Undersized Feeder Discovered During Modernization
During a controller modernization on a 2,500 lb (1,135 kg) hydraulic passenger elevator, the crew finds the existing feeder to the machine room is a 3-conductor cable with a copper equipment grounding conductor sized for the original 1970s controller. The new solid-state controller adds a soft-start unit and a larger motor starter. The nameplate on the new controller lists a continuous input current of 48 A at 208 V three-phase. The existing feeder conductors are No. 6 AWG copper. The disconnect is a 60 A fused switch. The machine room has no spare conduit, and the building electrical contractor says pulling a new feeder would take two weeks and require a shutdown of the adjacent tenant space. The elevator is the only car serving a small medical office building.
1. Walk through how the mechanic should evaluate whether the existing No. 6 AWG feeder and 60 A disconnect can serve the new controller, and what information is missing.
Read the worked answer
The mechanic should not accept or reject the feeder on wire gauge alone. The evaluation needs:
- Actual load basis. The controller nameplate gives 48 A continuous input. Feeder sizing for elevator circuits follows the applicable code article and the manufacturer's marked input rating, not a guess from motor horsepower. The mechanic needs the manufacturer's installation instructions to confirm whether the 48 A is the value to use for conductor and disconnect sizing.
- Conductor ampacity in the actual installation. No. 6 AWG copper ampacity depends on insulation type and termination temperature rating, and on how many current-carrying conductors are in the raceway. The mechanic must confirm the insulation rating printed on the cable and the lug temperature ratings at both ends. A No. 6 conductor at a 75°C termination is commonly rated in the 65 A range, which may or may not clear the required feeder ampacity once the code-required multiplier for elevator circuits is applied.
- Disconnect and overcurrent protection. A 60 A fused switch must be evaluated against the required disconnect rating and the overcurrent device the controller manufacturer specifies. The soft-start unit may change the required branch-circuit protection.
- Grounding conductor. The equipment grounding conductor must be sized to the overcurrent device protecting the circuit, not to the original 1970s controller.
Missing information: cable insulation type and temperature rating, termination ratings, whether the 48 A figure is the marked input for feeder sizing, and the manufacturer's required overcurrent protection. Without those, the mechanic cannot responsibly declare the feeder adequate.
2. The electrical contractor proposes to keep the No. 6 feeder and simply change the disconnect to a 70 A switch. Is that an acceptable resolution? Explain the reasoning.
Read the worked answer
No, not as a standalone change. Increasing the disconnect rating does not increase the ampacity of the existing conductors, and it can make the grounding conductor and the overcurrent protection inconsistent with the circuit. The reasoning chain:
- The conductor ampacity is fixed by the wire's insulation and termination ratings and the installation conditions. A larger switch does not change that.
- If the feeder was marginal at 60 A, upsizing the disconnect to 70 A may allow a higher overcurrent setting that the conductors cannot safely carry continuously.
- The equipment grounding conductor must be sized for the overcurrent device actually protecting the circuit; changing that device without re-evaluating the grounding conductor is a defect.
- The controller manufacturer's required branch-circuit protection governs; the mechanic must verify the soft-start unit's requirements rather than assume a larger disconnect is 'more capacity.'
The defensible path is to compute the required feeder ampacity from the marked input and code multipliers, verify the existing conductor ampacity under actual conditions, and verify the grounding conductor against the protecting device. If any element fails, the feeder must be replaced or the installation redesigned, regardless of schedule pressure.