I just read the article about service conductors, feeders and branch circuits.
I'm guessing it's the tap conductor thing I'm having more problems with
I'm guessing it's the tap conductor thing I'm having more problems with
guitarboyled
Service conductors are the supply to a service. Taps come out of a service to power other things.
I read the definition but I guess it's hard for me to visualise what a tap conductor is.
"Basically a conductor is considered a tap conductor where it has overcurrent protection higher than its rated ampacity."
Does this mean for instance the rated ampacity of 14 AWG is 20 amps but if the conductor is protected by a 30 amp fuse or breaker it becomes a tap conductor?
"Basically a conductor is considered a tap conductor where it has overcurrent protection higher than its rated ampacity."
Does this mean for instance the rated ampacity of 14 AWG is 20 amps but if the conductor is protected by a 30 amp fuse or breaker it becomes a tap conductor?
No, not at all. Take a feeder that is run from a panel to a trough and from there smaller ampacity conductors are spliced or tapped onto the larger feeder. These smaller conductors are not fused at their given ampacity but are protected by the larger feeder conductor. These wires must meet certain conditions, as mention in the graphs above, to be allowed to be protected at the larger ampacity.
The CEC and NEC have somewhat different definitions.
What you read is not an unreasonable description, from a CEC point of view.
Say you have a disconnect feeding a trough. The disconnect is a 200A unit, fused at 200A, feeding a 200A trough. The wire from the disconnect to the trough's distribution blocks is 250kcmil, as is appropriate. Now, from the distribution blocks you are feeding half a dozen 60A disconnects fused at 50A, and powering motors. The wire from the distribution block to these 60A disconnects is #6, as is appropriate for the 50A load. The upstream disconnect on these conductors is the 200A disconnect. You have, in that respect, #6 wire protected by a 200A fuse, which is ridiculous. This situation - a conductor protected by a fuse of a higher rating than the conductor - is what makes it a tapped conductor. In practice it isn't protected by the 200A disconnect, but rather by the 50A fuse in the disconnect that it is powering. This is a somewhat dodgy way to do business, because if the tapped conductor shorts out along its length then its over-current protection won't work. This is why tapped conductors are treated specially in 28-110(2) which limits their length and application. The alternative would be to wire up all of your 60A disconnects with 250kcmil conductor. Nope - not gonna do it. The conductor wouldn't even fit in the lugs.
Take a look at the extra info on 28-100 in appendix B of the CEC. There is a diagram there that should be memorized by every Canadian sparky.
What you read is not an unreasonable description, from a CEC point of view.
Say you have a disconnect feeding a trough. The disconnect is a 200A unit, fused at 200A, feeding a 200A trough. The wire from the disconnect to the trough's distribution blocks is 250kcmil, as is appropriate. Now, from the distribution blocks you are feeding half a dozen 60A disconnects fused at 50A, and powering motors. The wire from the distribution block to these 60A disconnects is #6, as is appropriate for the 50A load. The upstream disconnect on these conductors is the 200A disconnect. You have, in that respect, #6 wire protected by a 200A fuse, which is ridiculous. This situation - a conductor protected by a fuse of a higher rating than the conductor - is what makes it a tapped conductor. In practice it isn't protected by the 200A disconnect, but rather by the 50A fuse in the disconnect that it is powering. This is a somewhat dodgy way to do business, because if the tapped conductor shorts out along its length then its over-current protection won't work. This is why tapped conductors are treated specially in 28-110(2) which limits their length and application. The alternative would be to wire up all of your 60A disconnects with 250kcmil conductor. Nope - not gonna do it. The conductor wouldn't even fit in the lugs.
Take a look at the extra info on 28-100 in appendix B of the CEC. There is a diagram there that should be memorized by every Canadian sparky.
Taps systems are commonly found in laundromats , I've ressurected old 6/3 kitchen feeds to cover both cooktop and wall ovens.
The illustration above (provided by Dennis) states the 3 following rules:
1) The tap conductor should have a maximum of 25 ft long
2) The tap conductor should have an ampacity of no less than 1/3 the rating of the protective device
3) The tap conductor should terminate in a single circuit breaker (disconnect safety switch) or set of fuses rated no more than the conductor
The second rule seems to imply that in the example I illustrated:
200 amp x 1/3 = 66.6 Amp
If I understand the NEC, copper wire ampacity with 60/75/90°C for #6 AWG is 55/65/75A
Therefore #6 wire wouldn’t be fully adequate for such an installation???
1) The tap conductor should have a maximum of 25 ft long
2) The tap conductor should have an ampacity of no less than 1/3 the rating of the protective device
3) The tap conductor should terminate in a single circuit breaker (disconnect safety switch) or set of fuses rated no more than the conductor
The second rule seems to imply that in the example I illustrated:
200 amp x 1/3 = 66.6 Amp
If I understand the NEC, copper wire ampacity with 60/75/90°C for #6 AWG is 55/65/75A
Therefore #6 wire wouldn’t be fully adequate for such an installation???
