Sizing Conductors

[Charles R. Miller]

I am currently writing a series titled "Sizing Conductors" in Electrical Contractor Magazine. Find the conductor size in this example before it is published in the September issue.

"What size THHN copper conductors are required to supply a branch circuit under the following conditions? The load will be a 39-ampere, non-motor, continuous load. These branch-circuit conductors will be in a raceway. There will be a total of eight current-carrying conductors and an equipment grounding conductor in this raceway. The terminations on both ends are rated at least 75?C. The maximum ambient temperature will be 40?C."

 

[Dennis Alwon]

Whenever I read these questions my first puzzle is whether that continuous load is included in the 39 amps. I will assume not-- Is that correct

 

[infinity]

Charles,

Welcome back, see that you haven't posted in a while. I have to say I have been reading your ongoing series every month and always find it informative.

Whenever I read these questions my first puzzle is whether that continuous load is included in the 39 amps. I will assume not-- Is that correct

I assumed that as well.

 

[Dennis Alwon]

I get a #6 thhn--Sorry meant #4

 

[Dennis Alwon]

I get 39*1.25*1.3*1.12=70.98amps 75C= #4thhn. I am sure I must be missing something

 

[Charles R. Miller]

Hi everyone, it has been a while since I've been on this forum. Thanks for welcoming me back. The 39 amps is the actual load before the application of the 125 percent.

 

[Smart $]

110.14(C)(1)Termination Limitations
39A load
75?C terminal ratings
minimum 8AWG Cu conductor required (50A; 10AWG is 35A)

210.19(A)(1) Conductors — Minimum Ampacity and Size
39A continuous load
39A ? 125% = 48.8A
8AWG minimum THHN Cu (55A; 10AWG is 40A)

310.15(B)(2)&(3) Ambient Temperature Correction and Adjustment Factors
39A load
40?C ambient temperature correction factor for THHN is 0.91
8 current-carrying conductors in raceway adjustment factor is 70% (0.7)
39A ? 0.91 ? 0.7 = 61.2A minimum table ampacity required
6AWG minimum THHN Cu (75A; 8AWG is 55A)

6AWG is the largest of the required minimum sizes

 

[don_resqcapt19]

110.14(C)(1)Termination Limitations
39A load
75?C terminal ratings
minimum 8AWG Cu conductor required (50A; 10AWG is 35A)

210.19(A)(1) Conductors ? Minimum Ampacity and Size
39A continuous load
39A ? 125% = 48.8A
8AWG minimum THHN Cu (55A; 10AWG is 40A)

310.15(B)(2)&(3) Ambient Temperature Correction and Adjustment Factors
39A load
40?C ambient temperature correction factor for THHN is 0.91
8 current-carrying conductors in raceway adjustment factor is 70% (0.7)
39A ? 0.91 ? 0.7 = 61.2A minimum table ampacity required
6AWG minimum THHN Cu (75A; 8AWG is 55A)

6AWG is the largest of the required minimum sizes

The ambient correction factors and the adjustment factors change the conductor ampacity. The minimum conductor ampacity for this circuit is 48.8 maps. The corrected and adjusted ampacity of the #6 is 47.775 and is too small for this circuit.
75 x.7 x.91 = 47.775

 

[Dennis Alwon]

Looks like my error on correction factor-- I used .88- don is correct. .91 Result is still the same #4

 

[Smart $]

The ambient correction factors and the adjustment factors change the conductor ampacity. The minimum conductor ampacity for this circuit is 48.8 maps. The corrected and adjusted ampacity of the #6 is 47.775 and is too small for this circuit.
75 x.7 x.91 = 47.775

The minimum conductor ampacity BEFORE adjustment and correction is 48.8A

210.19 Conductors — Minimum Ampacity and Size.
(A) Branch Circuits Not More Than 600 Volts.
(1) General. Branch-circuit conductors shall have an ampacity
not less than the maximum load to be served. Where
a branch circuit supplies continuous loads or any combination
of continuous and noncontinuous loads, the minimum
branch-circuit conductor size, before the application of any
adjustment or correction factors, shall have an allowable ampacity
not less than the noncontinuous load plus 125 percent
of the continuous load.

The ampacity of the conductor AFTER the application of adjustment and correction factors must only equal or exceed the actual load current (39A), not 125% of it.

 

[Dennis Alwon]

You are saying that we don't need the 125% as long as the correction factors take us above the calculated continuous load 48.8. I think I finally get it. You have stated this before and this makes sense

 

[Smart $]

You are saying that we don't need the 125% as long as the correction factors take us above the calculated continuous load 48.8. ...

I've never really looked at it this way, but yes.

 

[david luchini]

You are saying that we don't need the 125% as long as the correction factors take us above the calculated continuous load 48.8.

That's not quite the right way to look at it. If the 39A continuous load was in a conduit with 6 total ccc's in an ambient of 40degC, then:

39A*1.25 = 48.8A: Min conductor size is #8AWG.

#8 THHN adusted ampacity would be 55A * 0.8 (6 ccc's) * 0.91 (ambient correction) = 40.04A.

