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| Subject - Derating | |
| relbas |
Can you direct me were to get explanation on derating. NEC require for the derating of conductors if over 3 in the conduit, My question let say I am pulling 6 phase conductors with 2 shared neutrals, in this scenario can I use 20 amp breaker or I need to derate? |
| in2serenity | 310.15 (b)(2) A (2002) the breaker doesn't get derated, its the current carrying capability of the conductor that gets derated. 7-9 conductors would be derated to 70% of the origional ampacity. For this many conductors you would need #10's for 20 amps. |
| kbsparky | quote: When calculating Amperage with derating tables, you begin with the listed Amps in the table, and then apply your 70%. In the case of #12 wires, the listed Amps = 30. After applying the derating factor of 70%, you end up with 21. You would be allowed to use #12 conductors with 20 Amp breakers in such a scenerio. Even if your conductors were found in the 75° column, the allowable Amps = 17.5, and you are permitted to go to the next higher standard size breaker, which happens to be 20 Amp. However, in the original question posted here, it was given that 6 conductors were present, with 2 shared neutrals. If you look at 310.15(B)(4)(a) you will find that the neutral need not be counted in applying the dreating. That changes the percentage to 80. Of course, this is a moot point, as the limitations originally placed on #12 wires is 20 Amp, and that has already been established as sufficient for this case.  -Ken |
| in2serenity | Sorry for the bad info. 
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| relbas | From your explanation: (30 Amp raiting of#12 thhn) does it mean that I can load #12 thhn to 30 amp as long that I have 3 conductors in the raceway or less. I always use # 10 for 30 Amp loads and not #12. Correct me If Iam wrong: if I am pulling (3/4 conduit) 9 hots with 3 nutrals I am still under 70% derating(less than 10 conductors for 70%)? And thank you again |
| kbsparky | >>...From your explanation: (30 Amp raiting of#12 thhn) does it mean that I can load #12 thhn to 30 amp as long that I have 3 conductors in the raceway or less...<< No. The 30 Amp "rating" is what you begin with before applying your derating factors, for conductor fill or high ambient temperature. The Code explicity states that the max. overcurrent protection on that #12 is 20 Amps. That gives you some wiggle room to allow for semi-crowded conduits without having to increase your wire size, or decrease your overcurrent protection. >>...I always use # 10 for 30 Amp loads and not #12....<< This is correct. >>...Correct me If I am wrong: if I am pulling (3/4 conduit) 9 hots with 3 neutrals I am still under 70% derating(less than 10 conductors for 70%)?...<< You are correct again. As pointed out before, even with the 70% derating applied, you can still use #12 wires with 20 Amp overcurrent protection on such an installation. -Ken |
| relbas | Mr kbsparky. Thank you very much you explan it 100% |
| dcid420 | kbsparky, im not sure if i agree with you about using 310-15(B)(4)(a). This might just be a matter of interpretation, but I learned that you can use that article with a single phase system, like in residential. But in this case, I think you have to go by 310-15(B)(4)(c) "On a 4 wire, 3 phase wye circuit where the major portion of the load consists of nonlinear loads, harmonic currents are present in the neutral conductor, the neutral shall therefore be considered a current-carrying conductor". Then you have to count the neutral as a current carring conductor, so with 6 hots and 2 neutrals, you need to count 8 current carring conductors, and derate to 70%, which with thhn, you can still use #12s on a 20amp breaker. But with 9 hots and 3 neutrals, you have to count 12 current carrying conductors, and derate to 50%, which would mean you need to use #10thhn after derating. Which would then bring up the issue of pipe fill. If you are using EMT, Then you can only fit 10 #10thhn in a 3/4 conduit, and you would have to split the circuits into 2 separate 3/4 pipes, or run a 1" pipe. any commments from others? Derating seems to be a subject with many differing opinions, and with only 5 years in the trade, I'm not gonna try to profess that I am an expert on this subject, but that is the way I have learned it. Dave |
| KSsparky | Ken is correct, you only count the ungrounded conductors as current carrying under the circumstances in 310.15(B)4(b)&(c). There is no distinction, that I see, as to whether it is single phase or otherwise. |
