Motor Conductors

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petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
I have 150 horse continuous pump motor.
What is the procedure for sizing the conductors?


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Take a look at article 430 part II.

Part II. Motor Circuit Conductors
430.21 General. Part II specifies ampacities of conductors
that are capable of carrying the motor current without overheating
under the conditions specified.
 
The tables in 430 is where you start-- T. 430.250 for 3 phase and then you need the section stated above

Thanks That's what I needed.

Table says 180 amps for the 480v 150H but code req's 125% which is 225 amps. ch#3 table shows 3/0 min for the THHN at 90 degreesC.
Plan calls for 4/0 but EEOR says it might fly with the 3/0; follow-up paperwork must be provided. I just needed to know if this it's code.


Does my path look good?
 

texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
Thanks That's what I needed.

Table says 180 amps for the 480v 150H but code req's 125% which is 225 amps. ch#3 table shows 3/0 min for the THHN at 90 degreesC.
Plan calls for 4/0 but EEOR says it might fly with the 3/0; follow-up paperwork must be provided. I just needed to know if this it's code.


Does my path look good?

Math is good but 3/0 will not fly as it would be extremely rare that you would have an installation where you could use conductors at the 90 degree value.
You will need to use the 75 degree value which means a 4/0.
 

kingpb

Senior Member
Location
SE USA as far as you can go
Occupation
Engineer, Registered
Still in good shape for a 1987 model. I upgraded, now using a 2008 model. I think the only thing that changed is equipment model numbers, which I never use it for anyway.
 

SSDriver

Senior Member
Location
California
Occupation
Electrician
Code requires you to use 75* column for motors over 100 HP assuming the the motors starter/disconnect/vfd/etc is also listed as 75* or above. Most will be rated at 75*. And since it is continuous you have use 125% factor as stated above.
 

kwired

Electron manager
Location
NE Nebraska
Code requires you to use 75* column for motors over 100 HP assuming the the motors starter/disconnect/vfd/etc is also listed as 75* or above. Most will be rated at 75*. And since it is continuous you have use 125% factor as stated above.

Code says 100 amps and under you use 60C column unless equipment terminations are rated otherwise. Over 100 amps you use 75C conductors as general rule.

Not aware of anything that states similar rule that is dependent on horsepower.

For past ~30 years most equipment terminations - in particular circuit breakers and motor starters have been rated 75C even if under 100 amps.
 

SSDriver

Senior Member
Location
California
Occupation
Electrician
Code says 100 amps and under you use 60C column unless equipment terminations are rated otherwise. Over 100 amps you use 75C conductors as general rule.

Not aware of anything that states similar rule that is dependent on horsepower.

For past ~30 years most equipment terminations - in particular circuit breakers and motor starters have been rated 75C even if under 100 amps.

lol, brain fart. Yep I meant 100 AMP. The funny part is that I had to read your post twice, thinking "that's what I typed".
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
Where is that table in the NEC?
I think he's referencing 110.14(C)(1)(4) :
(4) For motors marked with design letters B, C, or D, conduc tors having an insulation rating of 75°C (167°F) or higher shall be permitted to be used, provided the ampacity of such conductors does not exceed the 75°C (167°F) ampacity.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
I think he's referencing 110.14(C)(1)(4) :
(4) For motors marked with design letters B, C, or D, conduc tors having an insulation rating of 75°C (167°F) or higher shall be permitted to be used, provided the ampacity of such conductors does not exceed the 75°C (167°F) ampacity.

Thanks Augie I figured that a few numbers were missing and I was looking for a table.
 

Besoeker3

Senior Member
Location
UK
Occupation
Retired Electrical Engineer
I know you guys follow NEC code but would actual motor nameplate current rating work?
At the tender stage of a project, I may not have that information but I can estimate the likely FLA from power, voltage, number of poles, probable efficiency etc.

That's generally what we do.

We have cable tables in BS7671 which specify ratings depending on type, installation method, and give voltage drop (mV/A/metre).
Like yourselves, there are discrete sizes so there is almost certainly going to be some spare capacity.

Just the musings of an idle Brit on a Saturday afternoon............
 

ActionDave

Chief Moderator
Staff member
Location
Durango, CO, 10 h 20 min from the winged horses.
Occupation
Licensed Electrician
I know you guys follow NEC code but would actual motor nameplate current rating work?
At the tender stage of a project, I may not have that information but I can estimate the likely FLA from power, voltage, number of poles, probable efficiency etc.

That's generally what we do.

We have cable tables in BS7671 which specify ratings depending on type, installation method, and give voltage drop (mV/A/metre).
Like yourselves, there are discrete sizes so there is almost certainly going to be some spare capacity.

Just the musings of an idle Brit on a Saturday afternoon............
Certainly using the actual motor nameplate current would work for that particular motor. The idea behind the NEC tables is that not all motors are created equal and so all the estimating you would do based on power, voltage, efficiency, etc is covered.
 

Besoeker3

Senior Member
Location
UK
Occupation
Retired Electrical Engineer
Certainly using the actual motor nameplate current would work for that particular motor. The idea behind the NEC tables is that not all motors are created equal and so all the estimating you would do based on power, voltage, efficiency, etc is covered.
Thank you for your reply.

I see that. Without actual nameplate data, you have to cover for worst case I suppose. For most of our projects, hardware (drives, enclosures, PLCs,) was the vast bulk of our hardware costs. Having to go up a cable size was not a big issue.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
The NEC states that for most motor types with an HP rating you are not allowed to just the nameplate amp data. You must use the table. Interpolation and extrapolation are not explicitly allowed.
 
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