FFieldDeskElectrical and HVAC

Article 430 · screening

Size the motor branch circuit.

Use table full-load current for the wire, the breaker, and the disconnect. Use the nameplate amperes for the overload relay. The overload line below uses the table so you can see the percentage. Replace it with the nameplate before you set the relay.

Input values to calculate

Calculated results

Branch circuit, planning size. The wire and the short-circuit device use table current, not the nameplate. The overload line uses table current so you can see the percentage. Use the nameplate before you set the relay.

Table FLC
7.6 A
Minimum conductor amperes, 125%
9.5 A
Suggested conductor
14 AWG · 20 A
Short-circuit device, 250% then next standard
20 A
Calculated device before rounding
19 A
Overload planning value
9.5 A
Disconnect ampere rating, 115%
8.7 A
Voltage drop at table FLC
2.07 V · 0.4%
Report calculation

What this sheet does

  1. You pick the motor: one phase or three, the horsepower, and the voltage.
  2. The sheet looks up that motor’s full-load current in the table. The wire is sized from this number, not from the nameplate.
  3. It multiplies that current by 1.25. That is the smallest number of amperes the wire must carry.
  4. It multiplies the same current by the breaker or fuse percent you picked, then rounds up to the next standard size.
  5. It shows an overload so you can see the percent. Set the real relay from the nameplate amperes, not from this line.
  6. It checks how many volts you lose over the one-way length you typed.

Worked example: 5 hp, 460 V, three-phase

Table 430.250 lists 7.6 A. The wire is sized at 125%, so 9.5 A. A 14 AWG copper conductor in the 75°C column is listed at 20 A, which covers 9.5 A.

An inverse-time breaker at 250% is 19 A, then the next standard size is 20 A. The overload line at 125% is 9.5 A on table current. Replace that with the nameplate amperes before you set the relay.

Common questions

Why does the wire use table current?
The branch-circuit conductor starts from the table full-load current, then 125%. The nameplate is for the overload.
Why can the breaker be larger than the wire?
That device is short-circuit protection. The overload relay protects the conductor from a running overload.
Why must the overload use the nameplate?
The percentage is applied to nameplate current. This sheet shows the percentage on table current so you can see the rule.