Amp Calc conductor temperature

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jjavier

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Hi

We are using ampcalc software to do a preliminary calculation of cable, and we have a doubt:

We are selecting XHHW-2 aluminiun cable, and we are using specified temperature 90º (operation conditions) inside pvc conduit underground, Is this correct, because is wet environment? Really, cable datasheet shows 90º in wet and dry conditions.

Regards
 
jjavier said:
Hi

We are using ampcalc software to do a preliminary calculation of cable, and we have a doubt:

We are selecting XHHW-2 aluminiun cable, and we are using specified temperature 90º (operation conditions) inside pvc conduit underground, Is this correct, because is wet environment? Really, cable datasheet shows 90º in wet and dry conditions.

Regards
Click to expand...
Is the operating condition 90°F or 90°C?

If the latter, the insulation rating is 90°C and you must come up with a more specialized method of wiring (i.e. not the NEC).

If the former, you must properly correct for the ambient temperature. 90°F = 32.2°C
 
jjavier said:
Hi

We are using ampcalc software to do a preliminary calculation of cable, and we have a doubt:

We are selecting XHHW-2 aluminiun cable, and we are using specified temperature 90º (operation conditions) inside pvc conduit underground, Is this correct, because is wet environment? Really, cable datasheet shows 90º in wet and dry conditions.

Regards
Click to expand...

90C may be used for deratng however the final overcurrent protective device must not be higher than the 75C rating.
 
Smart $ said:
Is the operating condition 90°F or 90°C?
Click to expand...


90C [194F] refers to the peak temperature inside the wire, based on amps, conditions of use, and ambient temperature. No 90C rated wire should actually get to this temperature in a proper design, as there are safety factors involved in the calculation, but it means that the wire is tested to withstand 90C without failure.
90F [32C] is a common design ambient temperature


Both could be seen as "operating condition".
 
doubts

doubts

Dennis Alwon said:
90C may be used for deratng however the final overcurrent protective device must not be higher than the 75C rating.
Click to expand...

Hi
I don´t really understand the answer.

I am calculating the amps with ampcalc, with service temperature 90ºC, and this is correct (cable is listed to works 90ºC wet).

But we calculate the protection device with the nominal power of the inverters*1.25. What I have to do with the temperature here?
 
jjavier said:
Hi
I don´t really understand the answer.
Click to expand...
Your not providing a clear picture of your question as I/we see it. What is(are) the inverter(s) rated output current? If more than one, are you combining before or after this underground run.

We understand you want to use XHHW-2 aluminum conductors. Yes, these are rated for "up to" 90°C operation. However, under the NEC, we also have terminal temperature limitations to contend with [110.14(C)] coupled with the requirements for feeder or branch circuit sizing requirements. The short of this is, our circuits are limited to the ampacity value in the 75°C column of Table 310.15(B)(16) for the size we use. Only under special conditions can we use these conductors to carry the full 90°C rated amps.
 
Ok. That means that although cable rating is 90ºC, terminal limitation 110.14c (conductor higher than 1AWG or 100Amps), we have to calculate conductors with 75ºC. Is it true?






Smart $ said:
Your not providing a clear picture of your question as I/we see it. What is(are) the inverter(s) rated output current? If more than one, are you combining before or after this underground run.

We understand you want to use XHHW-2 aluminum conductors. Yes, these are rated for "up to" 90°C operation. However, under the NEC, we also have terminal temperature limitations to contend with [110.14(C)] coupled with the requirements for feeder or branch circuit sizing requirements. The short of this is, our circuits are limited to the ampacity value in the 75°C column of Table 310.15(B)(16) for the size we use. Only under special conditions can we use these conductors to carry the full 90°C rated amps.
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Dennis Alwon said:
Yes but you can use the 90C for derating purposes but the final ampacity cannot be larger than the 75C rating
Click to expand...


Thanks

Now is clear. We are going to install a connection busbar (with compresion lugs) before of the CB, so I think we could use 90ºC derating. Between this busbar and CB, we are going to calculate with cable 75ºC (really we have the big issue at solaredge inverters that the maximun cable is 6AWG and we are using aluminium....)
 
jjavier said:
Ok. That means that although cable rating is 90ºC, terminal limitation 110.14c (conductor higher than 1AWG or 100Amps), we have to calculate conductors with 75ºC. Is it true?
Click to expand...
Yes... but as already mentioned, you can use 90°C value for derating purposes. Also, a lot of equipment rated 100A and less is listed for 75°C terminations.
 
jjavier said:
Thanks

Now is clear. We are going to install a connection busbar (with compresion lugs) before of the CB, so I think we could use 90ºC derating. Between this busbar and CB, we are going to calculate with cable 75ºC (really we have the big issue at solaredge inverters that the maximun cable is 6AWG and we are using aluminium....)
Click to expand...

You must do two deratings of 90 degree conductors with 75 degree terminations.

1. Conditions of use: (90 degree ampacity)(ambient temperature derating, plus rootop adder if applicable)(derating for multiple current carrying conductors in a raceway)
2. Continuous use: (75 degree ampacity)(0.8)

Both of these ampacities must be more than your nameplate maximum current from the inverter(s).

If your aluminum conductors are too big for your lugs you can use Polaris blocks to change over to copper for the last bit as long as the copper has sufficient ampacity for the application.
 
Are you using the software to calculate the conductor size based on load and conductor length? If so, the temperature used in that type of calculation would be the ambient temperature of the location where the conductors are being installed.
 
FWIW, I do these calculations by hand. It takes about 15 seconds and I can see all the moving parts so I trust the answers I get.
 
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