Newbie motor calculation question

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rkrieger

rkrieger

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I'm a retired (tired) vocational instructor who changed employment venues and who is now probably one of the oldest licensed electrical apprentice's in Michigan. Can someone lay out the proper steps and NEC references for calculating OCP breaker and circuit conductors for most fractional horsepower motors used in small power tools/compressors/etc. Most motors I see are "internally and thermally protected", so I wouldn't need a starter or in some cases a magnetic switch. Please give me a step by step procedure using motor data plate info and NEC reference tables to accomplish this.

Thanking those who post in advance, I remain, thankful and looking forward to learning.
 
step by step ? do we start by taking the coffee cup off the code book?
:smile:
my references are 2002

First I guess you have to decide continuous or non-continuous..or intermittent
Continuous would be 430.32, Intermittent would be 430.33
Continuous (430.32) would then be (B) for one HP or less, automatically started or; to (D) non-automatically started
In the automatically started catagory you basically have some type overload means required, either internal, or seperate (unless it's low impedance).
If the motor is nonautomatically started (430.33D)and in sight of the controller then the brach circuit short circuit device shall be the overload device.
Non-automaticvally started and NOT within sight of the controller takes you back to 430.32I(B)

That's abbreviated...there are other condistiosn to consider which are also listed in 430.32 and 430.33
 
Thanks for getting me started. One of the "internal" portions of my question was I am getting told that you should use the data plate info for HP, then use chart 430.248 with that HP info. to determine maximum OCPD and conductors. You should not use the FLA rating for this calculation. Also, does chart 430.248 factor in the increased percentage calculation for "inrush" current during startup??

Sorry, for my newness.......Just wanting to do it right. Most motors I deal with are under 10HP, most are internally protected, start with on/off buttons, operate non-continous, and may or may not be within sight of OCPD.

Thanks again.

PS- My coffee cup is usually in my hand !!!
 
I think large motor circuits are easier to design and fractional require more code to qualify.

Fractional can be on general-purpose branch circuits 430-42(A) if their TP (430-32(B)(2)) and not larger than 1/4hp @ 120V & 1/2hp @ 230V w/15A> protection (430-53(A)&(1)).

If your motor are larger than the listed above (430-53(A)) then 430-42(B) can be used meaning they can be TP & the OCPD ?will not open under the most severe normal conditions of service that might be encountered? 430-53(B)

In all cases the only item sized from your motor nameplate ampere is the overload at 125% everything else is sized from the tables 430-247, 430-248- 430-249, & 430-250 with their notes.
 
rkrieger said:
Most motors I deal with are under 10HP, most are internally protected, start with on/off buttons, operate non-continous, and may or may not be within sight of OCPD.
Click to expand...

Most often motors over 1.5hp do not have TP and are considered continuous (430-33), with this your circuit will include:
Overloads sized from the nameplate FLA x 125% (430-32(A)(1))
Branch-circuit conductors sized from Tables 430-247, 430-248, 430-249, & 430-250 with table notes (430-6 & (1))
Local disconnect sized from Tables 430-247, 430-248, 430-249, & 430-250 with table notes x 115% (430-110(A))
Breaker/Fuses, i.e. OCPD/branch-circuit short-circuit and ground-fault protection, sized from Tables 430-247, 430-248, 430-249, & 430-250 with table notes multiplied by 175% for fuses or 250% for breakers in Table 430-52
 
I'm sure to get blasted for this, but here goes anyway.

In 18 years of doing all, and I mean ALL types of electrical work, I've learned that I can spend a huge amount of time studying the finer points of the code, or I can adopt certain 'rules of thumb'. I'm not discounting time in the codebook, it's generally well spent. I'm just stating that time can be saved out in the field using short-cuts that comply with the code.

Here's how I size breakers that feed motors:
3 phase 460V...HP X2= breaker size.
3 phase 208 or 230V...HP X4= breaker size.
1 phase 230V...HP X10= breaker size.
1 phase 115V...HP x20=breaker size.

For fuses, use the above, and reduce to the next lower standard size.

For wire size, multiply the full-load current by 1.25, and use the 310.16 table, 60C column. Interpolate down. (I know you're supposed to use the tables in 430, but the difference is small enough to not cause a problem.)

I've installed hundreds of motors like this over the past 18 years, and have yet to have a problem.
 
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micromind said:
I've installed hundreds of motors like this over the past 18 years, and have yet to have a problem.
Click to expand...

Not having a problem and actually being code compliant are not always the same thing.

I have no doubt your way works but does it meet the code?
 
I haven't tested every possible example, (I'm sure someone around here will though..lol), but the ones I did compare to code were indeed compliant.

Just my opinion, but if someone has been doing something a certain way for many years, without problems, and it's later found to be out of compliance, the accuracy and application of the code needs a hard look. True enough, the code is supposed to be (and usually is) based on the sum of the experience of the entire electrical trade from day one, there are a few instances where bureaucracy has obviously prevailed. This seems to be getting exponentially worse as time goes on.
 
another question related to........

another question related to........

OK, I think I am getting some basic ideas, but here is a real life situation. Here is the motor data plate info. Would someone walk me through calculating the size of the OCP breaker and the branch circuit conductor size for this motor to make sure I have it right???

MARATHON: HP - 7 1/2, Hz - 60, VOLT - 240, FLA - 28.6, SERV.FACTOR - 1.15, CODE - F, DUTY - CONTINUOUS, PH - 1, TYPE - BDR, AMB - 40 C. DES - L, THERMALLY PROTECTED
 
You will need your code book, do you have one?

See Annex D example #D8 in the back for referance

Then go to article 430 for your scenario:

Motor FLA = 430-248 40A
Breaker size = 430-248 x 430-52 inverse time (250%) 100A
Overload = 28.6 x 125% and is met by 430-32(A) & (2) 36A
Branch circuit conductors = 430-248 x 125% (430-22(A) & 430-23(A)), then go to 310-16 & use the appropriate column 50A & #8?s cu @ 75?
Disconnect = 430-248 x 115%, may/may not be required 60A
 
micromind said:
Just my opinion, but if someone has been doing something a certain way for many years, without problems, and it's later found to be out of compliance, the accuracy and application of the code needs a hard look...there are a few instances where bureaucracy has obviously prevailed. This seems to be getting exponentially worse as time goes on.
Click to expand...

What do you mean?
 
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