| dcid420 | Hmmmmm, I had to go back to my school books after reading all the responses. I think there is a clear distinction in 310.15(B)4(b)&(c) that it is for a 3 phase, wye connected system. To me it seems clear that according to 310.15(B)4(b), when two out of three phases are sharing a neutral, you have to count the "common" as a current carrying conductor (unless I have the wrong assumption that the "common" is the neutral) Hope I'm not making a fool out of myself for assuming. Am I missing something hear??????????????????? As for 310.15(B)4(c), regarding whether the load is linear or non-linear. electronic equipment, electric-discharge lighting, adjustable speed drive systems, and similar systems may be non-linear, according to art.100 FPN. So in this day and age of electronics that you can plug into an outlet, shouldn't we assume that if these circuits are used for outlets, that non-linear loads will be plugged in (assume the worst), and therefore count the neutral as a current carrying conductor. And if the circuits are for fluorescent lighting, the same would apply for the electronic ballasts. I would think that unless the circuits are going to be used for purely resistive loads, such as heaters, then there will be harmonic currents in the neutral, and you should count it. I'm glad theres other people that enjoy discussing these things, its a darn good way for young grasshoppers like me to learn, one day I'll even teach an oldtimer a thing or two  Dave |
| kbsparky | >>..."On a 4 wire, 3 phase wye circuit where the major portion of the load consists of nonlinear loads, harmonic currents are present in the neutral conductor, the neutral shall therefore be considered a current-carrying conductor"....<< Sorry, I should have qualified my response for linear loads. I enjoy these discussions and exchanges. It's good to keep us in practice in using the Code and up to date on the issues.
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| lctrc789 | On 3 phase load all neutral conductors count as current carrying conductors, on single phase they do not. |
| stedder | kb spark I agree I find myself digging, I'm online w/ the 2005 right in front of me, it's good and refreshing, uggh |
| Ryan_J | quote: If the circuit is a multiwire with all three phases and it is balanced, it does not count. |
| lctrc789 | Ryan, I was under the impression that all 3 phase neutrals were counted in derating as taught by the Neca-Ibew and current code classes article 310-15 (b)4 a-b-c. In a single phase system the neutral will counteract the balance at zero. In three phase systems, a wye connected system Phase current and line current are the same. However a wye connected system the line voltage is higher then the phase voltage by a factor of the sq. root of three. They cannot truly balance as a single phase system. We are always taught to count all 3 phase neutrals as current carrying conductors in a conduit for that reason. |
| Ryan_J | Pat, the formula for three phase nuetral current is as follows, with (^) representing the sqaure. Total nuetral current = the sqaure root of (A^+B^+C+)-(AB+BC+CA). If you have the same numbers for A, B and C, you will have zero on the nuetral. |
| Ryan_J | Yes Dave, if you have a 4 wire wye system and you only use three of the wires, the nuetral current will as much as the ungrounded conductors current. Try plugging in some values into the formula I posted above. :) |
| relbas | "Yes Dave, if you have a 4 wire wye system and you only use three of the wires, the nuetral current will as much as the ungrounded conductors current. Try plugging in some values into the formula I posted above." The neutral will carry only the unballance load of 3 phase system. How did you come out that the neutral will carry the load of ungrouned conductor?Isn't this the basic of 3 phase system? |
| Ryan_J | Alex: If you have 10 amps on phase A, ten amps on phase B and 0 amps on phase C, the nuetral will carry 10 amps. Once again, put some numbers into the formula and try it. |
| Ryan_J | I'm getting E-mails saying that I am wrong. Here is an example of where phase A carries 10 amps, phase B carries 10 amps, phase C carries 10 amps, and the grounded conductor carries 10 amps: Sqaure root of (A^+B^+C^)-(AB)+(BC)+(CA)= Toal Nuetral Current (In). (10^+10^+0^)-(10*10)+(10*0)+(0*10)=In (100+100+0)-(100+0+0)=In 200-100=In 100. The sqaure root of 100 is 10 amps. |
