Enter the motor nameplate and starting data to get the overload setting, the trip class that fits, and the maximum fuse or breaker, starter and conductor sizes.
Pre-sizing and estimating tool: results do not replace design, calculations or reports signed by a licensed professional, nor verification against the applicable codes and standards.
Cold trip band of each class at the current the relay sees, compared with the starting time and the motor safe stall time.
Cold trip time versus multiple of the setting (log scale), with the starting point and the motor limit.
Selection rule and minimum value of each component; standard sizes are the most common catalog ratings, confirm with the manufacturer.
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It sets up the protection of a three-phase motor: overload relay setting, trip class, maximum fuse or breaker size under NEC Table 430.52, and starter size. Use it when building a new starter, replacing a motor or chasing nuisance trips at start.
To the nameplate full-load current. In the example, a 40 hp, 460 V motor with 48 A on the nameplate gets the relay at 48 A; NEC 430.32 allows a trip current up to 125% because its service factor is 1.15. On a wye-delta starter the relay sits in the winding leads: set it to 0.58 × 48 = 27.7 A.
The class is the maximum cold trip time at 7.2 times the setting: class 10 trips in 4 to 10 s, class 20 in 6 to 20 s and class 30 in 9 to 30 s. The relay must let the start finish and trip before the motor safe stall time. With a 3 s start and a 12 s stall time, class 10 works, tripping between 4.2 and 10.6 s.
As a percentage of the Table 430.250 full-load current: 175% for dual-element time-delay fuses, 300% for non-time-delay fuses, 250% for inverse-time breakers and 800% for instantaneous-trip breakers, rounded up to the next standard size. For 48 A, a time-delay fuse may be 90 A. If the motor will not start, the exception allows up to 225% for time-delay fuses.
The safe stall time is on the motor data sheet, usually 5 to 20 s hot. Starting time can be logged with a recording clamp meter or computed from load inertia in the motor sizing calculator. Small pumps and fans start in 1 to 5 s.
Common causes: a start too long for the trip class, a setting below nameplate current, low voltage stretching the start, a wye-delta relay installed in the line but set to 58%, or repeated starts with a hot motor. Compare the measured start time with the class band in the chart.
It does not check short-circuit current rating, type 2 coordination or starts per hour. Use the manufacturer SCCR and coordination tables, and have a qualified engineer review the final design under the NEC as adopted locally.
Important notice: Indus Metrics tools produce estimates and pre-sizing results from the data you enter and from published engineering methods. They do not replace design, calculations or reports signed by a licensed professional engineer (PE), nor verification against the applicable codes, standards and regulations, such as OSHA rules and the ASME codes. Material and fuel properties vary: check manufacturer data or your own process analyses before deciding. By using this site you agree to the Terms of Use.
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