Conductor sizing??

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FranklinMade

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Can some help me with a simple calculation please. If you are sizing a motor load and let’s say you figure your load will draw 28 but it will be let’s say 400’ away. How would you calculate for VD and so on.
 
FranklinMade said:
Can some help me with a simple calculation please. If you are sizing a motor load and let’s say you figure your load will draw 28 but it will be let’s say 400’ away. How would you calculate for VD and so on.
Click to expand...

Go to the iPhone or android App Store and download Southwire Voltage Drop calc. Fill out the info in the app.

Also, you said it draws “28” but the actual value should be found using the actual HP of the motor.

Feel free to post your results and we’ll check them.


Sent from my iPhone using Tapatalk
 
Npstewart said:
Go to the iPhone or android App Store and download Southwire Voltage Drop calc. Fill out the info in the app.

Also, you said it draws “28” but the actual value should be found using the actual HP of the motor.

Feel free to post your results and we’ll check them.


Sent from my iPhone using Tapatalk
Click to expand...
I’m basically asking for a VD formula to find Length
 
When sizing conductors for a motor. the motor's HP rating is used, not a calculated current draw. To figure out voltage drop, you need your load (in amps), one-way distance, voltage and phasing. Most wire suppliers have the calculator on their website. Use it.
 
FranklinMade said:
I’m basically asking for a VD formula to find Length
Click to expand...

%Vd = I*r*L*a/(n*V*1000) * 100%

where:
I is the current
r is the resistance of the wire size per unit length, units of Ohms/kft or Ohms/km, from either chapter 9/table 8 in DC, or table 9 in AC.
L is the 1-way circuit length
a is a value that quantifies how L relates to the effective round trip length. For single phase and DC, a=2. For 3-phase with phase-to-neutral voltage in the denominator, a=1. For 3-phase with the phase-to-phase voltage in the denominator, a=sqrt(3).
V is the operating voltage
n is the number of parallel sets, or 1 if not applicable
 
None of the above calculators consider motor loads, as required per NFPA-70.

Few electricians, much less arm-chair engineers on the internet use Motor Efficiency 220.18(A), or Power Factor (PF) adjustments required for motor-conductor size per 460.9

This results in large errors with all calculators, and the hottest wire found in thermal imaging reports.

Calcs that properly follow motor PF from Table 9 Note 2 formula, and 460.9 for conductors, still suffer operator errors that omit the 220.18(A) efficiency factor.
 
FranklinMade said:
If you are sizing a motor load and let’s say you figure your load will draw 28
Click to expand...

Before voltage drop is considered, a 28A motor load must be efficiency adjusted x 1.25 per 220.18(A)

28A x 1.25 = 35A

Then any voltage drop calc must include Table 9 Note 2 Power Factor (PF) formula, per 460.9, to match PF built into all NEC Tables, including 310.16 ampacity.

Skipping both requirements, without engineered plans, under sizes equipment loads shown in 220.18, and other inductors.

Typical resistive loads don't require these motor inductive adjustments, so it is possible that dumb luck gets those wire sizes right.

However, given the complexity and experience required to properly adjust NEC 310.15 ampacity tables for extra-environmental variables, few inspectors are capable of correcting these errors, much less inductive motor-loads errors.

Its too complex, so it perpetuates the cottage industry of thermal imaging.

Thermal imaging is the Industry standard for inspecting NEC ampacity-table errors, so wire can be ripped out and replaced immediately after installed.
 
Last edited:
ramsy said:
None of the above calculators consider motor loads, as required per NFPA-70.

Few electricians, much less arm-chair engineers on the internet use Motor Efficiency 220.18(A), or Power Factor (PF) adjustments required for motor-conductor size per 460.9

This results in large errors with all calculators, and the hottest wire found in thermal imaging reports.

Calcs that properly follow motor PF from Table 9 Note 2 formula, and 460.9 for conductors, still suffer operator errors that omit the 220.18(A) efficiency factor.
Click to expand...
The code requires, in 430.22, that the motor conductor be sized at 125% or more of the motor full load currents as found in Tables 430.247 through 430.250.
 
ramsy said:
None of the above calculators consider motor loads, as required per NFPA-70.

Few electricians, much less arm-chair engineers on the internet use Motor Efficiency 220.18(A), or Power Factor (PF) adjustments required for motor-conductor size per 460.9

This results in large errors with all calculators, and the hottest wire found in thermal imaging reports.

Calcs that properly follow motor PF from Table 9 Note 2 formula, and 460.9 for conductors, still suffer operator errors that omit the 220.18(A) efficiency factor.
Click to expand...
460.9 has nothing whatsoever to do with conductor sizing.
 
ramsy said:
I agree to disagree.
Click to expand...
For what reason do you disagree?
460.9 Rating or Setting of Motor Overload Device. Where a
motor installation includes a capacitor connected on the load
side of the motor overload device, the rating or setting of the
motor overload device shall be based on the improved power
factor of the motor circuit.
The effect of the capacitor shall be disregarded in determining
the motor circuit conductor rating in accordance with
430.22.
Click to expand...
 
FranklinMade said:
Can some help me with a simple calculation please. If you are sizing a motor load and let’s say you figure your load will draw 28 but it will be let’s say 400’ away. How would you calculate for VD and so on.
Click to expand...
233BAE90-11C9-45B6-A1CA-0B9015AD728D.jpeg
Despite all the places this thread has gone. Here are some over simplified VD calcs. Though some may not think ideal for motor loads due to starting current and power factor. You can still play around with and see different results.
 
ramsy said:
See 430.27
Click to expand...
430.27 directs you to 460.9, which says that the effect of the capacitor shall be disregarded in determining the motor circuit conductor rating in accordance with 430.22.
ramsy said:
Before voltage drop is considered, a 28A motor load must be efficiency adjusted x 1.25 per 220.18(A)
Click to expand...
Also, 220.18(A) doesn't say anything about motor efficiency
 
david luchini said:
430.27 directs you to 460.9, which says that the effect of the capacitor shall be disregarded in determining the motor circuit conductor rating in accordance with 430.22.
Click to expand...

The 1st paragraph of 460.9 allows fuse rating based on "improved power factor" of capacitors, but not conductor rating.

Here is Informative Annex D example of 460.9 requirements.

Informative Annex D Examples
Power Factor. Calculations in the following examples are based, for convenience, on the assumption that all loads have the same power factor (PF).

Not much help there, or anywhere.

If 430.27 => 460.9 demands motor-conductor rating based on power factor, how can reasonable persons do so, except perhaps by using an unenforceable fine-print note under one obscure table.

If this Code is only intended for qualified persons 90.1(A), perhaps engineering supervision may better explain that question.
 
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