| dcid420 | Thanks Ryan That formula works for me, as long as the loads are linear. Now hears a stupid question. Do you have to count the grounded conductor/s as current carrying if you run a sepatate grounded conductor for each phase on a 3-phase wye system, like you do on a single-phase system? I was under the impression that you do not have to count them on a 3-phase system, but all this discussion got me thinking. Hmmmmm |
| Ryan_J | Yes, you would have to count them as current carrying. The grounded conductor would once again be carrying as much current as the ungrounded conductor. |
| Energreen | First things first... What is the hottest ambient temperature at any point along the run? Will it run through a hot attic? The values listed in T310-16 must be corrected to compensate for ambient temperature before derating for more than 3 conductors in a raceway (see bottom of table). If the run passes adjacent to an oven, furnace or kiln, for example, or on a rooftop in a hot climate, that makes a difference! (Temperatures of 140 deg. F have been measured one foot above an asphalt/tar roof.) Also, how long is the conduit and where is it run? There are exceptions. EG |
| relbas | Ryan Sorry It was my mistake I was pluging 10 Amp 8 Amp and 0 amp |
| lctrc789 | I agree with Ryans formula, the problem is that you never know what will be a linear load and what will not be as well as you will not always have a balanced load an a 3 phase system if they are all pulling different currents at different times. That is why we are taught to count all 3 phase system neutral as current carrying conductors.ALWAYS |
| binney | I’ve never calculated out the current on the Neutral in 3-ph before. I’ve always assumed if my circuits were close to being balanced and used for linear loads that I had no need to count the N as current carrying. A B C N 10 10 10 0 10 10 0 10 6 15 9 63 10 10 2 64 16 16 0 256 16 16 2 196 16 16 14 4 Shut off a few switches, and the amps jump down on 1 phase and the N sky rockets. Am I doing the math right??????????? I can’t be!! What to harmonic loads do to the Neutral??? |
| binney | I didn't take the square root of the last # try these #'s. A B C N 10 10 10 0 10 10 0 3.16 6 15 9 8 10 10 2 8 16 16 0 16 16 16 2 14 16 16 14 2 These #'s make much more sense. By looking at these #'s the N current is never more than the difference between the phase currents. Seing heat is directly related to current and the current doesn't increase, I see no reason to add another current carrying conductor. If the conductors are loaded properly there is no current on the N, even if there is a temporary change in current there is still no more heat than originally. Just my thinking, I wouldn't bet my life on it at this time. I'll continue to reasearch this. 
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| PEI | The use of Pythagorean theorem to describe the magnitude of the current in these circuits is acceptable, but realize these are vector sums, meaning that they have both a value and direction (magnitude at an angle). For instance, 208 volts at 0 degrees; and 3-phase would have 2 more phases of 208 Vrms at 120 degrees and a 3rd of 208 Vrms at 240 degrees. What I am getting at is that when you add 2 phases of current, you are adding vectors of 2 currents as a result of applied RMS voltage of 208, where the phase angles are 120 degrees out of phase. Assuming balanced loading, the sum of 2 phase currents, 120 degrees out of phase of the other, would be the same magnitude of that current in either phase with the resultant angle in the middle of the 2 phase angles. Introducing the 3rd phase on that neutral, again assume a balanced load on the 3 circuits, will cancel the current to zero in the shared neutral. NOW THE BIG SITUATION. Can you guarantee that you will always have all 3 circuits on, that they are always balanced (exactly) and that each are linear loads - all so you can claim the neutral is non-current carrying ? Thus, my contention is that only a 3-phase, 4-wire circuit, going to a 3-phase motor, for example, could the neutral be considered non-current carrying. Realize that only in purely resistive loads (linear) do the phase angles of the currents stay at the same phase angle as the voltages. When the circuit feeds a capacitive (rarely) or inductive (motor) load, the phase angle of the current is shifted slightly. Thus, 2 circuits feeding incandescent lighting and a 3rd feeding a motor, even with your ammeter reading the exact same on each circuit, the current would NOT be zero in the shared neutral. Harmonics is a different problem as it accomplishes no useful work and creates heating due to non-sinusoidal wave forms and frequencies above 60 hertz, further complicated by the lighting ballasts operating more randomly in their generation of these harmonics such that cancellation could never be achieved. NOW, do you believe your inspector would catch it, or are we in pursuit of academia? |