The correction factors take us below 125% of the load (for continuous load - 48.8), but above the actual load (39.)

Since the adjusted ampacity is large enough for the load (40.04 > 39) and the conductor is not smaller than the minimum conductor size, the #8 THHN would be acceptable for the circuit.

 

[Dennis Alwon]

That's not quite the right way to look at it. If the 39A continuous load was in a conduit with 6 total ccc's in an ambient of 40degC, then:

39A*1.25 = 48.8A: Min conductor size is #8AWG.

#8 THHN adusted ampacity would be 55A * 0.8 (6 ccc's) * 0.91 (ambient correction) = 40.04A.

The correction factors take us below 125% of the load (for continuous load - 48.8), but above the actual load (39.)

Since the adjusted ampacity is large enough for the load (40.04 > 39) and the conductor is not smaller than the minimum conductor size, the #8 THHN would be acceptable for the circuit.

My way is easier :lol:

 

[Smart $]

That's not quite the right way to look at it. If the 39A continuous load was in a conduit with 6 total ccc's in an ambient of 40degC, then:

39A*1.25 = 48.8A: Min conductor size is #8AWG.

#8 THHN adusted ampacity would be 55A * 0.8 (6 ccc's) * 0.91 (ambient correction) = 40.04A.

The correction factors take us below 125% of the load (for continuous load - 48.8), but above the actual load (39.)

Since the adjusted ampacity is large enough for the load (40.04 > 39) and the conductor is not smaller than the minimum conductor size, the #8 THHN would be acceptable for the circuit.

You changed the conditions from 8 CCC (70% adjustment) to 6 CCC (80% adjustment)

#8 @ 55A * 0.7 (8 ccc's) * 0.91 (ambient correction) = 35.0A... which is too small!!!!!

It's easier to go the other way...

39A ? 125% = 48.8A
39A ? 0.7 ? 0.91 = 61.2A, greater than 48.8A
#8 THHN table ampacity is 55A < 61.2A... too small
#6 THHN table ampacity is 75A > 61.2A... smallest permitted.

 

[david luchini]

You changed the conditions from 8 CCC (70% adjustment) to 6 CCC (80% adjustment)

#8 @ 55A * 0.7 (8 ccc's) * 0.91 (ambient correction) = 35.0A... which is too small!!!!!

It's easier to go the other way...

39A ? 125% = 48.8A
39A ? 0.7 ? 0.91 = 61.2A, greater than 48.8A
#8 THHN table ampacity is 55A < 61.2A... too small
#6 THHN table ampacity is 75A > 61.2A... smallest permitted.

Yes, I changed to conditions to try to make a point...that the adjusted ampacity doesn't need to be greater than 125% of the (continuous) load. I'm not suggesting that #8 is the correct size for the OP, I agree with you that #6 is correct.

Let me change the conditions again to try to further clarify my point:

39A continuous load, 6 ccc's, in a 25degC ambient.

39A * 125% = 48.8A. #8 (ampacity of 50) will be the minimum allowable conductor size.
39 ? 0.8 ? 1.04 = 46.9A, lower than 48.8.

However, #8 THHN table ampacity is 55A. The adjusted ampacity for the #8 THHN will be 55 * 0.8 * 1.04 = 45.76 (also lower than 48.8.)

But, 45.76 is greater than the load of 39A, and is the #8 THHN is not smaller than the minimum allowable conductor size, so #8 THHN would be allowable for these conditions.

My way is easier :lol:

I agree

 

[infinity]

Now I see why Charles posted this question. :roll:

 

[david luchini]

Now I see why Charles posted this question. :roll:

To try to set a new record?hmy:

 

[Smart $]

...

Let me change the conditions again to try to further clarify my point:

...

I think I see the point you are trying to make. Like I said when Dennis first made the statement, I've never really looked at it this way. But to proceed with your point vs. his statement, he said...

You are saying that we don't need the 125% as long as the correction factors take us above the calculated continuous load

...which is true... but your point is that there are conditions where adjustment and correction may not take us above the 125% load value

So you wrote...

39A continuous load, 6 ccc's, in a 25degC ambient.

39A * 125% = 48.8A. #8 (ampacity of 50) will be the minimum allowable conductor size.
39 ? 0.8 ? 1.04 = 46.9A, lower than 48.8.

However, #8 THHN table ampacity is 55A. The adjusted ampacity for the #8 THHN will be 55 * 0.8 * 1.04 = 45.76 (also lower than 48.8.)

But, 45.76 is greater than the load of 39A, and is the #8 THHN is not smaller than the minimum allowable conductor size, so #8 THHN would be allowable for these conditions.

I agree with your result. However, when you get to the blue text, you keep turning the method around, and it's not necessary (provided you are confident in your math abilities ).

The following is all it takes...

39A ? 125% = 48.8A, #8 minimum.

39A ? 0.8 ? 1.04 = 46.9A, also #8 minimum.

There is no larger size of the two, so #8 minimum it is.

 

[Dennis Alwon]

Thanks Charles for an interesting problem. I too enjoy your articles on EC&M.

Thanks to our members for putting the effort to "learn" me something.