motor calculation/wire size 310-16

[dmangalo]

here is the question: what size wire copper thw is necessary for a 5 HP, single phase 230 volt motor?

here is my calculation:

motor = table 430-148 = 28 amps for 5 HP motor
28 * 1.25 = 35 amps

wire = Table 310-16 = #10 is listed @ 35 amps but has an astric which I beleive is for calculating derate fill of conduit not wire size
therefore I would have to use #8 which is rated @ 50 amps.

My question is: "Is this correct?"

Also whenever the FLA is given then the table in art 430 is disreguarded and the actual FLA is used to determine the wire size.
True or false?

[Wgoodrich]

Read 430.6.1 and 2. That should provide your answer when to use the charts or name plate. Basically the only time you use the amp rating of the name plate is when sizing the overload protection per 430.6 and 430.32.

You are correct using table 430.148 for normal single phase motors to find the full load current rate of that motor then referring to 430.22 telling you to size your conductor ampacity by increasing the full load current found in table 430.148 by 125%. Then referring to TAble 310.16.

When you get to 310.16 don't forget to consider 110.14.C telling you if your conductor size hits 1 awg or smaller to use the 60 degree column regardless what your temp rating of your wire insulation or if larger than 1 awg use the 75 degree column never using the 90 degree column unless you are calculating ampacity deration then again once your calculation is done for ambient temp Table 310.16 at bottom of chart or more then 3 current carrying conductors in a raceway in Table 310.15.B.2.A.

Now you were right up to this point what you missed was the meaning of the asterik. The asterik states that UNLESS OTHERWISE MENTIONED IN THE NEC. This OTHERWISE MENTIONED IN THE NEC is referring to Chapter 4 of the NEC for motors, generators, and transformers. When using Chapter for such as this project concerning motors then you would use the 35 amps for 10 awg and ignoring the asterik altogether if you are designing your wiring for products controlled by rules in Chapter 4 for motors, transformers, or generators. However you do not ignore the derations required in table 310.15.B.2.A or ambient temp at bottom of chart in table 310.16. These derations still apply only you use the value in the chart to start your derations ignoring the asterik. Again you have the rule in Article 110.14.C concering the 60 degree column use even with motors. That rule still appies.

Hope this helps

Wg

[Anonymous]

Once again you have proven to be very helpful.
This is a awesome website wich I am glad I found.
dmangalo

[Wgoodrich]

Pleased to see you have seen benifit from what we have to offer. Hope one and all learn at least a little something. That is our goal.

Good Luck

Wg