Sparky, The 5% in the NEC is not a rule, but only a FPN.
Stu, On large circuits, oversized wire may inpact the available short circuit current requiring breakers with a higher interupting rating. I can't see any advantage to oversizing the conducotrs too much. However many times the cost of making the conductor one size larger wil be payed back in a year or two in the reduced electrtical usage if the circuit is almost always on.
Re: Voltage drop & over sized conductors#812 03/20/0112:43 AM03/20/0112:43 AM
My questions were related to fault currents. I think under some very special circumstances that some Voltage drop (IR loss) might be desirable to reduce available fault currents. Vision if you will a Day Care Center located on the 1st floor of a very large building, supplied by a utility network with over 200,000 Amps of fault current available. Being located on the 1st floor would place the Daycare's distribution very close to the main distribution & Service. Normally only two things on the load side of a transformer will reduce fault current: Current limiting fuses & the impedance of the conductors between the supply & the load. It would be unlikely for the branch circuits suppling this Daycare Center to be protected by fuses. They would be protected by circuit breakers, the problem, is that circuit breakers by them self don't due a hell of a lot to reduce those deadly fault currents. A circuit breaker with a high fault current rating is generally design to sit there a take several cycles of fault current before it can react and open its contacts. A fast current liming fuse will open within a 1/4 of a cycle there by preventing the fault current form reaching it's maximum destructive forces. Does anybody know were I going with this? ....to be continued - got to go now
Re: Voltage drop & over sized conductors#813 03/20/0101:56 AM03/20/0101:56 AM
On large circuits, oversized wire may inpact the available short circuit current requiring breakers with a higher interupting rating. How? What if the existing installation has #6 wire and the new equipment requires #12? #6 won't fit under the terminals of a 20A breaker. Can't you pigtail smaller wire onto the #6? Or must you pull new wire?
Re: Voltage drop & over sized conductors#814 03/20/0104:43 AM03/20/0104:43 AM
200KAIC Available!! Jeeze! Hope that's only at the Transformer's terminals Would be like a 1500 KVA 600 VAC 0.5% Z transformer - plus a gang load of motor contribution to boot!
If the 200KAIC was available after the main service, and at least 1/2 as high at the first sub panel, I would not even think of using breakers for anything - even the branch circuits!! Would be more like RK5 fuses for everything!!!
On the serious side now, I am just messing around here, not trying to be a jerk, or make fun of your message. You have a valid point here. SCA is a major factor with larger conductors. So is the Skin effect. I wouldn't imagine the fault levels could be that extreme, especially at some distance from the transformer [not using LRA motor contributions], but they could reach as high as 65KAIC if there were large subfeeders [like 500 KCMIL] and there was a lot more available at the service.
The branch circuits would be limited by their conductor sizes and lengths, but even then with 65K available at that sub panel, the branch breakers would need to be 65K also.
That NEC article about voltage drop is not a mandatory thing, it's more a suggestion. Some equipment manufacturers have specific tolerances that they also suggest their equipment be connected to. They are usually within 10% +/- of the RMS voltage at x temperature, and so on. Some have less tolerance, like +/- 1% maximum. All depends on the integrety of the equipment and the cost to replace burned out pieces [overhead, downtime and the part it's self].
Too much voltage drop on conductors creates too much heat generated in the conductors them selves, so there is a limit that should be set to where one will say that the voltage drop is becoming excessive.
A happy medium between limiting the voltage drop to a good enough level that will not overheat the conductors, while keeping the SCA within some safe area and trying to avoid too much skin effect is the way to go. With the SCA, upsizing one or two sizes wouldn't make the available level majorly higher, but if the sizes were upgraded twice the ampacity size, that would have a major factor in the overall SCA.
On the other hand, skin effect would probably assist in limiting the SCA in that situation...
Feel free to launch napalm on this message
Scott " 35 " Thompson Just Say NO To Green Eggs And Ham!
Re: Voltage drop & over sized conductors#815 04/24/0107:20 PM04/24/0107:20 PM
SCA is a major factor with larger conductors. So is the Skin effect.
Say, Scott, the way I took the question was hypothetically wiring a house using #10, #8, or even #6 on 20 A circuits.
1. Even if 2/0 were used, I don't see skin effect coming into play. Is skin effect dependent on the current being carried? I thought it was dependent on frequency and conductor resistance and acted to limit current.
2. To me, 2/0 is all skin at 60 Hz. If an arc fault is a much higher frequency, then that might limit it. Is this actually an issue?
3. The voltage drop will be lessened by using the larger conductors. This makes larger conductors sound advantageous.
4. I would like to hear more about what happens when the circuit is shorted. This makes larger conductors sound disadvantageous. Is there a prohibition or advisory against using ungrounded conductors which are significantly over capacity for the length of the run and the rating of the branch?
5. Is a short circuit on #6 significantly more dangerous than with #12 given that the OPD is a 20 A CB?
6. How about comparing short circuits in 8-2-G (120 V) to that in 10-3-G (240 V)?
These are some cases where I would rather ask you than trust my intuition.
Re: Voltage drop & over sized conductors#816 04/24/0109:50 PM04/24/0109:50 PM
the longer the conductors, further the X-former, the more R, the less SCA
at least the way i calc it....
I totally agree. But how much less is my question. If with a 20 A CB at 20 feet you get SCA of 100 A with 12-2 wiring and 200 A with 8-2, that might be significant.
But if the numbers are 40 A @ 120 V for 12-2 and 50 A @ 120 V for 8-2, that might not be significantly more considering that the total joules might be nearly the same (i.e., the OPD opens more quickly at the higher current).
And if with 12-3 (240 V) you get 80 A @ 120 V (i.e., twice what you got with 12-2) and this is an acceptable risk, then the 50 A @ 120 V for 8-2 must also be an acceptable risk.
I haven't any real numbers to make a real comparison. I have never measured SCA (and I don't believe that I would want to).
[This message has been edited by Dspark (edited 04-24-2001).]
Re: Voltage drop & over sized conductors#818 04/24/0111:35 PM04/24/0111:35 PM
Originally posted by Don'tKnowEverythingYet: Quoting:On large circuits, oversized wire may inpact the available short circuit current requiring breakers with a higher interupting rating. How? What if the existing installation has #6 wire and the new equipment requires #12? #6 won't fit under the terminals of a 20A breaker. Can't you pigtail smaller wire onto the #6? Or must you pull new wire?
I think I see where we are talking about several things at once... at least it looks that way now that I read all the responses in sequence.
First of all, I think resqcapt19 meant on the line side - meaning what is available in the panel box. So the CBs would need a rating that was higher than 10000 A. When he stated that oversized wire may impact the available short circuit current, he meant that it may increase it (in fact, would increase it at least slightly if not a lot).
In talking about oversized conductors it appears that you and I and perhaps others were talking about the load side - meaning what is available somewhere down a distant branch circuit.
Therefore, my questions and responses should be interpreted in the context of being after the OPD in a residential service. I was not thinking of a panel with 65 KA SCA available and then running #8 from a 20 A CB out to my 15 A receptables. However, from Stu T's follow up it appears that this might parallel where he was going... just on the branch side.
[This message has been edited by Dspark (edited 04-24-2001).]