Single Motor Branch Circuit question.....

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threephase

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I've been studying to take my exam for the last 6 weeks or so and I have a question that's been bothering me.

It's my understanding that you size a motors thermal overloads based on nameplate amperage if provided. If not, you use the FLC tables. Then based on that figure, you can correctly size the heaters per 430.32.

Based on the FLC values, you add 125% of that value to get the ampacity needed for your branch circuit wiring. Then depending on your overcurrent protection choice, you multiply the FLC value by either 175%, 250% to obtain your overcurrent protection for short circuit, ground-fault protection. (T430.52)

Example:

Branch circuit feeding a single phase, 2hp, 115V motor.
FLC current = 24 amps.
Heater size = 24 * 1.15% = 27.6 min 24 * 130% max = 31.2
OCPD = 24 * 250% = 60amp (inverse time breaker)which happens to be a standard size.

Here's my question: From the OCPD, you are allowed to run size 10awg THHN which is good for 30amps but what happens in the event that the heaters short out or become faulty? Then in theory the motor can go into Locked Rotor Current and the next device set to trip would be the breaker set at 60amps.

How is it that you can have size 10awg wire leaving a 60amp breaker? Am I correct that the only way that motor will pull more than 24amps is if it goes into Locked Rotor condition? At which time, the current will far exceed 60amps in turn tripping the breaker and protecting the conductors??

Thanks.
 
Your numbers appear right with out crunching them myself. I will address the fear you project regarding a failure of the ol device

The fundamental philosophy of motor BC protection leans towards the concept that the OL device protects the entire circuit (in general) against currents in exces of normal operating conditions. motors are generally considered a continuious load thus the OL device and BC conductors are increased (ampacity) by 25%.

Catastrophic failure is guarded by the brach circuit short circuit and ground fault portection 430.52.

Do not assume that a designed componet will fail. You may render yourself incapable of a basic installation.

You must believe that the installation will function as designed.

If you reduce the size of the BCSC and GF protection because of a practice of protecting all conductors at there listed ampacity the motor may not start. Actually the motor BC conductors are portected by the OL device.


If you increase the OL the circuit will not be protected. The Code is a minimum standard,

It generally works good and lasts a long time.
 
From the OCPD, you are allowed to run size 10awg THHN which is good for 30amps but what happens in the event that the heaters short out or become faulty?
Click to expand...


Just for the record the #10 THHN conductors at 75? C are rated for 35 amps. The restrictions of 240.4(D) do not apply to motor circuits.
 
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I would add to Charlie's post:

threephase said:
what happens in the event that the heaters short out or become faulty?
Click to expand...
I'm not 100% sure, but I think heaters tend to fail open, don't they?

I do agree with the sentiment, motors definutely challenge rules that we take for granted, such as 240.4(D). :cool:
 
When the motor is first started its current draw can be 500 times the running current this is why you are allowed to over-size the over-current protection. To allow it to start with out tripping your over-current device.
 
Excellent responses guys.

Thanks for the info. regarding the limitations of 240.4 (D)

Good point regarding the fact that I should assume the equipment will function as designed.

In my experience, the heaters usually burn open or fail to be reset as opposed to shorting out. I was using t hat more for an example that it would be possible if that indeed occured for the 10awg wire to experience current in excess of 35amps (conductor ampacity).

If certain conditions occurred, the current could fall in the 35 - 60amp range which could possibly damage the conductors.

I was just more a less wanting to make sure I was thinking correctly. Before getting into this kind of stuff, my mind has always been programmed that #12 should have a 20amp breaker, #10 a 30amp breaker.

Many of the guys I work with (because of ignorance) would CRAP if they say #10 wire leaving a 60 amp breaker !!! Simple because they don't know any better.

Again, thank you !!
 
infinity said:
Just for the record the #10 THHN conductors at 75? C are rated for 35 amps. The restrictions of 240.4(D) do not apply to motor circuits.
Click to expand...

Trevor,
Little more info please. Is 240.4 (g) telling you that you can do this.I looked at the table and never got a specific on where this rule is stated. I have did it this way before in panels before but cant back it up with a specific code ref.
Thanks,Travis
 
Bea said:
When the motor is first started its current draw can be 500 times the running current this is why you are allowed to over-size the over-current protection. To allow it to start with out tripping your over-current device.
Click to expand...
Bea
I don't mean to nip pick you response but I think you meant to say 5 times the running current and not 500 times.
 
"Before getting into this kind of stuff, my mind has always been programmed that #12 should have a 20amp breaker, #10 a 30amp breaker."
I'll walk you thru it.
1.
240.4(D) Small Conductors. Unless specifically permitted in 240.4(E) through (G), the overcurrent protection shall not exceed 15 amperes for 14 AWG, 20 amperes for 12 AWG, and 30 amperes for 10 AWG copper; or 15 amperes for 12 AWG and 25 amperes for 10 AWG aluminum and copper-clad aluminum after any correction factors for ambient temperature and number of conductors have been applied.

2. 240.4(E) is tap conductors, not important for this
3. 240.4(F) is transformer secondary conductors, "
4. 240.4(G) is overcurrrent protection for specific applications, references the table
In the table find
Motor and motor-control circuit conductors
430, Parts III, IV, V, VI, VII

So in those parts, the basic rule of 240.4(D) does not apply

Part III and IV have to do with sizing the overloads and branch circuit short circuit protection (IE your 60 amp circuit breaker)

A couple of points.
this a good example of how to follow code rule. We often only read some of a rule. 99% of all electricians will state the ampacity of a 12 THHN is 20 amps.
240.4(D) used to be part of table 310.16 as the obilisk note and was moved.

We don't read or use the parts of a article. Part 1 rules apply to the entire article. rules in part only apply to that part.
 
travis301 said:
Trevor,
Little more info please. Is 240.4 (g) telling you that you can do this.I looked at the table and never got a specific on where this rule is stated.
Click to expand...

Well, the small conductor rule undoes itself for the items mentioned in the table.

Since (D) says that (D) applies except for the applications mentioned in the following subsections, then (D) is removing itself from those applications completely.

(D) is the rule that leads us to count as we do; #14 for 15A, #12 for 20A, and #10 for 30A. Otherwise, 310.15 governs how we determine ampacity. If (D) says that it does not apply to motors, we don't need to find a rule in 430 that confirms that - 240.4(D) already said so.

Does that make sense? :)
 
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