Sizing a Subpanel Feeder for a Detached Garage Workshop
A homeowner in a Wisconsin one-family dwelling wants a 100 A subpanel in a detached garage 60 ft from the house. The garage has a 3 hp, 240 V single-phase table saw (nameplate 17 A), a 5 kW, 240 V electric unit heater, four 20 A general-use receptacle circuits, and two 8 ft fluorescent lighting circuits at 400 W each. The feeder will be run underground in PVC conduit from a 200 A service panel. Ambient design temperature is 30 degrees C. The owner asks whether a 100 A feeder is adequate and what conductor and overcurrent protection should be used. The journeyman must evaluate load, voltage drop, and the applicable rules before quoting the job.
1. Calculate the garage load and determine whether a 100 A feeder is adequate.
Read the worked answer
Compute each load:
- Table saw 3 hp, 240 V: Table 430.248 gives 17 A for a 3 hp 240 V motor, so 17 A x 240 V = 4,080 VA.
- Unit heater: 5,000 VA (continuous, treat at 100%).
- Receptacles: four 20 A circuits x 1,500 VA (or 180 VA per yoke; use the larger practical basis) = 6,000 VA.
- Lighting: 2 x 400 W = 800 VA.
Total connected = 4,080 + 5,000 + 6,000 + 800 = 15,880 VA. At 240 V that is 66.2 A. Apply the largest motor 25% adder per 430.24: 17 A x 25% = 4.25 A, so 70.4 A. A 100 A feeder with 100 A overcurrent protection is adequate for the calculated load, but voltage drop at 60 ft must be checked. Using 2/0 aluminum or 1/0 copper gives roughly 3% drop at 70 A over 120 ft round trip; smaller conductors may exceed 3% recommended. Confirm terminations are rated for the chosen conductor and that the panel bus is rated 100 A.
2. What overcurrent protection and grounding/bonding arrangement applies at the detached garage?
Read the worked answer
The feeder overcurrent device at the service panel protects the feeder; the garage panel main is not required if the panel has no more than six service disconnects (it is a subpanel, so a main breaker is optional but common). Because this is a separate structure fed by a feeder, a grounding electrode system is required at the detached garage per 250.32: drive a ground rod (or use the concrete-encased electrode if present) and connect it to the equipment grounding conductor and the panel enclosure. The neutral must be isolated from the enclosure and the equipment grounding conductor at the subpanel; do not bond neutral to ground at the garage. Run a separate equipment grounding conductor in the PVC conduit. Size the EGC per Table 250.122 based on the feeder overcurrent device. The grounding electrode conductor is sized per 250.66 and is not required to be larger than 6 AWG copper for a rod electrode.
3. The owner wants to add a 7.5 kW, 240 V welder later. How does that change the analysis?
Read the worked answer
A welder is a noncontinuous load and Article 630 permits demand factors. For a 7.5 kW welder at 240 V, primary current is 31.25 A. Under 630.11, the duty cycle multiplier applies; for a typical 20-30% duty cycle, the multiplier can reduce the effective load to roughly 55-70% of rated primary current. Even so, add the welder load to the 15,880 VA existing load: 15,880 + 7,500 = 23,380 VA, or 97.4 A at 240 V before demand factors. That is at the edge of a 100 A feeder. Recommend upsizing to a 125 A feeder or applying 630.11 duty-cycle reduction, verify the panel bus rating, and recheck voltage drop at the new load. Document the calculation on the permit.