Ampacity deration & conduit capacity

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  #1  
Old 06-28-05, 09:00 AM
NomadTech
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Ampacity deration & conduit capacity

I apologize if this question has already been posted…

On 3/3/02 Wgoodrich stated that, “If you install that 7th 12awg copper conductor in a conduit you now are not allowed to install that 12awg conductor anywhere in the home with voltage rated over 50 volts at all because it is not even 15 amp rated which is the smallest ampacity rating allowed in a dwelling for 120 volts or more.” He continued, “The reason for the derating of ampacity is because… of heat…” On his own site Warren Goodrich (I’m assuming this is the same person) states, “you will find it much better if you install only ½ of the maximum number of conductors allowed by the NEC Code in a certain conduit.”

My confusion comes in that if you are going to see significant ampacity deration after running more than 3 conductors in a conduit, why would you ever run large conduit (who cares about capacity if the limiting factor is the ampacity deration)? Along those lines, my confusion continues in that many tables on the web indicate that you can safely run dozens of conductors through conduit. The following table indicates up to 29 12awg THNN conductors through a 1” conduit.

http://www.mc2-ice.com/popular_conve...s_conduit.html

How does amperage, the number of conductors, and the size of the conduit all work together? Personally, I’m trying to run wires for 5 circuits (a hot, neutral, and ground “conductor?” for each) where space is very limited and where nothing would be considered “low voltage.” I appreciate any feedback you can give.

Thanks.
 
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  #2  
Old 06-28-05, 09:23 AM
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The rules for derating based upon the number of conductors in a conduit are somewhat complex.

First you have to count the number of 'current carrying' conductors correctly. In particular there are situations where the effective number of current carrying conductors is fewer than the actual number of conductors. You don't count equipment grounding conductors. You don't count neutral conductors that carry the imbalance current between two evenly sized circuits. When you have switch legs or travellers where only one of the conductors can carry current at a time, you only count the one conductor, etc.

After you have counted up the correct number of current carrying conductors, then you apply the derating rules of NEC Table 310.15(B)(2)(a) http://www.houwire.com/catalog/techn...icle310_15.asp
These derating factors apply to the table 310.16 ampacity of the conductors, even if other rules normally restrict the current to less than this.

For example, 12ga wire has a thermal ampacity of _30_ amps. Different rules restrict the maximum circuit breaker used for 12ga wire to only 20A, but any derating factors are calculated from the 30A number. What this means is that if you have 9 current carrying 12ga conductors in a single conduit, the required 70% derating factor reduces the thermal ampacity to 21A...which is still more than the 20A protection required to follow other rules, net result being that 9 current carrying 12ga conductors in a single conduit is just fine.

For your situation, you have _3_ choices.

1) You can simply run your 5 circuits, using 12ga THHN wire and having a total of 10 current carrying conductors in this conduit. You will need to use 15A circuit breakers, because at 10 conductors the thermal ampacity of 12ga wire is reduced to 15A. Note that you need to use a 12ga ground wire even though this is a 15A circuit set.

2) You can run your 5 circuits using 10ga wire. 10ga THHN wire has a non-derated thermal ampacity of 40A, so after derating you are down to 20A. Then protect your circuits with 20A breakers. Note that if you do this, you must use a10ga ground wire as well.

3) You can use 'multiwire branch circuits'. These introduce significant additional complexity, and are not an appropriate solution unless you are willing to read up quite carefully on the topic. But in a multiwire circuit you have _two_ hot conductors from the two different service legs, both sharing a single neutral conductor. Because the current flow from one circuit balances the current from the other circuit, the set of three conductors (two hots, one neutral) count as only two current carrying conductors. This means that you could run your 5 circuits with a total of 9 wires (5 hot, 3 neutral, 1 equipment ground) with only 6 current carrying conductors. This would dodge the derating issue, but introduce significant new complexity.

-Jon
 
  #3  
Old 06-28-05, 10:03 AM
NomadTech
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Related question

Jon,

Thanks for the wisdom on this matter.

Another related question. What distance can you run a conductor before you have to start factoring in the ampacity deration due to the conductor's internal resistance? Also how do outlets affect this? I'm attempting to reduce my runs by connecting multiple wall outlets together. Over the course of a 20' span (breaker box is 30' from the start of this span), I'll need a total of 8 outlets, of which I'm guessing that at most 3 will be used at the same time - none of which should draw significant wattage.
 
