wire

[stev11]

hi

I have 3 phase motor with FLA=30A and the site temp is 50c and I need to use 6 conductors instead of three conductors
and I need to check f the below selection is correct:-

LOAD	FLA	CALCULATION	TEMP DERATING	no of conductor carrying ampacity	Selected Wire Size AWG
motor 1	30A	1.25x30=37.5A	0	0	8 AWG
		(30X1.25)/0.75=50A	0.75 @50C	0	8 AWG
		(30X1.25)/0.75/0.8=62.5A	0.75 @50C	0.8 @ 4-6 WIRES	6 AWG

so , is this mean that I need to use 1.25 for continuous load with temp derating and no of conductor carrying derating???(3 factors at the same time)???

thank alot.

 

[augie47]

In an indirect answer to your question, you do need to apply all derating factors when computing conductor ampacity.
There are so many errors (paralleling less than 1/0)and assumptions (what type insulation, what environment) with your exact question, it can't be answered.

 

[don_resqcapt19]

Why do you need 6 conductors?

 

[stev11]

In an indirect answer to your question, you do need to apply all derating factors when computing conductor ampacity.
There are so many errors (paralleling less than 1/0)and assumptions (what type insulation, what environment) with your exact question, it can't be answered.

ok, in this case the motor with 30A need to multiply it by 1.25 ==30*1.25=37.5A
than to select 6 AWG THWN(75C COLUMN) cable with 0.075 TEMP derating at 50c then to use ccc(4-6 wires)=0.8
so, the final ampacity for 6 AWG =75*0.8*0.75= 45A.

and, is it not acceptable to use paralleling wires that is less than 1/0??? why???

also I ask this question just to understand derating issue.

thanx

 

[stev11]

Why do you need 6 conductors?

really I do not need to use 6 conductor but I was trying to understand derating issue.


thanx alot.

 

[petersonra]

might be different in India.

 

[JDBrown]

and, is it not acceptable to use paralleling wires that is less than 1/0??? why???

This rule may be found in NEC 310.4(A). There are a couple of exceptions, but that's the general rule.

As for why, I believe it's because you can easily get the ampacity required for your application by using a larger size wire instead of paralleling two or more smaller wires. Smaller wires are more likely to be cut or broken by accident, leaving all of the current to be carried by the unbroken wires and causing them to overheat.

For example, let's assume I have a circuit with a 40 amp load. If I'm using the 60 deg. ampacity column in Table 310.16, I have to use #8 AWG conductors. If one of my #8 conductors gets cut or broken it becomes an open circuit and my equipment won't run until I find and repair the break.

If, however, I was allowed to use small parallel conductors, I could use 2 sets of #12 AWG conductors. Now if one of my #12 conductors breaks, my equipment will continue to run, but all 40 amps will be carried by ONE set of #12's (since the other set is broken). Those 40 amps will cause that #12 to get very hot to the point that it will break down the insulation and possibly cause a fire. Meanwhile, since it's connected to a 40 amp breaker, the breaker doesn't trip.

Of course, this is all just speculation on my part, but it seems to make sense to me. Going strictly by what's in the NEC and NEC Handbook, however, I would say we're looking at this the wrong way. It appears to me that the NFPA would prefer to not allow ANY paralleling of conductors, but it just becomes impractical when required ampacities get too high. As for why/how they decided on #1/0 as the cut-off, I have no idea.

 

[stev11]

This rule may be found in NEC 310.4(A). There are a couple of exceptions, but that's the general rule.

As for why, I believe it's because you can easily get the ampacity required for your application by using a larger size wire instead of paralleling two or more smaller wires. Smaller wires are more likely to be cut or broken by accident, leaving all of the current to be carried by the unbroken wires and causing them to overheat.

For example, let's assume I have a circuit with a 40 amp load. If I'm using the 60 deg. ampacity column in Table 310.16, I have to use #8 AWG conductors. If one of my #8 conductors gets cut or broken it becomes an open circuit and my equipment won't run until I find and repair the break.

If, however, I was allowed to use small parallel conductors, I could use 2 sets of #12 AWG conductors. Now if one of my #12 conductors breaks, my equipment will continue to run, but all 40 amps will be carried by ONE set of #12's (since the other set is broken). Those 40 amps will cause that #12 to get very hot to the point that it will break down the insulation and possibly cause a fire. Meanwhile, since it's connected to a 40 amp breaker, the breaker doesn't trip.

Of course, this is all just speculation on my part, but it seems to make sense to me. Going strictly by what's in the NEC and NEC Handbook, however, I would say we're looking at this the wrong way. It appears to me that the NFPA would prefer to not allow ANY paralleling of conductors, but it just becomes impractical when required ampacities get too high. As for why/how they decided on #1/0 as the cut-off, I have no idea.

Mr.JDBrown

thank you for your help.