Supply-Side Connection and Feeder Design for a Commercial Array
A 40,000 ft² warehouse has a 208Y/120 V, three-phase, four-wire service with a 1,200 A service disconnect and a CT cabinet ahead of it. The PV system is four 20 kW three-phase inverters with 208 V output, 80 kW AC total. The array is on the roof, the inverters are on a pad 240 ft from the service entrance, and a single feeder serves all four. The designer proposes a supply-side connection in the CT cabinet through a 300 A fused disconnect per 705.12(A). The utility's service manual requires a visible, lockable, gang-operated disconnect within sight of the meter and limits customer-side voltage drop to 1.5% at the point of interconnection. Conductors are copper in PVC conduit, all terminals rated 75 °C, design ambient 35 °C, and the designer wants the AC feeder to use the entire 1.5% allowance at rated inverter output.
1. Size the feeder overcurrent device and conductors, and verify voltage drop against the utility's 1.5% limit.
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Ampacity. Three-phase current: 80,000 W ÷ (208 V × 1.732) = 222 A. Continuous-output sizing: 222 A × 1.25 = 278 A → 300 A OCPD, and conductors with at least 300 A ampacity. 350 kcmil copper at 75 °C is 310 A, so it is the minimum compliant size.
Voltage drop. The utility's 1.5% of 208 V = 3.12 V at rated output current (222 A), over 240 ft one way, copper. Required resistance: R = 3.12 V ÷ (1.732 × 222 A × 0.240 kft) = 3.12 ÷ 92.3 = 0.0338 Ω/kft.
Approximate 75 °C copper resistances: 350 kcmil = 0.0389, 400 kcmil = 0.0338, 500 kcmil = 0.0270 Ω/kft. So 400 kcmil copper meets the limit almost exactly: VD = 1.732 × 222 × 0.240 × 0.0338 = 3.12 V = 1.50%.
That is a knife-edge pass. Two refinements: use the conductor's actual resistance at the operating temperature rather than the 75 °C table value, and add margin for the terminations and any pull-box length not counted. Specifying 500 kcmil gives 2.49 V (1.2%) and absorbs the measurement error. Note the trade-off: 500 kcmil needs larger conduit and may force a different lug kit at the disconnect, so confirm the equipment lugs before locking in the size.
2. The feeder is upsized to 400 kcmil for voltage drop. Determine the equipment grounding conductor and explain the rule that governs it.
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Start from the OCPD. 250.122 sizes the EGC by the rating of the overcurrent device protecting the circuit: 300 A → 4 AWG copper (41,740 circular mils).
But the ungrounded conductors were increased above the minimum required for ampacity, and 250.122(B) requires a proportional increase in the EGC whenever conductors are upsized for reasons other than ampacity - voltage drop being the classic case. The proportional rule works on circular mil area, not on trade size:
EGC area = 41,740 cmil × (400 kcmil ÷ 350 kcmil) = 41,740 × 1.143 = 47,703 cmil.
The next standard size at or above 47,703 cmil is 3 AWG (52,620 cmil). 4 AWG is therefore too small even though it matches the 300 A row of the table.
Double-check the minimum-size floor is not exceeded the other way: 3 AWG is larger than 4 AWG, so it satisfies both 250.122(A) and (B). Also confirm the EGC is copper or sized per 250.122 for aluminum, that it is run in the same raceway as the circuit conductors, and that the bonding at the supply-side disconnect is sized per 250.66 at the point where the PV system connects. Skipping 250.122(B) is one of the most common commercial plan-check failures, because the conductor schedule shows a 4 AWG EGC next to 400 kcmil feeders and nothing on the drawing flags the mismatch.
3. Evaluate the supply-side connection: disconnect location, utility requirements, and what to verify about grounding.
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Code basis. 705.12(A) permits the PV output to land on the supply side of the service disconnecting means. 705.31 requires the PV system disconnecting means for a supply-side connection to be located within 10 ft of the point of interconnection. The 2020 and later editions scope that requirement to one- and two-family dwellings, while the 2017 edition applies more broadly - so confirm which edition the AHJ enforces before assuming the CT-cabinet area needs the disconnect, or that a disconnect 60 ft away is acceptable.
Utility overlay. The service manual requires a visible, lockable, gang-operated disconnect within sight of the meter, which is a utility requirement, not a code one; the 300 A fused disconnect in the CT cabinet governs the PV circuit, and a separate utility-accessible disconnect may still be needed near the meter. Get their written approval for the location before fabrication, because relocating after the gear is ordered is expensive.
Marking. The service equipment needs a permanent plaque identifying the PV system and its disconnect location, and the CT cabinet needs field marking of the tap and fuse rating.
Grounding. Confirm the PV disconnect enclosure and equipment grounding conductors bond to the service grounding electrode system per 250.50 and 690.47, and that the supply-side tap does not inadvertently create a second neutral-ground bond. Verify the service's available fault current against the 300 A disconnect's interrupting rating, and check that a fault on the feeder is cleared by the fuse upstream - the tap conductors have no downstream overcurrent protection of their own.