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Old 07-11-2012, 04:36 PM   #1
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electrical wire sizing


wiring a water pump to draw lake water. From box to pump is 150' and the amp draw is 12 amps on 120 volt. 6 on 240. what should wire size be for both 120 and 240? It will be a dedicated line. Undecided at this point as to which power to use. Thank you--Floyd Carlson patpacrlsn@verizon.net

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Old 07-11-2012, 04:37 PM   #2
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Go with 240v and save on copper.

BTW, there are 'bots cruising the Web looking for valid e-mail addresses.

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Old 07-11-2012, 05:19 PM   #3
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Originally Posted by Floyd Carlson View Post
wiring a water pump to draw lake water. From box to pump is 150' and the amp draw is 12 amps on 120 volt. 6 on 240. what should wire size be for both 120 and 240? It will be a dedicated line. Undecided at this point as to which power to use. Thank you--Floyd Carlson patpacrlsn@verizon.net

You could run # 14 AWG 150' at 240v and sustain a voltage drop of 2.%, but personally, i would be running #12 AWG, at 240v.... Final answer.
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Old 07-11-2012, 06:09 PM   #4
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For sure run 240VAC, halves the current and thus voltage drop in lines. 12 AWG isn't a bad idea as prior post suggests, not much extra cost in grand scheme of things.
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Old 07-12-2012, 04:57 AM   #5
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...It all depends on how hot the conductors will get exposed to the highest ambient temperature that will be experienced by the circuit, and whether the pump motor will be working for more than 3hrs (continuous use) status. Both these contingencies need to be taken into consideration before any derration factors are applied to the circuit's dynamics.
What is the highest ambient temperature, and what is the longest operating time of the pump motor? The correct wire size cannot be properly calculated without these two scalar values.
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Old 07-12-2012, 06:08 AM   #6
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...It all depends on how hot the conductors will get exposed to the highest ambient temperature that will be experienced by the circuit, and whether the pump motor will be working for more than 3hrs (continuous use) status. Both these contingencies need to be taken into consideration before any derration factors are applied to the circuit's dynamics.
What is the highest ambient temperature, and what is the longest operating time of the pump motor? The correct wire size cannot be properly calculated without these two scalar values.
WOW, even with the wire selection I gave, those two values are already included into the equation, 80% of 15 = 12 AMPs, 80% of 20 = 16AMPS, His pump at 240v is only drawing 6 amps... And I seriously hope he is direct burying a conduit or UF wire out to the pump....
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Old 07-12-2012, 10:41 AM   #7
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Quote:
Originally Posted by kontoose View Post
...It all depends on how hot the conductors will get exposed to the highest ambient temperature that will be experienced by the circuit, and whether the pump motor will be working for more than 3hrs (continuous use) status. Both these contingencies need to be taken into consideration before any derration factors are applied to the circuit's dynamics.
What is the highest ambient temperature, and what is the longest operating time of the pump motor? The correct wire size cannot be properly calculated without these two scalar values.
All that is really needed is the fla of the pump motor at 120 volts and 240 volts.The ampacity of branch circuit conductors are calculated at 125% of fla for a single motor as stated on the nameplate or NEC tables which ever is applicable..

So at 120 volt and an fla of 12 you need conductors capable of 15 amps and at 240 volts 6 amps you need conductors capable of 7.5 amps.


For copper conductors

14 awg = 20 amps
12 awg = 25 amps

Continuous duty is already factored in as Stickboy stated. As for ambient adjustments it likely won't apply.
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Old 07-12-2012, 05:35 PM   #8
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All that is really needed is the fla of the pump motor at 120 volts and 240 volts.The ampacity of branch circuit conductors are calculated at 125% of fla for a single motor as stated on the nameplate or NEC tables which ever is applicable..
Stubie, You must use the NEC FLC Tables to size a motors Branch circuit and ground fault circuit protection, you can only use the nameplate to size the OCP.... Im pretty sure you know this, but just pointing it out.

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Old 07-13-2012, 12:25 AM   #9
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Stubie, You must use the NEC FLC Tables to size a motors Branch circuit and ground fault circuit protection, you can only use the nameplate to size the OCP.... Im pretty sure you know this, but just pointing it out.

Not exactly it depends .. thats why I said if applicable. My example is based on 430.6 (A)(1) exception 3 ( 2011 NEC ) . In residential we never used the tables if our motor appliance was listed (UL)(CSA) and showed both horsepower and fla on the motor appliance nameplate. When a motor is a listed motor (MANY 3 PHASE MOTORS ARE NOT) and part of an appliance the appliance nameplate ratings are accurate (it was tested) for branch circuit sizing and ocpd. You will find this to be true with the majority of 120 volt and 240 volt single phase motor appliances. IMO the grey area has always been what defines an appliance. The NEC does have a definition. An example would be a washing machine, but it is not a good example as they are cord and plug for residential.

To this date I have never been in a class or in the field where anyone knew how in the heck they came up with the ampacity values in the NEC motor tables.

In industrial applications (usually 3 phase) it was rare for us to use anything but the NEC motor tables.

Anyway your point was well taken and I should have used an example and explanation that used the tables also to show the differences.

I came back to add that in this case I would certainly agree that the NEC tables would be applicable as I understand the original post.

Last edited by Stubbie; 07-13-2012 at 07:00 PM.
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Old 07-13-2012, 01:10 AM   #10
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They came up with the motor tables in the NEC calculated using 6.28X10 to the power of -18 electrons passing a point in one second -- i.e., one coulomb.
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Old 07-13-2012, 04:29 PM   #11
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As stated, 14-2 would work just fine at 240 volts. Even if you multiply the motor by 1.25 and derate the 14 by 20%. You still end up only using 7.5amps of 12 available. 14 will work just fine any way that you want to slice it. As others have stated, running #12 isn't much more expensive and would future proof it.

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