| Ryan_J | If I have a four wire circuit that is balanced most of the time, I would not count the nuetral. Even if you turned one phase off and therefore put curent on the nuetral, all you did was add one current carrying conductor and subtract another. Either way, you would have three current carrying conductors, not four. |
| PEI | Excellent argument, do you think you could win that with an inspector? Does anyone ever consider say, 5%, to be a negligible imbalance and thus you do not have to consider the neutral as a current carrying conductor? I will never tell you that twelve identical, 2-tube fluorescent fixtures on each of the 3 phases, would not yield some small unbalanced current in the neutral - but it would be negligible. After all, 5% of 20 amps is only 1 amp (and the random variation between lamps, wire lengths and ballasts from this example would not even account for a 1 amp variation). Then, even if the conductor was 100 feet long, you wouldn't have 1/2 of a watt over the length. Not to mention not all of that 1/2 watt would even be dissipated as heat! Should I petition for a change in 2008 ? I think you guys (and ladies) are doing an outstanding job in this forum. The information is accurate and thorough. Keep it up. |
| Ryan_J | quote: I am an inspector. I also use this argument when I teach inspectors, so hopefully some of the inspectors in my area see it this way as well
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| Ryan_J | quote: I think it would be difficult to get it to pass. In my opinion, they don't go into too much detail in 310.15(B)(2) on purpose. They have some general rules, but they really are pretty general. I think you would have to have a strong technical substantiation to back the proposal, which might be difficult to do. |
| blackrd | Now that was a good discussion |
| RS377 | eh, sort of This NFPA article explains it without getting too brainy http://www.nfpa.org/itemDetail.asp?categoryID=559&itemID=18401&URL=Publications/necdigest®/necdigest®%20Magazine/necdigest®%20archives/Winter%202003/Ask%20the%20Experts&cookie%5Ftest=1 |
| skynrd | I've been in the trades long enough to remember seeing wires burnt up in a 3 phase circuit that had a shared neutral. It took the NEC panel to long to realize the potential hazards. The only good practice for sharing neutrals on a 3 phase system is when doing a fixed incandescent lighting installation. Otherwise just count it and save the arguement when the inspector shows up. |
| RS377 | quote: The NFPA seems to disagree with you. |
| Ryan_J | quote: Me too. |
| lucky1122 | When derating conductors for fill. Firstly the ampacity of the wire which is to be used is based on the insulation of the wire not just the gauge of the wire . You will notice that #12 THHN has an ampacity of 30 A in the 90 deg col. ; whereas,THWN in the 75 deg column is 25 A. That is key to derating . If you had 6 #12 THHN ungrounded conductors with 2 shared neutrals you would have to determine if the neutrals meet the requirements of 310.15b4 for a neutral with no harmonic loads, and I will assume that they will be properly loaded as to phase assignments so they are true neutral conductors . If they dont, they must be calculated on the basis of being current carrying conductors . that means you would be required to use the ampacity in the 90 deg col (for derating only) which is 30 . Multiply it by the percentage of wires in table 310.15(b)(2)a under 8 conductors which is 70% for from 7-9. The ampacity is 21 amperes after derating is complete . Now provided you donot have any ambient temperature issues which would necessitate further derating of the conductors you may protect such a run with 20 ampere breakers . Remember : you may use the insulation ampacity to derate for fill but you may never use it to increase the operating ampacity of the wire. This would violate the temperature requirements of 110.14(c) which would allow the temperature of the wire to operate above the 60 degree operating temperature of the wire. Its a little complicated but an important subject. If the neutrals were true neutrals and that would be largely determined by the loads served and the nature of the same. You would not need to count them. however the odds of that are slim seeing that ballasts,computers, printers ,all introduce the non linear load factor which necessitates treating neutrals at the very least current carrying conductors . In actuality it may be necessary to either oversize the neutral or run dedicated neutrals to compensate for the same. hope that helps. |
