I've posted on aspects of this before, and d-day is coming.
The situation is this: A fuse panel upstairs is being eliminated, and the #14 bx cable extended down about 10-15 feet to a new surface mounted breaker panel downstairs. Thirteen separate bx cables, 7 of which are 3 wire (Edison), for a total of
20 circuits.
Should I extend each bx cable with its own extension bx (or romex), or get one or two 3/4-1" greenfield cables with sufficient #14 conductors pulled through?
I'm leaning toward the greenfield, as it's just neater, less bulky, fewer knock-out connections, and I believe a lot more economical.
*I'd go with 3- 3/4" or 2-1" flexible conduits since you will have a total of 33 conductors plus a grounding conductor in each conduit. Using the conduits will make it easy to pull additional circuits in the future if needed. Single conductor wire is cheaper than BX cable.
Check tables C.3, 310.15(B)(2)(a) and 310.16 for more information.
in that scenario would you really need individual grounds? I'm thinking that you'd have one large grounding conductor between the panel and the trough and have a terminal strip in the trough to land all your existing "field" grounds. Is there some reason why that wouldn't work?
I ASSume the steel version does as well, I bought some premade fixture whips a while back and they had ground wires in them despite being basically steel Greenfield.
Now the really really old BX in my house, that just has a bonding strip inside the armor, no ground conductor, but that also has cloth covered wiring in it...
There are a couple locations where I'm really really tempted to try to use the two old wires to pull two plus ground new THHN through the BX armor, because it would save SO much work rewiring. However, I don't have a "plan B" if things go wrong...
*A grounding conductor is required for Type FMC (Flexible Metallic Conduit, AKA Greenfield) regardless of the metal composition for anything over six feet. Check 348.60, 250.134, 250.118(5)
I believe that a grounding conductor is required for each conduit run even though one ground may be sufficient. Someone else can probably confirm or refute this.
310.16 gives the allowable "ampacity" for the wire. Assuming it is #14 with *THHN* insulation the ampacity is 20A (but cannot be used at over 15A).
310.15(B)(2)(a) requires the ampacity to be reduced if there are more than 3 wires in a conduit. If you have 6 "current-carrying" wires, the ampacity is reduced to 80%. For #14 *THHN* the ampacity is then 16A and you can still use it on a 15A circuit. You can not have more than 6 "current-carrying" #14 *THHN* wires in a conduit and connect them to a
15A breaker.
310.15(B)(4) says that the neutral of a 3 wire "Edison" circuit does not count as one of the "current-carrying" conductors.
310.15(B)(5) says that ground wires do not count as "current-carrying" conductors.
You can run 3 "Edison" circuits, 9 - #14 *THHN* wires (6 "current-carrying" conductors) in a single conduit. Add a #14 ground wire.
Appendix C.3 says the 10 - #14 conductors can be run in a 1/2" Greenfield (aka flex).
For 2 wire circuits (if you do not combine them to be "Edison" circuits), you can run 3 - #14 *THHN* circuits in the same conduit - 6 "current-carrying" wires plus ground = 7 wires.
The rules are somewhat more complicated, but this is what should apply to what you are doing, assuming the installation complies with the NEC.
Since you are posting fairly often, do you have a copy of the NEC? Are you reasonably proficient at reading it?
For multiple circuits in one conduit you only need one ground wire for each conduit (where the conduit can not be used as the grounding conductor). The ground wire is sized for the maximum circuit breaker amp rating for any circuit (#14 for 15A) (NEC250.122). If circuit conductors are increased (such as to limit voltage drop) the size of the ground wire must also be increased.
Very helpful info, thank you. I have a 1996 NEC, and I *could* be proficient at reading it if the essentials weren't drowned in thousands of other words. But yeah, the tables in App. B and C do get to the point.
It seems that many others have trouble reading the NEC, as there is a whole cottage industry of workbooks and guides in how to read, understand, and use the NEC. wow....
I just came home with 100 ft of 3/4 greenfield, two 500 ft rolls of #14 wire -- which, btw, just jumped $5 a roll in the past 3 days. Man, I hate gving money to HD -- who nevertheless charge in some cases quite a bit less than electrical supply houses for wire -- but a hair more for greenfield.
*The supply houses that I deal with adjust their wire prices daily according to the market. Home Depot does it I presume as they replenish. I have found Home Depot higher and lower depending on which way copper is going. Unfortunately I don't always have time to shop around.
The NEC has been revised 4 times since your 1996 copy. Revisions can change often used sections. And numbering in some articles has been changed, so references may not match your copy.
The NEC is not laid out to be easy to learn (it is a reference). Information on installing an outlet, for instance, is in buried in several articles. Good reason for the cottage industries.
What I wrote was based on THHN insulation. That is probably what you bought, but not necessarily. The wire is probably marked THHN/THWN. If not, the number of wires in a conduit is likely different.
But I managed to read the wrong column for ampacity. Particularly annoying because the value I read was not what I remembered. What I wrote is revised below.
310.16 gives the allowable "ampacity" for the wire. Assuming it is #14 with *THHN* insulation the ampacity is 25A (but cannot be used at over 15A).
310.15(B)(2)(a) requires the ampacity to be reduced if there are more than 3 wires in a conduit. If you have 7-9 "current-carrying" wires, the ampacity is reduced to 70%. For #14 *THHN* the ampacity is then 17.5A and you can still use it on a 15A circuit. You can not have more than 9 "current-carrying" #14 *THHN* wires in a conduit and connect them to a
15A breaker.
310.15(B)(4) says that the neutral of a 3 wire "Edison" circuit does not count as one of the "current-carrying" conductors.
310.15(B)(5) says that ground wires do not count as "current-carrying" conductors.
You can run 4 "Edison" circuits, 12 - #14 *THHN* wires (8 "current-carrying" conductors) in a single conduit. Add a #14 ground wire.
Appendix C.3 says the 13 - #14 conductors can be run in a 1/2" Greenfield. It is the maximum number and you would have to pull the wire carefully. With 3/4" Greenfield you shouldn't have a problem
For 2 wire circuits (if you do not combine them to be "Edison" circuits), you can run 4 - #14 *THHN* circuits in the same conduit - 8 "current-carrying" wires plus ground = 9 wires.
When you are talking about 14, 12 and 10 gauge, don't forget 240.4(D) that effectively limits that to 15a, 20a and 30a respectively, except for certain fixed loads. You always have to use 240.4(D) if receptacles or lamp holder are involved since the installer has no control over what the user will attach..
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