the 75 degree column is used). It looks to me like 3/0 copper would be sufficient (200 amps), so your existing 4/0 is fine. Not sure how you came up with 350kcmil, especially if you are using the 90 degree column.
Click to expand...


Hello thanks for the reply,
The 350kcmil is what is being used for other existing 125HP motors. I think if I use the existing 4/0 I might be pushing it. When multiplied by the PF (per 430.250), we'll have 214 amps, and then on top of that, I think the de-rating is considered. As we get close to 40C around summer time (we're down south), the cable ampacity is multiplied by 0.82 (if 75 deg. column is choosen), per 310.16, a 4/0 cable only gives us about 188 amps. Maybe this was one of the reasons they used 350kcmil cables for 125HP as well as 100HP machines.
 
R

rcwilson

Senior Member
Location
Redmond, WA
Is the cable run underground in a duct bank? A good design includes derating for the depth of burial, proximity of other conductors in the duct bank and ambient temperature.

Also, make sure the ME's realize they have to change the motor starter to go from 75 to 125 HP. I have had oil field ME's tell me they had the explosion proof starter ready to go for the relocated pump. At the site they gave me the NEMA 7 start/stop pushbutton station that they thought was a 250 HP motor starter.
 
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Designer69

Senior Member
Im thinking the wiring for these goes thru alot of cable trays, including fire wall penetrations possibly thru kaewool blankets which require serious de-rating.

cable tray heat rise plays a factor and also VD. I can certainly see why the large cable size so be careful especially if it is a certain critical or safety related pump.
 
H

haskindm

Senior Member
Location
Maryland
tankfarms said:
Hello thanks for the reply,
The 350kcmil is what is being used for other existing 125HP motors. I think if I use the existing 4/0 I might be pushing it. When multiplied by the PF (per 430.250), we'll have 214 amps, and then on top of that, I think the de-rating is considered. As we get close to 40C around summer time (we're down south), the cable ampacity is multiplied by 0.82 (if 75 deg. column is choosen), per 310.16, a 4/0 cable only gives us about 188 amps. Maybe this was one of the reasons they used 350kcmil cables for 125HP as well as 100HP machines.
Click to expand...

OK, so using your figures we have a total calculated load of 214 amps. 4/0 copper at 75 degrees is good for 230 amps so it still should be fine. I suspect that the existing installation may have sized the conductor to the overcurrent device instead of to the actual motor load, unless it is an extremely long circuit and voltage drop is a big consideration.
 
T

tankfarms

Member
haskindm said:
OK, so using your figures we have a total calculated load of 214 amps. 4/0 copper at 75 degrees is good for 230 amps so it still should be fine. I suspect that the existing installation may have sized the conductor to the overcurrent device instead of to the actual motor load, unless it is an extremely long circuit and voltage drop is a big consideration.
Click to expand...

230 amps is without taking the ambient temp de-rating factor; where we're at does get hot during summer, that was why I used 0.82 for temp de-rating. The circuit is fairly long, from the MCC to the motor is approx. 300 to 400 ft. one-way.
 
T

tankfarms

Member
Designer69 said:
Im thinking the wiring for these goes thru alot of cable trays, including fire wall penetrations possibly thru kaewool blankets which require serious de-rating.

cable tray heat rise plays a factor and also VD. I can certainly see why the large cable size so be careful especially if it is a certain critical or safety related pump.
Click to expand...

thanks. Indeed, we're at a refinery, cable tray/metallic conduit is typical for circuit feeding motors in the field. like mentioned in the previous reply post, we have a good 300-400 ft. one-way distance from the MCC to the motor. This might be another reason 350kcmil cable was chosen for all other 125HP machines. I think to be on the safe side, and considering all the de-rating factors, I'll recommend to them 350 cable also to be consistent with other equipment also leave some reserved capacity for the future.
 
mbeatty

mbeatty

Senior Member
Location
Illinois
tankfarms said:
Hello thanks for the reply,
The 350kcmil is what is being used for other existing 125HP motors. I think if I use the existing 4/0 I might be pushing it. When multiplied by the PF (per 430.250), we'll have 214 amps, and then on top of that, I think the de-rating is considered. As we get close to 40C around summer time (we're down south), the cable ampacity is multiplied by 0.82 (if 75 deg. column is choosen), per 310.16, a 4/0 cable only gives us about 188 amps. Maybe this was one of the reasons they used 350kcmil cables for 125HP as well as 100HP machines.
Click to expand...

If you get close to 40 degrees C, your derate should be based on a 0.88 multiplier if 75 degree C column is appropriate. :roll:
 
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