Posted: 6/5/2017 3:57:15 PM EDT
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I have an old Belsaw planer with a 3 HP motor.
120 / 240 volts.. 32 / 16 amps. Currently wired for 240 and is fed with 12 / 3 with ground. Is the current listed on the tag the starting current or the running current? Will a 20 amp DPST switch handle this? Thanks.. |
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You do not use the motor label for current.
Table 430.248 Full-Load Curreent in Amperes, Single -Phase Alternating-Current Motors must be used. A 3 HP motor at 230 Volts is 17 amps. Wiring is 125% percent of that so 21.25 Amp rated. So #12 is not large enough and you need #10. While some AHJs might let #12 slide it will result in longer starts, higher current, and more wear to the start switch contacts built into the motor. An "inverse time breaker" (AKA 'thermal-magnetic) rating can be 25 Amps up to 53 Amps. This is to allow for starting of the motor. If you routinely start and stop the motor for each cut you usually have to go a little higher on the breaker since it will warm up a little with each start cycle. The starting surge on that motor can easily be up into the 50 A range. The starting current is found from the Locked Rotor Code Letter and table 430.70(B) Locked-Rotor Indicating Code Letters. It tells you kV-A (kilovolt-amperes) per horsepower. You then convert it back to current. Typical breakers used in home panels are inverse time. It means they trip faster for larger currents. There is a thermal trip that allows shorter small overloads but a magnetic trip that goes very fast (milliseconds) on large currents like a short. The wire size is no longer tied to the breaker size on Article 430 motor circuits. A fractional HP motor has built in thermal overload protection and cannot over load a rated circuit. The breaker is for short circuit protection of the branch circuit conductors. Table 430.7(B) Locked-Rotor Indicating Code Letters Code Letter Kilovolt-Amperes per Horsepower with Locked Rotor A 0–3.14 B 3.15–3.54 C 3.55–3.99 D 4.0–4.49 E 4.5–4.99 F 5.0–5.59 G 5.6–6.29 H 6.3–7.09 J 7.1–7.99 K 8.0–8.99 L 9.0–9.99 M 10.0–11.19 N 11.2–12.49 P 12.5–13.99 R 14.0–15.99 S 16.0–17.99 T 18.0–19.99 U 20.0–22.39 V 22.4 and up |
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Quoted:
Aren't 14AWG and 12AWG already derated to 15A and 20A respectively? Meaning the 12 AWG can actually handle more than 20A safely? Unless the run is very short #10 will give faster starts and less pitting of the start winding throw out switch. Decent TEFC motors are not cheap. |