Code Question from Mike Holt Book

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chuckgassmannm

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I'm sorry this doesn't have anything to do with this post. I don't know how to post a new topic,so if you can help me out. My question is. I'm studying for my journeyman's Electrician course. Mike Holt exam prep. Page 187 conductor ampacity example 1 . On the ambient temperature correction factor it says for 150 degrees it's .58 but when I look it up it says .50 Thanks

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chuckgassmannm said:
I'm sorry this doesn't have anything to do with this post. I don't know how to post a new topic,so if you can help me out. My question is. I'm studying for my journeyman's Electrician course. Mike Holt exam prep. Page 187 conductor ampacity example 1 . On the ambient temperature correction factor it says for 150 degrees it's .58 but when I look it up it says .50 Thanks

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I moved this post to a new thread.

Chuck we need to know the entire question as we are not affiliated with Mike Holt offices. We can help but we need more info to answer the question and most of us don't have the exam prep. Thanks
 
chuckgassmannm said:
I'm sorry this doesn't have anything to do with this post. I don't know how to post a new topic,so if you can help me out. My question is. I'm studying for my journeyman's Electrician course. Mike Holt exam prep. Page 187 conductor ampacity example 1 . On the ambient temperature correction factor it says for 150 degrees it's .58 but when I look it up it says .50 Thanks

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Make sure you are using the correct table for the factory test ambient temperature rating of the conductor, which is usually 30C [86F]. Most applications in my familiar work, use Table 310.15(B)(2)(A). Avoid using the next table that is much larger and stands out visually, as it is for more exotic types of wire that are factory tested at 40C ambient. Draw an X through it, if it doesn't apply to your scope of work.

You can also use the square root formula, if desired. 310.15(B)(2). sqrt((Tc - Ta)/(Tc - T0)). It may get you slightly different results, because it is evaluated with more precision than the 5C temperature ranges in the tables.

Tc is the temp rating of the conductor, such as 90C [194 F]
Ta is the effective ambient temperature
T0 is the factory test ambient temperature, usually 30C [86 F]
 
I have a 5 HP 3 phase 480 v motor. According to 430.250 the flc. Is7.6 A x1.25=9.5A but according to 430.5 2 it says to multiple the flc×2.50=19A so the motor would need a 20 Amp breaker. Is that correct? Thanks

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chuckgassmannm said:
I have a 5 HP 3 phase 480 v motor. According to 430.250 the flc. Is7.6 A x1.25=9.5A but according to 430.5 2 it says to multiple the flc×2.50=19A so the motor would need a 20 Amp breaker. Is that correct? Thanks

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Yes, the 1.25 x the FLA gives you the minimum conductor size but the 2.5 x FLA gives the max overcurrent protective device.
 
chuckgassmannm said:
I have a 5 HP 3 phase 480 v motor. According to 430.250 the flc. Is7.6 A x1.25=9.5A but according to 430.5 2 it says to multiple the flc×2.50=19A so the motor would need a 20 Amp breaker. Is that correct? Thanks

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Please note that section is telling you the maximum size breaker you can use (unless it won't allow for the motor to start). Most of the time a 15 amp breaker will be just fine on a 5 hp 480 volt motor, and is still less then the calculated maximum so is code compliant. I have seen several times where a 10 HP motor starts fine on 15 amp breakers though they could be on as high as a 35.
 
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