I wouldnt recommend making your own cord to anyone unless they have some experience. Poor connections are a huge factor in fires because that is where all the heat will be generated. 12AWG wire will handle the draw of a GG polisher no problem, the issue is the voltage loss over X distance. Less voltage loss with bigger gauge wire and it will handle more amperage. Money well spent IMO to get the best you can afford for whatever length you need.
Im gunna go full geek here for you...
Vd=Voltage Drop
I= Amps (current)
L= One-way length of circuit (feet)
Cm= Area of conductor (Circular mils)
K= Resistance in ohms of 1Cm (circular mil again) foot of conductor
Now K value for copper we will use 12.9 @ 167 degrees F ( im talking about temperature of the conductor here not the ambient and 12.9 is a standard for this calculation)
the voltage drop formula looks like this
Vd= (2K * L * I) / Cm
So...we will use 7.08 amp draw (Gets us to the 850W quoted power), 50ft and 12AWG wire (Cm of 12AWG is 6530Cm)
Vd= (2 * 12.9 * 50ft * 7.08) / 6530
aaaaand we get Vd= 1.4V
Cool not to much huh lets see then...
120V * 7.08A = 850Watts (7A gives you 840watts not the 850 quoted)
Well then if i lost 1.4 volts what power do i get now?
118.6V*7.08A=839.68 watts... hmmm man I just lost like 10Watts of power!!!!
BUT WAIT! what about that 16AWG 25ft cord on the polisher???
Lets do that as well then!
Vd = (2 * 12.9 * 25ft * 7.08A) / 2580Cm
Vd = 1.77V.... uh oh
So now by the time the voltage actually gets to the motor its
120V - 1.77V - 1.4V = 116.83V
Hmmm the plot thickens!
116.83V * 7.08A = 827 watts!!! down 23 watts of power after having to go through all those cords!!!
The lesson here is you cant change the tools characteristics but you can control the Length and Gauge of the cord. Increasing the gauge lowers voltage drop and increases the power you can use. Increasing the length uneccesarily will do the opposite and decrease the power you can use.
If anyone is curious I referenced my little Ugly's electrical manual 2011 edition which is based on the NEC. If I have made any errors feel free to laugh a little at my expense!
Feel free to do this calc with the other mentioned Gauge wire as well here are the Cm for each
16AWG = 2580Cm
14AWG = 4110Cm
12AWG = 6530Cm
K for copper is a standard 12.9 as mentioned
K for aluminum is 21.2
The 2 multiplier is a constant for the formula as well for single phase calculations.
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