Rule of thumb for motors

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Buck Parrish

Senior Member
Location
NC & IN
Is their a rule of thumb for single phase motors? Some thing like. 1.25 amps per horse power.
Im dealing with chopper pumps.
But curious about all types
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
Single phase is usually 120V or 240V. The increase in amp is not a ratio. For instance a 1hp motor at 230V= 6.9 amps, 2 HP = 12, 3 hp =17, 5 HP = 28

Get an Ugly's book. It appears once you know the amps at 1 hp you could almost add 5 amps to each extra HP and get the results. At least this is true for single phase- 230V
 

sgunsel

Senior Member
No. The efficiency of single phase motors varies greatly by HP. It would be easier to pick a manufacturer and look at the FLA for 0.5, 1.0 and 2.0 HP motors at 120 VAC. Current will be half at 240. There is less variation between manufacturers for a given size motor than there is between a 0.5 and a 2 HP motor.
 

renosteinke

Senior Member
Location
NE Arkansas
Forget the math formulas and talk about 'efficiencies.' There's no substitute for the nameplate information.

This is because, not to put too fine a point on it, motor makers, pump makers, compressor makers all LIE. It's that simple. As an example, I have just received a quote for a 15hp motor/gearbox combination where the provider has re-labled a 20hp. motor as 15.

When a motor is relabled, the FLA and the (maybe) frame sizes are your only clues.

Sinning in the other direction, do you really believe that your hand-held circular saw has a 5hp. motor? It's amazing how the term 'service factor' can be stretched to cover any advertising claim.
 

Buck Parrish

Senior Member
Location
NC & IN
Thanks, Ive got an ABS single phase Resi. SFD septic pump. So would that be light duty, intermitent light start
230V
Nema code H
3400 RPM.
FLA 29.3
P2: 6.7 HP
60 hz
Amb Temp. 40 c

I would like to use conductor size #12 with ocp 35 . Am I in the ball park?
 

raider1

Senior Member
Staff member
Location
Logan, Utah
Thanks, Ive got an ABS single phase Resi. SFD septic pump. So would that be light duty, intermitent light start
230V
Nema code H
3400 RPM.
FLA 29.3
P2: 6.7 HP
60 hz
Amb Temp. 40 c

I would like to use conductor size #12 with ocp 35 . Am I in the ball park?

A #12 copper conductor is not going to work as at 75 degrees C it only has an ampacity of 25 amps.

Also what type of OCPD are you using a fuse or circuit breaker?

Chris
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
Thanks, Ive got an ABS single phase Resi. SFD septic pump. So would that be light duty, intermitent light start
230V
Nema code H
3400 RPM.
FLA 29.3
P2: 6.7 HP
60 hz
Amb Temp. 40 c

I would like to use conductor size #12 with ocp 35 . Am I in the ball park?
How do you think a #12 wire would work here. #12 is only good for 25 amps at 75C that means you are not using nm cable. Sorry I should have refreshed my browser before posting-- Chris got it.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
That is one huge sewer pump. Is that the one that is used at the Governor's Club because if so then #10 is what they called for. I believe it was 10/3 with ground if I remember correctly.
 

Buck Parrish

Senior Member
Location
NC & IN
That is one huge sewer pump. Is that the one that is used at the Governor's Club because if so then #10 is what they called for. I believe it was 10/3 with ground if I remember correctly.


Yes, some thing to do with being at the end of the block .
I have done a lot of homes out thier with much smaller pumps. I used 14-3 on some.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
746 watts = 1 HP

By itself, this all but a useless formula. It deals with the conversion of output power from one system to another. You should primarily only use it to compare a European KW rated motor to a North American HP rated one.

Motors are always rated in output power, but to size the feeder circuits we need to know the input power. The difference is in the Power Factor (PF) and the Efficiency (Eff) of the motor. there are no good rules of thumb to estimate these values, especially for motors less than 25HP. This is one reason the NEC contains required tables of values.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
I suppose you use the nameplate if the tables don't have a corresponding nameplate. I have never seen a 6.7 HP motor as Buck has.
 

renosteinke

Senior Member
Location
NE Arkansas
I thinka little refresheris needed here ... and I apologize for not doingit in my first post!

The NEC tables arewhat you use to size your wire. I don't have a codebook handy, but I'm pretty sure that the single-phase table has a 7-1/2hp entry. The table then gives you the amps you use to size the wire.

The nameplate amps are what you use to size your overcurrent protection - your fuse or breaker. Since this is a motor load, you will most likely size them quite a bit larger than your FLA. How much larger depends on the exact type of OCPD.

If the pump is really that big, you won't be controlling it directly; there will be some manner of starter. The various float switches just aren't made for the starting current (appx. 150 amps) that the thing will draw.

With a starter, you have 'heaters,' or overload devices. These you size / set just a bit more than the maximum running current that you actually measure. The most important factor influencing the motor load is the pump 'head,' or how high the pump must pump the water. This amount will not change.

A final note: it is essential to set your floats so that the pump is always submerged (assuming a submersible pump is used). The waterisneeded to cool the motor. You also want to set the floats for as large a gap between 'pump off' and 'pump on' as posible; you don't want the pump short-cycling. If there are two pumps, choose pumps large enough that any one can handle 'normal' loads on its' own.

I've never seen a nameplate that gave the 'horsepower' in any but the 'usual' values. Examples would be 5hp, 7-1/2hp, 10hp. I've never seen, for example, a 6hp motor nameplate - let alone a '6.7hp' tag. In any event, a sewage pump that size would be too large to place by hand, and have at least a 3" discharge pipe. Check your facts.
 
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