Ambient temperature

[hmspe]

I'm in the Phoenix area, so temperature derate for outdoor circuits is a must. As has been noted in other threads here the NEC is very ambiguous on what to use for ambient. The FPN in 310.15(B)(2)(c) suggest getting the "average ambient temperature" from ASHRAE Fundamentals. A couple of problems here. First, the text of the Code does not use the word "average". Second, ASHRAE Fundamentals lists an absolute maximum (which in the case of Phoenix is not the same as what the National Weather Service reports), and 2%, 1%, and 0.4% maximums.

I'm thinking that if the CMP chooses to be ambiguous then the design community needs to try to come up with a consensus on what to use for ambient so that we can have a "generally accepted standard" to cite when we're asked why we did the calculations as we did (especially if we have to defend a design in court or before a registration board).

My thoughts:
Using a 2% value (which means 2% of the hours in the year are above the temperature given) or a 1% or 0.4% value makes some sense for HVAC since it just means that the indoor temperature may be a tad higher than normal for a few hours. I'm not sure that that makes sense for electrical since it means we could be running the conductors above rated temperature.

On the other hand, using the maximum recorded temperature can raise costs significantly. Example: feeders to 200A residential panels in this area are generally #4/0 AL SER cable. With a 122 degF (highest recorded temperature) the derating is 205A X 0.82 = 168A and a 175A breaker must be used. With a 110 degF (ASHREA 2% temperature) the derate is 205A X 0.87 = 178A, and a 200A breaker can be used. The 400A "all in one" services that are generally used in this area come with a 200/2 main for the second panel. 175/2 breakers are not cheap.

Any and all comments are welcome.

Martin

 

[kingpb]

YOu bring up a very good point of contention. I would think a good place to start is this question needs to be posed to the AHJ.

You might want to use a fake name though because you may potentially be opening a huge can of worms (or snakes)

 

[winnie]

The other side of the coin to keep in mind is that the temperature rating for a conductor is not really a hard limit above which the conduction will suddenly fail and burst into flames.

The temperature rating is one at which you can expect a reasonably long useful life if the conductor is at that temperature on a continuous basis. Exceeding this temperature for short periods of time now and then is not really a problem.

Because chemical reactions (including the thermal decay of insulation) tend to proceed in a fashion that is exponentially related to temperature, my _guess_ is that the proper way to calculate 'average' temperature for the purpose of ampacity calculations is to use the 'log mean exponential' of the absolute temperature.

-Jon

 

[beanland]

Temperature Limit

Temperature Limit

Thermoplastic insulation softens as it heats. Many soften at 100C so there is no much margin above 90C. Thermosetting plastics do not behave in the same fashion. I have samples of LLDPE jacket material that ran at 115C.

So, you need to consider the temperature at the location of use (appropriate for use) and adjust ampacity accordingly. Even not in AZ, a conduit on a roof can be subject to ambient temperatures well above normal.

 

[ramsy]

With a 110 degF (ASHREA 2% temperature) the derate is 205A X 0.87 = 178A, and a 200A breaker can be used.

It appears anything over 150A, factoring conductor adjustments for HVAC motor loads, overheats equipment using #4/0 AL SER. Are you using Tbl 310.15 for residential feeders only?

Your max conductor operating temperature should be 75c, considering panel listings & terminations. Engineers might replace the NEC's 1.25 motor adjustment with real current rise @ 0.85pf, using above ground conduit, & max 3 ccc's. Assuming equivalent engineering supervision, here's my spreadsheet calcs of temp-rise at peak loads:

Example A: 151A Peak Motor load @ 0.85pf = 178A @ 30c on #4/0 AL SER
The default ambient of (30c or 86f) NEC Tbl 310.16 operates at 75.1c.
Fails UL/NEC for 75c listed equipment, using any ambient over 30c.

Example B: 136A Peak Motor load @ 0.85pf = 160A @ 50c on #4/0 AL SER
Adjusted ambient (50c or 122f) operates at 74.5c. Barely compliant with UL/NEC

Example C: 153A Peak Motor load @ 0.85pf = 180A @ 25c on #4/0 AL SER
Underground installs require engineering supervision.
Adjusted ambient (25c or 77f) operates at 75.1c.

With a 122 degF (highest recorded temperature)

1) Rooftop ambient adjustments should be higher than max above ground.
2) Highest record temps. are on an upward trend, increasing each year. Most installs should be good for several years.

 

[hmspe]

YOu bring up a very good point of contention. I would think a good place to start is this question needs to be posed to the AHJ.

You might want to use a fake name though because you may potentially be opening a huge can of worms (or snakes)

Any place but Arizona it would probably be easy to do so. Arizona is a "home rule" state, so each town, city, and county adopts Codes independently and sets standards independently. We probably have at least 20 different jurisdictions just in the Phoenix metro area. Some jurisdictions may have addressed this, but I'm sure most have not.

 

[hmspe]

Are you using Tbl 310.15 for residential feeders only?

No. The job that brought up the issue happens to be residential.

2) Highest record temps. are on an upward trend, increasing each year.

Temperatures here are actually trending downward.

 

[ramsy]

Temperatures here are actually trending downward.

No payment for the other points?

Table 310.15(B)(6) does not change equip. temperature limits.

 

[hmspe]

No payment for the other points?

Table 310.15(B)(6) does not change equip. temperature limits.

I do appreciate the posts.

Still thinking over the calcs. I didn't comment earlier because they really aren't applicable to residential, which is what I originally referenced.

As to the temperature limits, all I can say is that basing derating on the actual insulation temperature rating is the accepted method. I suspect the reason it's accepted is that there's a high probability that any given circuit will be running below the amp load that would be required to heat the conductor to the limit, but that's just a guess.

Martin