vd 5 hp pump is this right?

[-marty]

I need to figure wire size for a 5 hp well pump that is 500' from the source of power.

Is this right?

cm= 2*K*I*D/volts dropped

K = 21.2 alum
I = 28 per T430.148
D = 500'
volt dropped = 240 * .003 = 7.2

cm= 2 * 21.2 * 28 * 500 / 7.2

cm = 82444
2 awg = 66360
1/0 = 105600

2 awg and 1/0 are the common sizes here for 3 wire direct burial.

I should choose 1/0 right?

The pump salesman told my customer he could use 12 awg or 10 awg (copper).

 

[physis]

Re: vd 5 hp pump is this right?

2AWG Al. has a voltage drop of 3.7 percent at 28 amps. I think most guys use 5 percent as a voltage drop ceiling. I think 2 should be ok. Your math looks good to me, exept you made a typo on the voltage drop percentage .03 you put .003.

Most of the other guys have more experience with your application than I do, see what they say.

 

[bennie]

Re: vd 5 hp pump is this right?

What is the nameplate voltage of the pump?

 

[bob]

Re: vd 5 hp pump is this right?

I do not caculate VD the way you do but here are the results:

1. #2 AL VD = 8.3v 3.5%
2. 1/0 AL VD = 5.4v 2.3%
3. #2 CU VD = 5.2v 2.2%
I think I would use 1/0 if you are going with AL.
You need to verify that you have 240 volts at the
start or the voltage level will be lower.
Tell you customer to inform the pump salesman he
need to stick to selling. You couldn't use #12
or #10 if the distance was 1 ft. Neither is large
enough.

 

[-marty]

Re: vd 5 hp pump is this right?

Thanks for the replies. Now my customer has more faith in me.

Bennie, I'm sure on Mike Holt's Motor Calculations video he said you only go by the nameplate for overloads but not for calcs. But, my memory is not the greatest.

 

[bennie]

Re: vd 5 hp pump is this right?

This is a different situation but questions using nominal voltage instead of NEMA rating for motors.
A motor can function on a short time basis with a 10% above and below tolerance. The low number is more destructive than the higher amount.

The 10% is based on the nameplate rating which is normally 230 volts. 10% of 230, is 23, which leaves 207 volts. This eliminates a 230 volt motor from being operated on 208 volts.

Using 240 volts will incorrectly imply the motor will operate on 208 volts.

 

[physis]

Re: vd 5 hp pump is this right?

Bennie,

Is 10 percent really pushing the envelope on a motor? Not that 10 percent isn't a little sloppy but I've seen voltage change almost 5 percent at one outlet within a few minutes in commercial buildings. That doesn't leave much.

Sam

 

[jtb]

Re: vd 5 hp pump is this right?

Physis,

Hi. The motor is going to draw the power it needs, or attempt to do so no matter what the voltage. P=V*I, so decreasing the V increases the I draw and heats the motor up.
Increasing the V above rating can shorten the life of the motor also, but due to different reasons. Probably not the most concise description, but hope it helps.

 

[tom baker]

Re: vd 5 hp pump is this right?

Marty, 10 or 12 may be ok for the drop cable, but not for the circuit to the well head.

NEC voltage for calculations is 240 V
Nominal voltage that the motor is rated at is 230 V.
NEMA voltage tolerance is +/- 10 %. I would go for 5% VD from controller to motor. You have to allow for the entire ciruit length including the drop cable to the submersible.

It may be less expesive to use a larger, more common conductor from the panel to the well head, and a smaller drop cable. It depends on how deep the pump is set. I have installed submersibles that the well head is 1/4 mile from the motor controller. The pump mfg will have tables on volt drop you can use.

As pointed out, low voltage is worse than high voltage, but a high voltage above 10% reduces the magnetic saturation of the motor windings and power drawn goes up.

 

[rey-man]

Re: vd 5 hp pump is this right?

check out thesite for Electrical Designer's Reference. you can download EDR from there. its the easiest way to compute for VD's. its all automatic, it will give you the wire size.