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Old 06-28-05, 10:24 AM
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Distance has nothing to do with ampacity deration. The longer the wire, the more its resistance, and the more total heat generated in the wire...but the larger the area it has to dissipate the heat.

Distance figures into 'voltage drop'. The longer the distance, the greater the resistance, and the greater the voltage drop. But this voltage drop also depends upon the actual current flow; the more the current flow the more the voltage drop. The normal rule of thumb is to aim for less than a 3% voltage drop in your branch circuit.

If we presume the worst case, using 12ga wire with a full 20A load at the end of the run, on a 120V circuit, then you get a 3% drop with a run about 55 feet long. So the situation that you describe, 30 feet of circuit with low wattage loads, should be just fine.

-Jon
 
  #5  
Old 06-28-05, 11:00 AM
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Ampacity deration & conduit capacity

Winnie,
In your note #2 you state that if using #10 hot & neutral for a 20A circuit, you must also use #10 ground. Why?
In my project I plan to use #10 for all my 20A circuits, but due to physical constraints in the conduit, I had planned to use #12 grounds (each circuit with its own ground conductor - no tying together).
If I run 3 circuits in a conduit, does that mean I have to run 3 #10 grounds instead of 3 #12s?
The reasons for using #10 for my 20A circuits is both for derating (hot attic runs) and voltage drops. I never, ever want to see lights flicker when turning on a vacuum, etc.
 
  #6  
Old 06-28-05, 11:06 AM
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Why woulsd you want to run seperate grounds, one is all thats needed for all the circuits, its one of the advantages of wiring in pipe. In fact with emt you dont need any ground wire.
 
  #7  
Old 06-28-05, 11:09 AM
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If you increase the size of the circuit conductors for any reason, you are required to increase the size of the equipment ground conductor proportionately. So if you use 10ga circuit conductors when 12ga would normally be used, then you need to use 10ga equipment grounding conductors.

The reason for this is performance of the ground system. If you are worried about voltage drop in your circuit conductors, then you have to be similarly concerned about voltage drop in your equipment ground conductors.

Running separate equipment grounds for each circuit is a waste and potentially a code violation. All grounds in a given box need to be tied together, so if you are not joining these grounds then you have a code violation, and if you are joining these grounds then you are wasting material running several when one (properly sized) will do.

-Jon
 
  #8  
Old 06-28-05, 06:13 PM
NomadTech
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Relationship with conduit diameter?

I'm still a bit confused...

I easily followed along with Winnie’s explanation and understand the NEC tables. However, none of this seems to take into account the diameter of the conduit that the conductors are being run through.

It seems that the NEC guidelines were established to limit the amount of heat generated (and held) by the conductors, which makes sense as heat causes fires, exactly what the NEC is trying to limit. (To clarify my own thoughts) - the more conductors, the more heat… the more amps, the more heat. Therefore, if we limit the conductors and the amps then everyone should be happy. However, it also seems to make sense that if the conductors are less crowded within the conduit then the heat will more easily dissipate. With less heat we should have less voltage drop (less heat = less resistance = less voltage drop... right?). So how do the various charts that simply list the maximum number of conductors allowed for the various sizes of flexible conduit relate to the NEC’s table 310.15(B)(2)(a) and 310-16?

- Shane
 
  #9  
Old 06-28-05, 07:39 PM
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The answer is that the tables do not take this information into account. These tables are only approximate, with considerable safety factor built in. But yes, in theory a much larger conduit will dissipate heat better, and thus permit a higher thermal ampacity, but the NEC tables will not show this.

The particular NEC article that talks about ampacity is 310.15, and it has two sections. One describes the use of tables, with all the limitations discussed above. The second section permits the use of the appropriate heat generation equations and heat dissipation equations 'under engineering supervision'. This means that if a suitable licensed engineer does all of the appropriate calculations, you can take into account all of the various heat dissipation factors and arrive at the exact ampacity for a given conductor in a given situation. Generally this analysis costs far more than simply using a larger conductor , so everyone uses the tables with their big safety factors.

