Feeder Protection -WA Admin

[charlie b]

Re: Feeder Protection -WA Admin

Several members have stated that 430.62(A) and its Exception prohibit us from ?rounding up.? They do not. Look at them more carefully.
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• <font size="2" face="Verdana, Helvetica, sans-serif">430.62(A) says to start with the highest allowable rating (of the short-circuit device) for the biggest motor, per 430.52.</font>

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• <font size="2" face="Verdana, Helvetica, sans-serif">430.52 allows rounding up to the next higher size, in certain cases.</font>

<font size="2" face="Verdana, Helvetica, sans-serif">QED

But there is more:
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• <font size="2" face="Verdana, Helvetica, sans-serif">Exception 1 to 430.62(A) only comes into play when the circuit breakers are of the instantaneous type. Even so, it does not tell you to ?round down.? What it tells you is that the number you use for the ?highest allowable rating (of the short-circuit device) for the biggest motor? can be no higher than the percentage (from Table 430.52) times the full load current.</font>

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• <font size="2" face="Verdana, Helvetica, sans-serif">Note that in my solution I continued to use the value 325 amps (I did not round it down to the next lower standard value). I finished the calculation by adding in the other three motors. Then I picked the next lower standard value.</font>

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[dillon3c]

Re: Feeder Protection -WA Admin

Good eye Mr.B sir,
That would bring ampere to a total of:
398 amperes

In all due respect,taking 430.62(a)in account,would the Motor Branch-Circuit Short-Circuit and Ground-Fault Protective Device of table 430.52 be:
350 amperes?---(applying to test question)

[ March 24, 2005, 01:34 PM: Message edited by: dillon3c ]

 

[snapco]

Re: Feeder Protection -WA Admin

Great input and point of view. Personnaly the way I read it I have to agree with Charlie B, I obvously can not tell you which way is right or wrong.

I think the answer has to be 'C', but more important I guess is what the State of Washington thinks is the correct calc to use- and good luck finding that information as they do not allow you to question them-

 

[wirenut1980]

Re: Feeder Protection -WA Admin

Thank you all for the corrections. I am definately learning today!

dillon3c, I think I just added the two 10 hp motor FLC amps together and so I added 42 and 56 (28*2) Amps to my OCPD for the 50 hp motor.


Charlie B,

That is not true for the OCPD of a motor circuit. If I can select a circuit breaker for more than double the full load amps of a motor, then that circuit breaker is not going to protect the conductors from an overload. Protect from a fault? Yes. Overload? No. Rather, the overload protection (i.e., for the conductors) comes from the motor?s control circuit (e.g., from thermal overloads).

Absolutely correct, I should have specified short circuit protection only.

I disagree. You get the value of a motor?s ?full load current? from Tables 430.147 through 430.150. Table 430.22(E) does not alter that value. What Article 430.22(B) does is to say that if you are sizing a conductor, you select it with an ampacity no smaller than the percentage (of ?full load current?) as shown in Table 430.22(E). I say again, this does not change the value of the ?full load current.?

When you work with Table 430.52, you start with the ?full load current.? Nothing in Table 430.52 refers you back to Table 430.22(E), nor to any other place in the code. I reiterate my view that the 85% factor (for the 5 minute intermittent motor) does not come into the present calculation (of the feeder OCPD).

I guess I am still fuzzy on this as I thought that the OCPD was chosen based on the size of the conductors so it could protect them (in this case for motors from only short circuit protection ). I suppose in this calculation, we are only interested in the full load current for feeder OCPD. I think I got it now.

 

[dillon3c]

Re: Feeder Protection -WA Admin

Snapco,
Brother the Main thing is to beat the test.Mike Holt teaches that in videos that I have of him. If need be, pick the closest answer but understand the path in getting there.Once you get you test behind you, theres always time for contined education..Maybe the path of being another respected Mr.B,if you so choose sir..

dillon3c