Temperature/more than three wires derating question

[kenaslan]

I think this is one of the most confusing parts of the NEC. I quess this question has two parts.

#1 does anyone know of any good software that will calcuate a combimed temperature derating with more than three wires in a conduit?

#2 I have two small moters, one a 1/2 hp Maint Hoist, that will almost never be used, and a 30 hp lubercation pump motor. 480V, ambient temperature of around 100 Deg F in an Arizonia copper mine. THHN wire. I want to put the 6 wires in the same conduit. 150' total distance from the MCC so voltage drop is not a factor.

No motor derating on the 1/5 hp motor because it will be almost never used.

So here goes, do I use 75 or 90 degree table to derate the wire and then use the min wire size based on 75 degree lugs? Then deratte for temp and then for 6 conductors in the same conduit?


So confused
Ken

 

[charlie b]

I think this is one of the most confusing parts of the NEC.

That’s a category in which there are a lot of top flight contenders! :grin:

#1 does anyone know of any good software that will calculate a combined temperature derating with more than three wires in a conduit?

No software is needed. This is a simple math problem.

No motor derating on the 1/5 hp motor because it will be almost never used.

Was that a typo? Did you mean “1/2”?

You can’t declare that a motor need not be taken into account, unless there is an interlock in place, or some other technique, that will prevent that motor from ever being on at the same time as the other loads. So you do have six wires in the conduit, and all are “current-carrying conductors.” That gives you a derating factor of 80%.

You have an ambient temperature of 100F, and that gives you a derating factor (using the 90C column, since you have 90C rated wire) of 91%. The two derating factors together give you a total derating of 72.8%.

Next step is to calculate the load. A 30HP 480V motor has a current of 40 amps, per table 430.250. A 1/2 HP motor has a current of 1.1 amps. You add 25% to the larger of the two, and you get a total of 51.1 amps as the value you need for the ampacity of the conductors.

Looking at Table 310.16, without yet accounting for derating factors, we see that a #8 is not adequate, but that a #6 is adequate. Now apply the derating factor of 72.8% to the ampacity of a #6, as shown in the 90C column, and you get 0.728 times 75, or 54.6 amps. That will be the ampacity of a #6, in the “conditions of use” for your installation. That exceeds the required ampacity of 51.1 amps, so you are done.

 

[kenaslan]

oops yep a typo I ment 1/2 hp darn 5 an 2 are two close together with my wrist brace on. As for not counting the 1/2 hp motor I ment because it was a noncontinus motor that it need not be increased by 125%

But I knew you would have the answer Charley. My boss thinks that because the terminations are 75 degrees you MUST ONLY use the 75 degree column

Ken

 

[kenaslan]

I must say you lost me on the 72.8% can you please show me how you got that from the two derating factors please

 

[don_resqcapt19]

I must say you lost me on the 72.8% can you please show me how you got that from the two derating factors please

You multiply the factors by each other. In this case 91% times 80% to give a final derating factor of 72.8%.

 

[charlie b]

My boss thinks that because the terminations are 75 degrees you MUST ONLY use the 75 degree column

Show your boss the last sentence in NEC article 110.14(C).

But let's be clear. If the conductors are rated for 90C, then you can start with the 90C ampacity value, and then apply the derating factors for temperature or for over 3 current carrying conductors. The value you get to use for ampacity is the lower of two numbers: the value you jsut calculated, and the value shown in the 75C column.

 

[kenaslan]

If you were in CO, I'd apply at your place. LOL

 

[ramsy]

ambient temperature of around 100 Deg F in an Arizonia copper mine.

Mines aren't governed by NFPA 70, per 90.2(B)2, but if proper code has similar methods its a good exercise.

I must say you lost me on the 72.8% can you please show me how you got that from the two derating factors please

Perhaps (0.8+0.91)-1 = 71%, but the resulting 53A still covers 51.1

Even though this exercise shows your conductors operate right around 75c / 165f, the max for NEC motor loads, the proper code for Mines might not permit conductor temps operating over 60c for loads under 100A.

When replacing NEC's 25% motors adjustment with 0.85 Pf, I have a spreadsheet that shows 7.3A rise in current --48.4A total-- and 72c for #6cu. I still don't have the right IEEE standard to verify this temp.rise function.

 

[kenaslan]

What code would cover mines then? This is for a medium size rock crusher, around 5 stories tall. I know under ground MSHA is the safety law but what about electrical codes?

 

[ramsy]

I know under ground MSHA is the safety law but what about electrical codes?

If your employer isn't providing required resource or training then perhaps the failure to comply with MSHA standards should be of more concern.

If you were in CO, I'd apply at your place. LOL

If that reflects the confidence in your working environment, you can't run far. Construction / contracting / and industry are enjoying great latitude with self-regulatory policy. Perhaps, with less compliance enforcement, there's less perception of risk to outweigh rewards of cheating.

 

[charlie b]

Perhaps (0.8+0.91)-1 = 71%, but the resulting 53A still covers 51.1

Derating factors are "factors," in the mathematical sense, meaning that you multiply them, not add them. 0.8 times 0.91 is 0.728, or 72.8%.

 

[ramsy]

Derating factors are "factors,"

Thanks Charlie. Found one published example. Pg. 719 of the 2005 NFPA 70 Handbook, Step 1 in example under Table 400.5(B).

Using the form:
Max90c >= Load/derate1/derate2

You are using the form:
Max90c*derate1*derate2 >= Load

Is that two ways to arrive at similar "factored" results?