explain de-rating please

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  #1  
Old 07-11-06, 09:37 AM
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explain de-rating please

Can someone explain de-rating to me? This came up in another thread and I want to understand what to do. For example, I'm looking at an existing 3/4" conduit (emt) and I would like to pull 3 circuits through it, one 20a and two 15a. I'll have more than 3 conductors in there so I'm supposed to derate them, but I don't know what that means. I just looked in my book and saw that I could have 11 wires if they were 12g so I figured I'm fine with 9 (and that includes grounds). help?

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Last edited by mattison; 07-12-06 at 05:54 AM.
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  #2  
Old 07-11-06, 09:47 AM
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Derating

Originally Posted by newoldhouse
Can someone explain de-rating to me? This came up in another thread and I want to understand what to do. For example, I'm looking at an existing 3/4" conduit (emt) and I would like to pull 3 circuits through it, one 20a and two 15a. I'll have more than 3 conductors in there so I'm supposed to derate them, but I don't know what that means. I just looked in my book and saw that I could have 11 wires if they were 12g so I figured I'm fine with 9 (and that includes grounds). help?
The rating of conductor by the NEC is given in table 310.16 for
a max of 3 conductors in conduit. When you have more that 3 conductors the rating of the conductor must be reduced.
Table 310.15.B.2 give you the % reduction. If you have 3 circuits I assume you will have 6 conductors. Using the table,
the reduction in rating is 80%. You take the rating given in table
310.16 and multiply by 0.80. If you are using a conductor with 90C insulation rating such as THHN, you use the 90C rating for the reduction. Example if you use #12 thhn, the 90C rating is
30 amps x 0.80 = 24 amps. However you may not exceed the 20
amp rating at 75C. The reason is that the breaker terminal is rated at 75C.
Derating has nothing to do with the conduit fill.
 
  #3  
Old 07-11-06, 09:51 AM
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One other comment. When running in conduit, you only run one ground wire, sized for the largest current carrying conductor.
 
  #4  
Old 07-11-06, 12:19 PM
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Electric current flowing through a wire generates heat, for any given wire size the greater the current flow the greater the heat generated.

Electrical wire is insulated with various materials and the various combinations are given somewhat arcane names. The most common insulation is thermoplastic and it has the term T. Type T wiring may only be used at a maximum insulation temperature of 60 degrees C. or 140 degrees F. For higher temperatures different formulas for the plastic are used and the most common type of single conductor wire today has the designation THHN/THWN This denotes a thermoplastic insulation (T) that has been compounded to be used at a temperature of 90 degrees C. (HH) and has an overall jacket of nylon (N). This wire is also suitable for use in wet areas or in underground conduit as denoted by the W in the THWN descriptor.


De-rating of conductors is necessary in two major instances, the first is where the ambient temperature, that is the temperature of the immediate surroundings, exceeds 30 degrees C. (86degrees F.) and the second is where more than three current-carrying conductors are contained in the same raceway or conduit.

The reason for the ambient temperature correction is because the higher the temperature of the surroundings the slower the internal heat of the conductor can be dissapated which leads to the conductor's internal temperature rising above the failure point of the insulation. For the multiple conductor correction it is because more wires simply means more heat and less ability to radiate that heat into the surroundings.
 

Last edited by Furd; 07-11-06 at 03:35 PM.
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Old 07-11-06, 01:06 PM
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Originally Posted by furd
...has been compounded to be used at a temperature of 90 degrees C. (TT)
I know it was just a typo on your part, but since you were explaining an acronym, I'll point out that you meant the "HH," instead of a "TT."
 
  #6  
Old 07-11-06, 01:39 PM
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It is generally recommended for residential to keep to 9 conductors or less in the smaller guages (14 and 12 awg) when running in conduit. This negates any derating that would result in an ampacity less than the rating of a 12 awg or 14 awg branch circuit. In your case you will have 6 conductors so you have no problems with derating. Ground wires do not count as conductors nor do true neutrals.
It is also generally recommended to run a ground wire as Bob mentioned when using EMT in a DIY installation. Note.. however... that the Emt metal conduit when properly installed can act as your ground wire and no ground wire is required. I only say this for your education I recommend you run your equipment ground wire sized as Bob stated.

Roger
 
  #7  
Old 07-11-06, 02:17 PM
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thanks everyone, that was educational. The first one referencing the tables was just confusing.

am I correct in saying that de-rating basically (I said basically) means is more conductors can equal more heat which means that the conductors can't carry as much electricity as they might otherwise and you have to account for that with larger wire size, but also have to pay attention to not melting the insulation and starting a fire?

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Last edited by mattison; 07-12-06 at 05:55 AM.
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Old 07-11-06, 03:36 PM
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Originally Posted by MAC702
I know it was just a typo on your part, but since you were explaining an acronym, I'll point out that you meant the "HH," instead of a "TT."
Thank you, absolutely correct. I have edited my original post.
 