| lucky1122 | When derating conductors for fill. Firstly the ampacity of the wire which is to be used is based on the insulation of the wire not just the gauge of the wire . You will notice that #12 THHN has an ampacity of 30 A in the 90 deg col. ; whereas,THWN in the 75 deg column is 25 A. That is key to derating . If you had 6 #12 THHN ungrounded conductors with 2 shared neutrals you would have to determine if the neutrals meet the requirements of 310.15b4 for a neutral with no harmonic loads, and I will assume that they will be properly loaded as to phase assignments so they are true neutral conductors . If they dont, they must be calculated on the basis of being current carrying conductors . that means you would be required to use the ampacity in the 90 deg col (for derating only) which is 30 . Multiply it by the percentage of wires in table 310.15(b)(2)a under 8 conductors which is 70% for from 7-9. The ampacity is 21 amperes after derating is complete . Now provided you donot have any ambient temperature issues which would necessitate further derating of the conductors you may protect such a run with 20 ampere breakers . Remember : you may use the insulation ampacity to derate for fill but you may never use it to increase the operating ampacity of the wire. This would violate the temperature requirements of 110.14(c) which would allow the temperature of the wire to operate above the 60 degree operating temperature of the wire. Its a little complicated but an important subject. If the neutrals were true neutrals and that would be largely determined by the loads served and the nature of the same. You would not need to count them. however the odds of that are slim seeing that ballasts,computers, printers ,all introduce the non linear load factor which necessitates treating neutrals at the very least current carrying conductors . In actuality it may be necessary to either oversize the neutral or run dedicated neutrals to compensate for the same. hope that helps. |
| lucky1122 | When derating conductors for fill. Firstly the ampacity of the wire which is to be used is based on the insulation of the wire not just the gauge of the wire . You will notice that #12 THHN has an ampacity of 30 A in the 90 deg col. ; whereas,THWN in the 75 deg column is 25 A. That is key to derating . If you had 6 #12 THHN ungrounded conductors with 2 shared neutrals you would have to determine if the neutrals meet the requirements of 310.15b4 for a neutral with no harmonic loads, and I will assume that they will be properly loaded as to phase assignments so they are true neutral conductors . If they dont, they must be calculated on the basis of being current carrying conductors . that means you would be required to use the ampacity in the 90 deg col (for derating only) which is 30 . Multiply it by the percentage of wires in table 310.15(b)(2)a under 8 conductors which is 70% for from 7-9. The ampacity is 21 amperes after derating is complete . Now provided you donot have any ambient temperature issues which would necessitate further derating of the conductors you may protect such a run with 20 ampere breakers . Remember : you may use the insulation ampacity to derate for fill but you may never use it to increase the operating ampacity of the wire. This would violate the temperature requirements of 110.14(c) which would allow the temperature of the wire to operate above the 60 degree operating temperature of the wire. Its a little complicated but an important subject. If the neutrals were true neutrals and that would be largely determined by the loads served and the nature of the same. You would not need to count them. however the odds of that are slim seeing that ballasts,computers, printers ,all introduce the non linear load factor which necessitates treating neutrals at the very least current carrying conductors . In actuality it may be necessary to either oversize the neutral or run dedicated neutrals to compensate for the same. hope that helps. |
| lucky1122 | I think personally that the time for thinking of true neutrals as non current carrying conductors is fading fast |
| lucky1122 | I think personally that the time for thinking of true neutrals as non current carrying conductors is fading fast |