10' tap rule
- Thread starter mannyb
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jaggedben
Senior Member
- Location
- Northern California
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- Solar and Energy Storage Installer
Yes, assuming 800A refers to the rating of the breaker protecting the feeder (rather than the wire size).
Elect117
Senior Member
- Location
- California
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- Engineer E.E. P.E.
There is no 10' tap rule for transformers.
240.21(B)(3) for the feeder supplying the transformer and 240.21(C) for the secondary conductors from the transformer. 240.21(B)(3) applies to the feeder on the primary side of the transformer and the total tap length of secondary and primary. The rules for protecting the secondary conductors are in 240.21(C). If the feeder supplying the transformer is not a tap, then only 240.21(C) applies to the secondary conductors.
And remember, there is no rounding up with OC when protecting conductors at 800A or over and when protecting a tap conductor.
The exception being, if you provide protection between the tap and the transformer, as in, a 800A feeder, 80A tap, to a fuse disconnect that is 80A or less, then to a transformer that is adequately protected by it. The conductors for the 80A tap must be of equal or higher ampacity. So #3 AWG Cu or greater.
The exception is a design that makes the transformer's feeder after the OC of the tap, which means it is no longer a transformer tap.
240.21(B)(3) for the feeder supplying the transformer and 240.21(C) for the secondary conductors from the transformer. 240.21(B)(3) applies to the feeder on the primary side of the transformer and the total tap length of secondary and primary. The rules for protecting the secondary conductors are in 240.21(C). If the feeder supplying the transformer is not a tap, then only 240.21(C) applies to the secondary conductors.
And remember, there is no rounding up with OC when protecting conductors at 800A or over and when protecting a tap conductor.
The exception being, if you provide protection between the tap and the transformer, as in, a 800A feeder, 80A tap, to a fuse disconnect that is 80A or less, then to a transformer that is adequately protected by it. The conductors for the 80A tap must be of equal or higher ampacity. So #3 AWG Cu or greater.
The exception is a design that makes the transformer's feeder after the OC of the tap, which means it is no longer a transformer tap.
- Location
- Wisconsin
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- PE (Retired) - Power Systems
Are you tapping off of an 800A conductor to feed a transformer primary side OCPD? Use 240.21(B)(1) or (2).
Or
Are you tapping off an 800A conductor and feeding a transformer without dedicated primary side protective device? Use 240.21(B)(3).
Or
Are you tapping off an 800A conductor and feeding a transformer without dedicated primary side protective device? Use 240.21(B)(3).
- Location
- Connecticut
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- Engineer
Voltage wasn't mentioned, but I don't see how an 800A ocpd would protect a 10kVA transformer..
Elect117
Senior Member
- Location
- California
- Occupation
- Engineer E.E. P.E.
Me either, but the OP doesn't mention any OC other than 800A.
The question was about a tap, but rules for other sections still apply, like 408.36 and 450.
jaggedben
Senior Member
- Location
- Northern California
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- Solar and Energy Storage Installer
Maybe I'm the only one who thought it was obvious, but I thought that the transformer is the load being supplied by the tap. And that the unmentioned OCPD for the load is obviously less than 80A, hence the question if the tap conductors need to be sized for 80A.
- Location
- Connecticut
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- Engineer
The OP mentioned the 10' tap rule (240.21(B)(1)), which requires an OCPD (or equipment containing an OCPD) at the end of the tap.
I don't think there as any conceivable way that 240.21(B)(3) would apply to the installation.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
I agree it is unlikely.
For the most part it is hard to conceive a 240.21(B)(3) compliant installation except where small transformers are being installed on street/yard lighting circuits to either power the light itself or a convenience outlet. I think I was involved in one installation for pole mounted power boosters for 'cable TV' distribution.
Elect117
Senior Member
- Location
- California
- Occupation
- Engineer E.E. P.E.
I also agree that there is no conceivable way to make a complaint install applying 240.21(B)(3) to the OP's situation. And as acrwc10 pointed out, there is the hurdle of possible parallel runs.
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