Cable Derating Questions

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  #1  
Old 05-14-11, 10:44 PM
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Cable Derating Questions

Would someone mind explaining cable derating to me, specifically as it applies to cable in stud bays?

For example, I'd like to pull some cables from my basement to the second floor. My studs are 16" OC, and 4" deep.

What are the factors that go into cable derating, what does it mean if you have to derate (use different breakers?), etc.?

If someone has the time, I'd love a complete explanation so I could really understand it, and not have to repost next time I have a differant sized chase or more/less cables

Thanks in advance.
 
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Old 05-14-11, 11:20 PM
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First, it is conductor derating, not cable derating. Essentially it is a code requirement that when more than three current-carrying conductors are run in a cable or raceway then you must follow certain rules to derate the Ampacity of the individual conductors. This derating is a percentage of the maximum Ampacity of the conductors, which varies according to the type of insulation on the individual wires and how many current-carrying conductors in total.

What this means is that equipment ground conductors are NOT counted for derating purposes. Neutral conductors, when part of a multi-wire branch circuit are also not counted since in these specific instances the neutral wire carries ONLY the unbalanced current of the "hot" wires.

For all practical purposes when using type NM-B cable you do not have to be concerned about derating until you have more than three cables. If using type THHN individual wires in conduit you can go as high as nine current-carrying conductors before derating becomes an issue. Understand that in a normal two-wire circuit the neutral IS a current-carrying conductor, the exemption of the neutral only occurs in a properly wired multi-wire branch circuit.
 
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Old 05-15-11, 01:21 PM
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OK, so just to make sure I understand this correctly, conductor derating for NM-B cable is not an issue when the conductors are located outside of a raceway?

Meaning that a stud bay or area between joists can contain x amount of conductors with no derating issues? I thought I'd heard otherwise....

Thanks.
 
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Old 05-15-11, 02:09 PM
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Originally Posted by Furd View Post
For all practical purposes when using type NM-B cable you do not have to be concerned about derating until you have more than three cables.
To expand on Furd's post, when you have more than three cables through the same hole of a joist or stud for 24" or longer is when you have to start derating cables.
 
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Old 05-15-11, 02:54 PM
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I would like to pull a #8 for a small stove, and three 12-2 cables through an area between joists that are 16" OC, for a length of approximately 8'.

Do I need to derate, and if so, what is the formula and what steps do I need to take based on how much derating occurs?

Thanks.
 
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Old 05-15-11, 03:05 PM
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Put the #8 in a separate hole and you will be fine.
 
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Old 05-15-11, 03:59 PM
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Do not "bundle" the cables. This means to not use the nylon wire ties to hold all cables close together for neatness. Do not run all cables through the same holes in the framing. Space the cables out in the stud space using staples or straps if the stud space is open.

Will this stud space be insulated? if no insulation then no problem. If it is insulated then there is a very slight possibility of future problems but in most cases it can be ignored.

The formula is for three or less current-carrying conductors no derating is required. For four through six current-carrying conductors the individual conductors need to be derated to 80% of their ultimate Ampacity as determined by table 310-16 of the National Electrical code. Seven through nine current-carrying conductors require derating to 70% and I don't off hand remember past nine. In all cases the ultimate Ampacity is based upon the type of insulation used on the individual conductors but the Ampacity of the conductor/insulation combination at a specific usage may not be exceeded.

Now I suspect that has thoroughly confused you so I will give an example. Type NM-B cable has the individual conductors insulated with a material that is rated at 90 degrees C. although it is not marked as such. For a number 12 conductor the ultimate Ampacity is 40 amperes but the Code also stipulates that #12 may not be protected at any greater than 20 amperes. If you had three 12-2 type NM-B cables you would have six current-carrying conductors and would be required to derate to 80% Ampacity if the cables were bundled. Using the ultimate Ampacity of 40 and taking 80% of 40 amperes gives you an ultimate Ampacity for the bundled cables of 32 amperes BUT the Code allows only 20 amperes for #12 conductors so in effect no derating is necessary.

If you have more questions just ask. The concept of derating is not all that difficult but it definitely can be confusing at first.
 
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Old 05-15-11, 07:55 PM
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Thank you. That is exactly what I was looking for. What had me hung up was the difference between ultimate Ampacity and what code required protection levels were. I guess I never really thought about ultimate Ampacity, but it would make sense to build in a generous safety factor.

I wasn't planning to bundle, was planning on separating with staples, and no insulation.

Thanks again. Interesting stuff.
 
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Old 05-16-11, 10:10 AM
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To expand a little on Furd's explanation there are also derating factors for environmental temperature. In many situations it can be ignored (certainly in my climate anyway), but if you are running through a very hot attic for example you should derate a cable knowing that it will operate in a high ambient temperature.

The code tables are based on assumed indoor ambient temp of 30C (86F); however in our example Arizona attic you might want to check the table for up to 60C (140F) ambient which would require a derating factor of 71% right off the top before considering any additional derating factor due to bundling. Likewise, cables known to be installed in a cool environment could actually have ampacity increased by up to something like 10% to account for the lower ambient temp.
 
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