termination provisions

[tstrouse]

When determining the correct conductor for terminations do you have to derate the ampacity because of a higher ambient temperature? For example I have a 69 amp load, I will be using 90C wire and my lowest termination is 60C. My ambient temperature is 40C. Following the rules for sizing my wire for the termination I come up with #2 wire if I don't derate for temperature. 69 times 1.25.

If I do derate for temperature I come up with #1.
(69 times 1.25)/0.91

What wire size should I use?

Does the change to 110.14 (C) (1) in 2002 make a difference? It tells me to use table 310.16 so I assume I have to derate the terminal for temperatures above 30C or does the derating only affect the conductor?

 

[bob]

Re: termination provisions

A few questions:
1. Is the 69 amps continuous (3 hours or more)? If so the 1.25 multipler is correct.
2. New equipment has 75 degree terminations. What do you mean by "my lowest termination is 60C"?
3. If the equipment is rated 75 degree C then the factor for 40 degrees is 0.88
Assuming load is continuous then
69 x 1.25/0.88 = 98 amps or #3 thhn 75 degree C

 

[websparky]

Re: termination provisions

Hi Bob,
My understanding of derating based on ambient is as follows.
The ambient correction is applied to the conductor based on the insulation.

To apply ambient derating we select the type of conductor insulation. For this arguement let's choose THHN. 2AWG THHN has insulation rated at 90C which gives the conductor an effective ampacity of 130A. If we then apply the derating for ambient based on the insulation of THHN, we would have;

130A x .91(40C ambient) = 118.3A

The load is: 69A x 1.25(continuous) = 86.25A
2 AWG = 95A(60C column, based on termination rating).

Please advise!

[ March 12, 2004, 06:25 PM: Message edited by: websparky ]

 

[bob]

Re: termination provisions

Dave
I think your method is correct. Thats what I get for trying to watch "Patton" on TV and do this at the same time. I would still question the 60 degree rating of the terminals unless this is old equipment.

 

[tstrouse]

Re: termination provisions

Thanks Bob and Dave. Yes my load is continuous. Based on Dave's response it doesn't appear like I have to apply the correction to the terminal for an elevated ambient temperature. Can I clarify with another example at a higher continuous current?
Step 1
69.8A x 1.25 = 87.25A
87.25/.91=95.9Amps (90 degree derating factor)
#3AWG, 90C wire appears to work
Step 2
Since my lowest termination is 60C I would check that table and see what size wire will work with 87.25A.
#2AWG appears to work
Since I need to work with worst case than #2 appears to be the conductor to use.

Is this correct or do I have to look at the 60C table and determine what wire size will handle
95.9Amps (69.8 x 1.25/.91). That's the real question because that would drive me to now use #1 instead of #2. Do I have to derate the terminal for an elevated ambient? Thanks Tim.

 

[websparky]

Re: termination provisions

Hi Tim,

87.25/.91=95.9Amps (90 degree derating factor)

This is not the proper sequence. The 87.25 is the rating for the load. It does not change!
What needs to be adjusted is the conductor's ability to safely carry the load.

Since my lowest termination is 60C I would check that table and see what size wire will work with 87.25A.
#2AWG appears to work

This is correct.

Is this correct or do I have to look at the 60C table and determine what wire size will handle
95.9Amps (69.8 x 1.25/.91). That's the real question because that would drive me to now use #1 instead of #2. Do I have to derate the terminal for an elevated ambient?

This is using the wrong sequence of calculation.
If you want to be sure that this conductor size will cover with room to spare, calculate the ambient at 50C and derate from 130A which will give you 106A. Remember, you are derating the conductor's current carrying capacity.

Thanks Bob for your reply! Patton is a very entertaining film!