#6 would be fine if the calculated load is 55 amps or less or if you use 75C or 90C conductors. At 75C it is rated 65 amps so why would that not work.
The NEc allows you to use the next higher size breaker if the wire ampacity
does not correspond to a standard size breaker and the load is not larger than the ampacity of the wire.
The NEc allows you to use the next higher size breaker if the wire ampacity
does not correspond to a standard size breaker and the load is not larger than the ampacity of the wire.
The CEC allows for two situations for a tap. The first is different from the NEC. It allows up to 3m (about 10 feet) of conductor of any size as long as it's protected at the downstream end by a single set of fuses of a size appropriate to the conductor, and run entirely in metal. The other situation is virtually identical to the NEC: up to 7.5m (roughly 25 feet) if it's 'suitably protected' and no less than 1/3 the ampacity of the feeder (still terminating in one set of fuses).
What’s this 60/75/90°c rating by the way? Are these different types of wires? Are we talking about the heat generated by the electricity passing through the wire?
When I go to the store can I purchase 2/14 rated for 60°c and 2/14 rated for 75°c and 2/14 rated for 90°c?
By the way, thanks to everyone helping me out. I really really appreciate this.
When I go to the store can I purchase 2/14 rated for 60°c and 2/14 rated for 75°c and 2/14 rated for 90°c?
By the way, thanks to everyone helping me out. I really really appreciate this.
I cannot answer for CEC but the NECstates that NM cable is rated 60C. The wire in NM is rated 90C which means it is a higher temp insulation and thus the conductor may be used at a higher ampacity.
In all cases we are always limited by the weakest link. For instance if you use 90C wire and terminate at a breaker then the circuit is really only as good as 75C since almost all breakers today are rated 75C. You can use the 90C for derating but you generally can't protect the wire with OCPD at the higher ampacity.
!2 thhn is rated 90C so it is good for 30 amps. Say we have to derate our conduit because of many wires and we must derate at 70%. 30amps x 70% is still 21 amps so you can still use a 20 amp breaker to protect this wire.
In all cases we are always limited by the weakest link. For instance if you use 90C wire and terminate at a breaker then the circuit is really only as good as 75C since almost all breakers today are rated 75C. You can use the 90C for derating but you generally can't protect the wire with OCPD at the higher ampacity.
!2 thhn is rated 90C so it is good for 30 amps. Say we have to derate our conduit because of many wires and we must derate at 70%. 30amps x 70% is still 21 amps so you can still use a 20 amp breaker to protect this wire.
This may clear up the tap conductor confusion
A Tap conductor is defined in 240.2 as a conductor, other than a
service conductor, that has overcurrent protection ahead of its
point of supply that exceeds the value permitted for similar
conductors that are protected as described in 240.4. Tap
conductors are permitted provided they adhere to the requirements
of 240.21(A)-(G). In addition, 240.21 prohibits tap conductors
from supplying other conductors, except through an overcurrent
protective device meeting the requirements of 240.4. This means
“you can’t tap a tap”.
One common misconception about tap conductors is that
overcurrent protection is not required. However, it should be
noted that in most instances, overcurrent protection is required at
the termination point of the tap conductor by either 240.21(A)-
(G), transformer protection requirements per 450.3, or panelboard
protection requirements per 408.16. In addition, the use of tap
conductors should be limited where possible since tap conductors
have limited short-circuit protection. In fact, besides meeting the
requirements of 240.21, a short-circuit protection analysis of the
tap conductor should also be done per NEC 110.10. The primary
applications of interest to most include feeder taps per 240.21(B)
and transformer taps per 240.21(C). This issue will focus on
feeder taps, the next issue will focus on transformer taps.
A Tap conductor is defined in 240.2 as a conductor, other than a
service conductor, that has overcurrent protection ahead of its
point of supply that exceeds the value permitted for similar
conductors that are protected as described in 240.4. Tap
conductors are permitted provided they adhere to the requirements
of 240.21(A)-(G). In addition, 240.21 prohibits tap conductors
from supplying other conductors, except through an overcurrent
protective device meeting the requirements of 240.4. This means
“you can’t tap a tap”.
One common misconception about tap conductors is that
overcurrent protection is not required. However, it should be
noted that in most instances, overcurrent protection is required at
the termination point of the tap conductor by either 240.21(A)-
(G), transformer protection requirements per 450.3, or panelboard
protection requirements per 408.16. In addition, the use of tap
conductors should be limited where possible since tap conductors
have limited short-circuit protection. In fact, besides meeting the
requirements of 240.21, a short-circuit protection analysis of the
tap conductor should also be done per NEC 110.10. The primary
applications of interest to most include feeder taps per 240.21(B)
and transformer taps per 240.21(C). This issue will focus on
feeder taps, the next issue will focus on transformer taps.
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