  #9  
Old 07-11-06, 03:44 PM
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Originally Posted by newoldhouse
thanks everyone, that was educational. The first one referencing the tables was just confusing.

am I correct in saying that de-rating basically (I said basically) means is more conductors can equal more heat which means that the conductors can't carry as much electricity as they might otherwise and you have to account for that with larger wire size, but also have to pay attention to not melting the insulation and starting a fire?
That's pretty close, more conductors mean more electric power flow which means more heat in the concentrated space of a conduit. To reduce the additional heat the individual conductors are limited to lower current/power flows than would be normal for only three conductors.
 
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Old 07-11-06, 04:23 PM
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Originally Posted by Roger
Ground wires do not count as conductors nor do true neutrals.
Roger
By "true neutral" I assume you are referring to a load that is normally balanced and has no current flowing in the neutral lead. Since that rarely happens in a single-phase circuit the neutral is indeed a current-carrying conductor.
 
  #11  
Old 07-11-06, 10:48 PM
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Since that rarely happens in a single-phase circuit the neutral is indeed a current-carrying conductor.

Not at all. I'm talking about a 120/240 single phase residential supply for a home that is using multiwire branch circuits. The neutral does not count as a conductor. These are common as red apples on a tree in October. What about a 120/240 stove or dryer? Would you count their neutrals? A Neutral does not have to have zero current only that it serves more than one ungrounded conductor and carries the unbalanced load of those conductors of the same circuit. There are exceptions but not in single phase 3 wire 120/240.
If you have a 208Y120 with 6 hots and 2 neutrals would you count the neutrals as conductors assuming the loads are linear?

Technically a 2 wire with ground 120 volt single phase circuit has no neutral.
 

Last edited by Roger; 07-12-06 at 10:07 AM.
  #12  
Old 07-12-06, 02:59 PM
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I see your point and I agree with the physics behind it. Unfortunately the code specifically states a normally balanced load and single phase loads are rarely balanced, either on a 240/120 three-wire circuit or a four-wire three-phase circuit. As a result there usually is a current, admittedly only the "unbalanced current", flowing on the neutral conductor.

Yes, when I run 208/120 three-phase, four-wire for multiple single phase loads I do count the neutral.

Maybe I am wrong but it is an error on the side of safety.
 
  #13  
Old 07-12-06, 03:18 PM
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The number of current carrying conductors for purposes of 310.15 is different than the total number of conductors with some current in them. The code really should use a phrase such as 'effective number of heat producing conductors' or some such, rather than 'current carrying'.

Derating is all about heat production. When you have a multiwire branch circuit on a 120/240V supply, the state of maximum heating occurs when both circuits are fully loaded. In this situation, the neutral carries no current and produces no heat. In any other condition, say with one circuit fully loaded and partial load on the second circuit, then all three conductors will be carrying some current, but the _total heat_ produced by all three conductors will be less than or equal to the heat produced with both circuits fully loaded. So from a thermal point of view, when you have three conductors in a multi-wire branch circuit, even though all _three_ usually have some current flow, the heating effect is that of two wires.

-Jon
 
  #14  
Old 07-12-06, 04:01 PM
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I see your point and I agree with the physics behind it. Unfortunately the code specifically states a normally balanced load and single phase loads are rarely balanced, either on a 240/120 three-wire circuit or a four-wire three-phase circuit. As a result there usually is a current, admittedly only the "unbalanced current", flowing on the neutral conductor.
I thought we we discussing 310.15 and whether the neutral is counted as a current carrier for deration purposes??? Not that it carries current...So what are you saying that because it is a current carrying wire it counts for deration purposes for multiwire circuits? Fortunately the code is also quite clear about this also.

Yes, when I run 208/120 three-phase, four-wire for multiple single phase loads I do count the neutral.
Then your boss wouldnt be overly joyed because you were installing extra conduit runs or upsizing wire due to incorrectly counting the neutral in your circuit design for the linear loads.

Maybe I am wrong but it is an error on the side of safety
I dont see where there is anything unsafe to be worried about in the situations we have been discussing.

This thread is probably a little to argumentive but I just dont follow your logic to count wires for deration purposes that are not required to be considered.

Jon..... I really like that explaination..simple but accurate.


Roger
 
  #15  
Old 07-12-06, 07:47 PM
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As I stated, I understand the physics and I understand what winnie explained concerning the heating effects due to current flow.

Certainly the case can be made for counting the neutral conductor when a large (whatever the definition of large may be) portion of the load is non-linear. The code also points out that loads served by two phases of a three-phase four-wire system does indeed count the neutral as a current-carying conductor.

I'm really not trying to be aurgumentative, just trying to understand the wording of the code and why my interpretation of the letter of the code does not follow the physics of the real world. I understand that barring significant non-linear loads that the neutral current will only be the unbalanced portion of the load served by the "hot" conductors.


As in so many areas of the NEC the wording could be better.
 
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