18 ft Feeder Tap to a Second-Floor Panel with an Unverified Series Rating
A 400 A, 480Y/277 V switchboard in a first-floor electrical room feeds a 100 A panelboard on the second floor. The tap conductors are 1/0 AWG copper THWN-2 in EMT, 18 ft of conductor length, terminating in the 100 A main breaker of the panelboard. The switchboard's 400 A frame and breaker are rated 35 kAIC. Available fault current at the switchboard is 22,000 A. The 100 A panel's main and branch breakers are 10 kAIC. A previous renovation removed the panel's factory marking, which had identified the breakers as part of a listed series-rated combination with the specific 400 A upstream device. The electrician argues the run is 'short enough' to be a tap and wants to energize.
1. Which tap rule applies, and does the installation comply?
Read the worked answer
The 18 ft length rules out the 10 ft tap rule (which caps tap length at 10 ft and requires the tap ampacity to be at least 1/10 of the upstream device rating). The applicable rule is the 25 ft tap rule.
Conditions and check:
| Condition | Requirement | Actual | Result |
|---|---|---|---|
| Length | <= 25 ft | 18 ft | Pass |
| Ampacity | >= 1/3 of 400 A = 133 A | 1/0 Cu at 75°C = 150 A | Pass |
| Termination | Single OCPD or single fuse set | 100 A main breaker | Pass |
| Physical protection | In raceway or other approved means | EMT | Pass |
| Extent | Does not extend beyond the panel it supplies | Terminates in the panel | Pass |
The tap complies, assuming terminations are rated 75°C (if the breaker lugs are 60/75°C and the conductor is used at 75°C, 150 A still applies; if any termination is 60°C only, the ampacity drops and the 1/3 check must be redone).
Note the tap conductor ampacity (150 A) is less than the upstream 400 A device - that is allowed only because every one of the tap-rule conditions is met. Remove any single condition and the tap is unprotected and illegal.
2. Evaluate the interrupting rating situation at the 100 A panel.
Read the worked answer
The 10 kAIC panel breakers are exposed to 22,000 A available. They are only usable if they are part of a listed series-rated combination with the specific upstream 400 A device, and the equipment is marked to indicate the series rating (with the caution marking that the system is series rated and the downstream breakers are not to be replaced with non-series-rated units).
With the factory marking gone:
- The listing may still exist in principle, but it cannot be verified in the field from the equipment as installed.
- The marking requirement is part of what makes the installation compliant; an unmarked series-rated system is an inspection failure.
Options:
- Obtain the original series-rating documentation from the manufacturer for that exact combination and re-mark the equipment. If the exact combination cannot be confirmed, this is not an option.
- Replace the panel's breakers with fully rated 22 kAIC (or higher) devices, and confirm the panel bus is rated for it.
- Add a current-limiting device ahead of the panel to reduce the available fault current.
Do not accept: 'the main is 35 kA so everything downstream is fine.' A breaker's own interrupting rating does not protect a downstream device from the available fault current at its terminals.
3. The tenant now needs a 200 A panel in the same location. What has to change?
Read the worked answer
A 200 A panel changes two things:
1. The tap conductor ampacity. 1/0 Cu at 75°C is 150 A. The tap must terminate in an overcurrent device, and the 200 A main would exceed the tap conductor ampacity by 50 A. Also, 150 A is only 37.5% of 400 A, so the 1/3 check still passes - but that check is not the binding constraint here; the conductor must still be adequate for the load it serves and for the device it lands on.
2. Conductor size. For a 200 A tap, use conductors with at least 200 A ampacity at the termination temperature - 3/0 AWG copper (200 A at 75°C) or an equivalent aluminum size. Verify 1/3 of 400 A = 133 A is still satisfied (it is).
Also re-check:
- Box fill and bending space at both ends for the larger conductors.
- Raceway fill - 3/0 Cu in the existing EMT may exceed fill or jam ratio; a raceway change may be needed.
- Working clearance and dedicated space at the new, wider panel.
- Interrupting rating - a new 200 A panel will need to be fully rated to 22 kA or be part of a verified, marked series-rated combination.