In this installation it carries the full load on it back to the distribution point. a single 120V single phase branch circuit, such as a general purpose receptacle outlet circuit conductor, it counts as a current carrying conductor. If the (neutral) grounded conductor is NOT shared between phases, i.e. If you have electronic fluorescent ballasts or heavy computer loads (etc.) on a shared branch circuit neutral, it will have harmonic distortions that will disrupt the cancellation of loads between phases and can actually cause larger loads on a neutral that are found on any single ungrounded phase conductor in the circuit. Where non-linear loads are supplied however the neutrals DO count. If it is shared between two different phases in a multi-wire circuit, it usually does not count as a current carrier, as it only carries the unbalanced load in the circuit. It counts as a "current carrying conductor" in some cases, but not in others. The grounded conductor (neutral) can also cause confusion sometimes. You can quickly see that in our earlier example, if we put 16, #12 THHN conductors in that 3/4" conduit, we would have to drop the circuit breaker to a 15 ampere size! Now seriously, how many electricians in the field really do that? You understand now how dangerous the casual use of Annex "C" can be.įor all practical purposes, only 9 current carrying conductors could be placed in that conduit without having to drop the OCPD (circuit breaker) size: "Adjustment Factors for more than Three Current carrying conductors in a Raceway or Cable" The de-rating table T310.15(b)(2)(a) on page 145 shows us: When de-rating wire, we begin with table 310.16 (page 147, 2008 NEC ©) and de-rate for our ambient temperature AS WELL AS the number of current carrying conductors. It does NOT take the ampacity of the wire into consideration! Annex C is only concerned with the protection of the wire during installation. this is only for the protection of the wire from a physical stand point. Annex C tables show that we can safely place a total of 16 of this size (Type THHN, THWN, or THWN-2) in a 3/4" EMT conduit. We normally use a 20 amp circuit breaker to protect #12 AWG cu wire. Annex "C" in the back of the 2208 NEC © is a good tool, but it can be very dangerous to use it without considering the impact of de-rating factors found in Chapter 3. There seems to be a lot of common confusion out there when it comes to wire fill. This week I want to take a minute to review our wire ampacity and conduit fill (wire fill) calculation. Wire Ampacity and Conduit Fill Calculation
0 Comments
Leave a Reply. |
Details
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |