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#54041 - 07/14/05 03:16 PM Motors calcs for intermittant use?  
walrus  Offline
Member
Joined: Jul 2002
Posts: 680
Bangor Me. USA
I have a location that has a 5hp 230 volt single phase motor. FLC is 28 amps. Used for pumping propane. The operator tells me the pump runs for a maximum 20 to 40 minutes at a time. Presently its wired thru a size 0 motor starter [Linked Image], no 12 wire and a 20 amp 2 pole breaker. Tell me how this works(and it has for 10 years) when the motor running draws 32 amps(from my clamp on).
If continous it should have size 2 starter, # 8 wire and as high as a 70 amp breaker,. Is this intermitant duty as it migh run for 40 minutes and then not again for hours? Can I use 430.22 duty cycle service?? and am I reading 430.33 right if intermittant I can use the breaker as my overload protector?? How do I size wire and breaker? per 430.22???

Realistically if the customer approves the money I'm going to continous duty calcs but otherwise how do I go about sizing for intermittant.

I have pictures of one shoddy install I'll post later.

Thanks Jon


Work Gear for Electricians and the Trades

#54042 - 07/14/05 06:02 PM Re: Motors calcs for intermittant use?  
Ray97502  Offline
Member
Joined: Oct 2004
Posts: 55
Central Point,OR
From your description, Art.430.33 last paragraph would indicate that the motor is to be considered as continuous.
“Any motor application shall be considered to be for continuous duty unless the nature of the apparatus it drives is such that the motor cannot operate continuously with load under any condition of use”
Is it the operator’s option not to use the motor for more than those few minutes or is there an operational limit that prevents continuous use? If it’s a judgment call on the part of the operation, not all operators use the same standards of judgment.


#54043 - 07/14/05 06:42 PM Re: Motors calcs for intermittant use?  
Alan Belson  Offline
Member
Joined: Mar 2005
Posts: 1,803
Mayenne N. France
Also, the running amperes and volts you quoted add up to way more than 5hp.

5hp x 0.746 = 3.73kw @ 230v = c. 16A ?
Rough and ready calc; 4A/hp = 20A

Alan


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#54044 - 07/14/05 08:11 PM Re: Motors calcs for intermittant use?  
renosteinke  Offline
Cat Servant
Member
Joined: Jan 2005
Posts: 5,316
Blue Collar Country
Since you have a starter present, the breaker is only used for "overcurrent" protection.
"Overload" protection is provided by the "heaters" in the starter.

Now, let's look at your information...like Alan, I have some problems with your numbers.
The largest "heater" for a size ) starter will handle 18 amps- quite a bit less than what you say you have. A size 1 goes to 27 amps, and a size 2 up to 46 amps.
Since you say you measured 32 amps, and the nameplate is marked 28 amps, you need a size 2 starter.

Now, let's look at table 430.248 of the NEC. According to this table, a 5 Hp 230 v single phase motor has a full load current of 28 amps. This is the value used for selecting your wire; since the application precludes continuous operation, I am comfortable using 28 as the basis for selecting #10 as the wire- IF this is a 5 hp motor!!!
The "problem" is that table 430.248 typically shows a FLA much higher than what is on the motor nameplate....there should be no way a 5 hp motor would draw 32 amps. If you are drawing 32 amps, then you need a 7 1/2 hp motor, which in turn leads us to #8 as the wire size.

Now for sizing the circuit breaker and disconnect.
The disconnect must be rated to 115% of the 40 amps from the table, or 47 amps. The next size disconnect is rated 60 amps, so tha't what you need.
The circuit breaker can be as much as 250% of the 32 you measure...this comes to 80 amps, which is considered a standard breaker size.

(I'm sure you already knew most of this- I'm just being thorough for the others out there)

Table 430.33 allows you to size your wire based upon the nameplate FLA, rather than the NEC table, and sometimes might allow the use of a smaller wire. In your case, 28 x 1.5 = 42, so you'ld have to use #8 anyway.

In theory, you could use the breaker as your sole disconnect and overload protector- but that would make it "continuous," as circuit breakers have no contacts that allow the controls to operate.

If the pump is controlled by, say, a float....then you can consider it as intermittent. If someone is actually flipping a switch, then it's continuous. That's how I understand the issue here.

A note about horsepower ratings: these things are best ignored, as manufacturers have been lying about them for decades. The FLA on the nameplate is the "real" number. If you are exceeding that number, there is a problem somewhere, and the motor is being damaged. (Take the reading after the motor is up to speed, and not during the start-up period).

Selecting the appropriate heater will allow you to best protect the pump against over-heating.
That a size 0 starter is being used suggests to me that the heaters have been bypassed- or that the amp readings we have are way off.


#54045 - 07/14/05 09:17 PM Re: Motors calcs for intermittant use?  
walrus  Offline
Member
Joined: Jul 2002
Posts: 680
Bangor Me. USA
Its definetely a size 0 Sq. D starter. The heaters installed do not correspond with any of the heaters on the chart inside the cover. The chart goes to 12 amps??? The heaters installed right now are labeled B28.
Its possible I read the HP wrong, I'll check it again.The motor nameplate definetly said 28 amps. The amp draw I read with clampon was with motor running. I'm wondering if wire size(currently#12 and approx 65 ft. in length)) has anything to do with higher amp reading than expected?

Wasn't sure as how one figured intermittant or continous, thanks for the explanation.


#54046 - 07/15/05 02:30 AM Re: Motors calcs for intermittant use?  
Ray97502  Offline
Member
Joined: Oct 2004
Posts: 55
Central Point,OR
Great job on the calcs renosteinke.


#54047 - 07/15/05 02:58 AM Re: Motors calcs for intermittant use?  
mxslick  Offline
Member
Joined: Oct 2004
Posts: 803
Atomic City, ID USA
Quote
The motor nameplate definetly said 28 amps. The amp draw I read with clampon was with motor running. I'm wondering if wire size(currently#12 and approx 65 ft. in length)) has anything to do with higher amp reading than expected?


Anybody else see the red flags here? [Linked Image]

#12 wire, 65ft long
28 amps FLA
32 amps actual read

Measure the voltage at the motor location with it running, I think you'll see the answer. [Linked Image]

Wire size too small for even a short total length, most likely excessive voltage drop, resulting in higher that rated current draw.

I'd install the #2 starter, #8 wire and 80 amp breaker as reno has calulated, and do it soon before the customer has to replace the motor when it burns up. Not a good thing around propane!!

Where's Hank Hill when you need him? [Linked Image]

Edit: Noticed the reference to the 2p 20 amp breaker. Two guesses here on why it doesn't trip:

Duty cycle of motor too short to cause overload trip or:

It's FPE or Zinsco! [Linked Image]

[This message has been edited by mxslick (edited 07-15-2005).]


Stupid should be painful.

#54048 - 07/15/05 06:13 AM Re: Motors calcs for intermittant use?  
walrus  Offline
Member
Joined: Jul 2002
Posts: 680
Bangor Me. USA
Quote
Not a good thing around propane!!

As bad as the sealtight thats feeding the motor? [Linked Image]

Quote


Edit: Noticed the reference to the 2p 20 amp breaker. Two guesses here on why it doesn't trip:

Duty cycle of motor too short to cause overload trip or:

It's FPE or Zinsco!


Westinghouse



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