wire derating

[wdalrymple]

I found the situation,created by an electrical contractor, listed below and need some input on what they can do to fix the problem.

1 100 amp panel with 2 2" conduits 5' in leangth, 1 has 17 current carrying conductors
the other has 18 current carrying conductors.
they are on 20 amp breakers.

this makes the ampacity of each wire 12.5 amps
can you by code attach a 20 amp duplex receptacle to this conductor.

to fix this they can add more conduit?"how many"?
or they can increase the wire size "how much"?

each circuit runs to 2 gfi's in the bathroom and 2 other receptacles in the dorm rooms" total 4 duplex receptacles"
this is work that is still withen the 12 month warrenty period.

thanks for your input and ideas

Will Dalrymple

 

[iwire]

Re: wire derating

Originally posted by wdalrymple:
1 has 17 current carrying conductors
the other has 18 current carrying conductors.

10 to 20 conductors will be a 50% derate.

this makes the ampacity of each wire 12.5 amps

Yes if the wires are 75 C rated, this is new work so I would say 90 C rated which means after derating you have a 15 amp conductor.

can you by code attach a 20 amp duplex receptacle to this conductor.

No

to fix this they can add more conduit?

"how many"?

To use 20 amp breakers on 12 AWG if it is rated 90 C you can not exceed 9 current carrying conductors per raceway, you would need 4 raceways

or they can increase the wire size

"how much"?

10 AWG @ 90 C is rated 40 amps apply a 50% derate and you are left with 20 amps.

 

[steve66]

Re: wire derating

I may be in the minority, but I'm not sure I agree. The fine print note to table 310.15(B)(2)(a) says "See annex B, Table 310.11 for adjustment factors for more than 3 current carrying conductors in a raceway or cable with load diversity". The handbook states "The factors in the second column are based on no diversity, meaning that all conductors in the raceway or cable are loaded to their maximum rated load."

If the loads are for receptacles or lights, they will not be fully loaded all the time. And, branch circuits conductors already have some measure of diversity included (ie. 12Ga 30A wire can only be protected at 20Amps, and continuous loads are limited to 16 amps.)

Steve

 

[iwire]

Re: wire derating

Steve you must be looking at the 99 code, the FPN is now gone.

What you say makes perfect sense, look at what these conductors are feeding almost never will you have a raceway with all the conductors maxed out at the same time.

That said FPNs are not code and Annex B is not code so I do not see any options to apply load diversity.

 

[iwire]

Re: wire derating

By the way, in MA the table we use for derating all conductors is the same as Table B-310-11.

Bob

 

[charlie b]

Re: wire derating

Originally posted by steve66: The fine print note to table 310.15(B)(2)(a) says "See annex B, Table 310.11 for adjustment factors for more than 3 current carrying conductors in a raceway or cable with load diversity".

There is no easy way to take advantage of that statement. The footnote to Table B310.11 says the following:

* These factors include the effects of a load diversity of 50 percent.

We should take note that the NEC never defines ?load diversity.? Therefore, we are left to use the ?industry standard? definition for the term. But there is no industry standard definition for ?load diversity.? The nearest standard term is ?diversity factor.? That term can be defined as ?average divided by peak.? A circuit that is fully loaded (let us say at 20 amps) for 12 hours a day, and that is turned off the other 12 hours, with have a peak of 20 amps, and an average of 10 amps. Therefore, it will have a diversity factor of 50%.

But here?s the difficult part. Suppose you have 9 circuits (18 conductors) in a conduit. Suppose that one circuit is fully loaded (let us say at 20 amps) for 24 hours a day. Finally, suppose that the other have essentially no load (i.e., nothing plugged in). In this case, the peak and the average will be the same value. Therefore, you have a diversity factor of 100%, and Table B310.11 will not help you at all!

If anyone wants to use Table B310.11, I think you will need a professional engineer?s seal on the calculation. Speaking as a PE, I?m not sure I would want to take on that assignment. I say that mostly because I am not certain I understand the intent of the term ?load diversity,? as used in the NEC.

 

[thunder15j]

Re: wire derating

Derating of wire ampacities due to the larger numbers of current carrying conductors in a raceway makes sense because of the potential for heat buildup. The more current conductors that are in a raceway, the more heat buildup. (Dah dah)The reality of this installation, however, is that the 2" conduits are only 5 feet long and if they were 24" or less derating would not have to be adressed. The conduits are hardly approaching their maximum fill allowance, which is 109 #12 THHN conductors. The reality is that there is probably not a problem. What if they had been installed in 4" conduit? Would you have the same heat build up in the oversized 4"? I assume the same derating adjustments would be required. You could install a couple of pieces of 3/4" or 1" smurf (ENT) inside of the 4" and divide the wires to meet or beat the code. This installation would meet code but would be more likely to trap heat in each subconduit than if the conductors were all together without the dividers. Now after all this gab, comes my question that I wanted to ask all along: Does the NEC consider conduit size and conduit fill when applying conductor derating percentages due to a larger number of conductors in a raceway?

 

[iwire]

Re: wire derating

Now after all this gab, comes my question that I wanted to ask all along: Does the NEC consider conduit size and conduit fill when applying conductor derating percentages due to a larger number of conductors in a raceway?

No

[ May 10, 2004, 04:02 AM: Message edited by: iwire ]

 

[steve66]

Re: wire derating

Thunder:

Raceway fill is a point I thought of also. Like Iwire said, per the NEC it doesn't matter how much you actually fill a conduit. I think the reason is that even in a very large conduit, the wires may bunch up at the bottom of the conduit.

This is a little off the subject, but consider article 376.22 for metal wireways. If I have 50 conductors in a 6" x 3.5" wireway, I have to derate them per 376.22. But if I put a divider in the wireway, and put 20 conductors on one side, no derating is required. That seems odd.

Steve

 

[Saferguy]

Re: wire derating

When de ratting conductors...let us not forget, that THE LAST SAY SO belongs to the " device terminations ".... They will see the real thing....meaning, temperature created by the AMPACITY.....!