Replacing a Parallax transfer switch? Match the full part number first. An ATS301 serves a single 120V/30A supply, while an ATS503 serves a two-leg 120/240V/50A system. They are not interchangeable on amperage alone, and their terminals differ.
Identify the shore, generator and load connections
One transfer switch accepts two separate AC sources and selects one to feed the downstream AC distribution. It prevents the connected sources from backfeeding each other; it does not combine their available power.
Shore or utility AC
Generator or alternate AC
Confirm ATS301 versus ATS503 from the label
| Model | Current | AC service | Switching design | Connection identification |
|---|---|---|---|---|
| ATS301 | 30A | 120 VAC, single leg | DPDT relay; generator command | Shore / GEN / LOAD groups and protective ground |
| ATS503 | 50A | 120/240 VAC, two legs | Heavy-duty relays; generator command | UTILITY / GEN / LOAD L1, L2, neutral and auxiliary terminals |
ATS301 and ATS503 connection identification
ATS301: The manufacturer labels the three main circuits A (LOAD), B (GENERATOR input) and C (SHORELINE input), plus D for the equipment ground. Its 30A supply is 120V; it does not handle two independent 50A hot legs.
ATS503: The factory drawing distinguishes UTILITY, GEN and LOAD groups with L1, L2, NEUTRAL and auxiliary terminals. Its NJ1–NJ3 neutral jumpers and timer jumper are specific to different operating configurations, so installation must follow the actual wiring label rather than a generalized diagram.
Generator power is normally the command source. A stabilization delay is provided before transfer, while the default position supplies loads from shore/utility power.
Identify the exact model in our catalog
The product cards link to the model identified on the existing equipment. Both parts are separate models; check each listing for current availability.
Original manufacturer connection drawings
Parallax's factory drawings show the source terminals, load circuits and specific ATS503 jumper configurations. Use the drawing attached to the actual unit for conductor order and torque requirements.
Parallax ATS301 official model page · Parallax ATS503 official model page · Parallax ATS301 owner and terminal drawing (PDF) · Parallax ATS503 owner drawing (PDF)