-Jon
 
  #10  
Old 06-28-05, 07:43 PM
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The ampacity rules ignore conduit size because gravity means that all the wires sit on the bottom of the conduit no matter how big it is. This means that how "crowded" the conductors are is independent of conduit size. Ampacity rules are suspended if you have some way of enforcing spacing between the wires.
 
  #11  
Old 06-29-05, 03:18 AM
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Ampacity rules are suspended if you have some way of enforcing spacing between the wires.
John, I've never heard nor seen reference to this. It makes sense to me that there should be a way to distribute the heat of (for example) wires in a large gutter, but I've never seen a code reference for this. Any pointers?

Thanks.

Jon
 
  #12  
Old 06-29-05, 05:01 AM
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Hey All,

Basically my take on the derating and number of wires in a given coduit has been factored by the engineers who assist in code panel reviews and make suggestions based on responsive feedback from peers.

The derating is set to counter act the heat issues the best we can which is why we have the derating for conductors in a conduit over 3 and when dealing with ambient temps in a given area.


On hot gutters...well i find that the bend ratios and requirements for clearance inside from conductors based on the conduit size and so on tend to give ample room for heat reduction...about the best we can.

In other words would I run (4) 14 AWG in a 2" conduit just to reduce the heat....ahhh...nope...lol....I would do what the code allows and trust in those that have WAY more education than I do and sit on a panel of NEC advisors and Misers.

Also when I get to the point of having to use a hot gutter like in a set of appartments I did just recently...I have to say the wires were stiff and sturdy enough to position the wires in the gutter with ample spacing because the gutter was sized correctly....I think that is the issue....do we all know how to properly size a hot gutter....my guess is we do so usually it takes care of itself.

Just my OPIN......Hey John...are Moderators allowed to give Opinions..thehehe..I dont want to step in the so called Pulling LUBE...
 
  #13  
Old 06-29-05, 06:42 AM
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Winnie, the only area that the code talks about enforced spacing is in the article on cable trays, because most other raceways have no means to do it.
 
  #14  
Old 07-04-05, 01:14 PM
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One more....

Somewhat related to what I've inquired about in the past postings... You've explained to me how the voltage drops as the length of a conductor is increased and I've used the calculators to figure out the max lengths I can use with the different sized conductors I desire to run through conduit. What I've found is that I'm limited to roughly 70' to 100' depending upon 12 or 10awg use. So, how is it that a 4000 square foot ranch home can have one breaker box and outlets hundreds of feet from that source without exceeding the 3% voltage drop?
 
  #15  
Old 07-04-05, 04:11 PM
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There are several things one could do, you could locate the panel to be helpful, near the heavy loads, you could upsize the wires, you could run multi wire circuits and some places see so light of loads most of the time that voltage drop may not be that much of a factor. In bedrooms the biggest load may be a vaccuum on occasion, the rest of the time it is an amp or 2. I saw a house a while back that had the panel located in a poor spot, a sub would have made great sense, would have saved a fortune on wire and every run was 75 ft into the living area. Most houshold circuits are not loaded to the max most of the time.
 
  #16  
Old 07-05-05, 08:04 AM
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I like all of sberry's ideas. Here are some other thoughts.

Suppose that 4000-square-foot ranch is 40 feet by 100 feet square. Put the panel in the middle of the long exterior wall and the farthest distance to anywhere (with right-angle turns only) in the house would be 90 feet. Use subpanels with generously sized feeders and the distance drops more. Run the cables through the attic where you can go direct instead of right angles and the farthest distance drops even further.

Also, keep in mind that 99% of the time, a 20-amp circuit is carrying less than 20 amps. This is especially true if you put in more circuits than you really need. Furthermore, if you occasionally exceed 3% drop, it's not the end of the world, and it's not normally a safety issue.
 
  #17  
Old 07-05-05, 08:37 AM
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Presuming that this is "new" constuction----- Much depends on the location of the Service panel relative to the locations of the loads. The S-P may be on the West end of the structure, and the A-C compressor-motors 100 ft distant on the East end.

I agree with John N that Feeders to sub-panels are the solution.One possible arrangement, since you are discussing conductors in raceways, are raceways laid on the "rough" cellar floor , since a "ranch" suggests a simple rectangle which lends itself to a straight run, end-to-end.This presumes the Service-panel will be located in the basement

Good Luck & Enjoy the Experience!!!!!!!!!!
 
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