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#191643 - 01/05/10 09:38 PM Motor Calcs
watersparkfalls Offline

Registered: 03/08/05
Posts: 210
Loc: Washington...Not DC
1/2 HP single phase 240 volt(4.9A)TBL 430.248.

430.52(B) Capable of carrying start current. How do I determine start current?

A little confused about TBL 430.52 800% for maximum short circuit. I am using a standard cutler hammer breaker.
4.9*800%=49 Amps maximum short circuit? If for some reason I wanted to put 1/2 HP on a 50 amp breaker? Conductors would have to be sized according to 310.16

Confused what septic motor rating would be (430.22 5 min, 15 min?) and what classification would be (short time, intermittent?), septic motors pump some ridiculous Gal. per Min. and guessing it pumps for 30 seconds.

Appreciate help.

2014 / 2011 NEC & Related Books and Study Guides
#191644 - 01/05/10 10:05 PM Re: Motor Calcs [Re: watersparkfalls]
gfretwell Offline


Registered: 07/20/04
Posts: 9045
Loc: Estero,Fl,usa
That "standard" breaker is going to be "inverse time" I bet. Does it say HACR on it?

At any rate, we usually assume the 250% number applies without any other guidance. That gets you 12.25a. A regular 15a breaker and 14ga wire should be more than you need.
Greg Fretwell

#191647 - 01/05/10 10:15 PM Re: Motor Calcs [Re: gfretwell]
gfretwell Offline


Registered: 07/20/04
Posts: 9045
Loc: Estero,Fl,usa
As a side note, the conductors only have to be sized to 125% of FLA using 310.16 if the motor has supplemental overload protection.
The classic example that you see on tests is a 1 hp 115v motor with a 16a FLA.
125% of 16 is 20a and 310,16 says you can use 14 ga copper for that.
You can size the breaker at 250% of 16 or 40a. so you have a 40a breaker protecting 14 ga wire.
It might not seem right but any other answer on the test will be scored as wrong.
Greg Fretwell

#191657 - 01/06/10 08:35 AM Re: Motor Calcs [Re: gfretwell]
testguy Offline
New Member

Registered: 06/14/07
Posts: 7
Loc: IN, USA
The 40 amp breaker is for branch circuit protection from short circuit and ground fault (fast current rise / short duration event). The motor overload device is for slow current rise / longer duration protection.

#191658 - 01/06/10 10:23 AM Re: Motor Calcs [Re: testguy]
renosteinke Offline
Cat Servant

Registered: 01/22/05
Posts: 5305
Loc: Blue Collar Country
Just a few general comments on the application ...

"Septic pumps" aren't that severe of an application. Let's look at their operation.

A motor has the greatest current draw at start-up. This factor is worst where the motor starts off against a full load, as would happen with a compressor. An ordinary fan, by comparison, starts off with very little load. A septic pump usually has little load as well; it just has to pump against the "head" (which is usually well under FLA), and there's usually a 'weep hole' near the pump, and before the check valve.

The pump operates for short periods of time, so it's not continuous duty. You MIGHT have other issues if the pump is 'short cycling,' or operating more than six times an hour.

Your enemy is heat - whether from the start-up load or short cycling. With a sewage pump, this is mitigated by the cooling effect of the sewage. I am assuming these are submerged pumps - as discussed in another post. Not that it's pretty important for the pumps to shut off while still submerged; you do not want to pump that pit dry every time the pumps cycle.

At any rate, we over-size the breaker to allow for that moment of high start-up current. As others have mentioned, the "heaters" in the starter are there to shut things down if the motors are continually drawing excessive current.

You need to MEASURE the actual current drawn by the pumps. One of the best indications of what is happening in the pit is the current drawn by the pump. When the system is new and clean, measure current, write it on a piece of tape, stick it in the panel. This gives you a reference for your next visit.

One cause of high current draw is simple the accumulation of crud in the discharge line. Over time, that 2" line will be reduced to 1/2". Discharge lines need periodic cleaning. I've been down this road with reluctant customers and sceptical plumbers; in one instance the problem festered for three years, and several pump replacements, before the necessary clean-outs were added. In that case, cleaning the pipe cut the amps drawn to 1/3 of the previous level.

Edited by renosteinke (01/06/10 10:31 AM)

#191697 - 01/07/10 09:48 PM Re: Motor Calcs [Re: renosteinke]
watersparkfalls Offline

Registered: 03/08/05
Posts: 210
Loc: Washington...Not DC
Thanks guy's, been helpful as usual!!


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