Motor Overload and Short-Circuit Protection: Worked Sizing

Sizing a motor circuit on paper is one thing. Knowing why the motor keeps tripping, whether a bigger breaker is allowed, or whether the panel will survive a fault is something else. Those are the calls a senior electrician or lead makes, and they are the ones inspectors and plant engineers ask about. This works through real sizing cases, including the ones where the normal answer is wrong. It assumes you know the difference between table full-load current (FLC) and nameplate full-load amps (FLA).

What You Will Be Able to Do

• Size conductors, protection, overloads, disconnect and grounding for one motor.
• Choose an overload trip class from the acceleration time.
• Use the exceptions in NEC 430.52 only when they apply.
• Handle the conductors and overloads in a wye-delta starter.
• Check short-circuit current rating and coordination.
• Protect a control transformer and find the limits of the code.

Numbers follow recent National Electrical Code (NEC) editions, and Article 430 sections have stayed fairly stable. Your state or city may enforce a different edition, with local changes. Confirm yours. Sizing and settings on a real motor are approved by the qualified person responsible for the job.

A Quick Reminder

Table FLC, from Table 430.250, sizes the conductors, the short-circuit device and the disconnect. Nameplate FLA sets the overload. Protection is split on purpose: the overload stops slow heating, and the fuse or breaker clears short circuits and ground faults.

Worked Example: A 50 HP Pump

A 50 HP, 460-volt, three-phase motor has a nameplate FLA of 62 amps and a service factor of 1.15. Terminals are rated 75 degrees C. Table FLC is 65 amps.

Notice how far apart the numbers are. A 175-amp breaker sits on wire good for 85 amps, and that is correct, because the overload protects the wire from overload. A plant electrician who sees this and swaps the breaker for a 90-amp one will get a motor that trips on every start.

NEC 430.32 caps the overload at 125 percent of FLA for a motor with a service factor of 1.15 or more, or a marked temperature rise of 40 degrees C or less. All other motors are capped at 115 percent. That cap is a maximum, not a target. Many plants set the overload closer to the FLA to protect the motor better. If you use heaters, use the maker's table and do not add your own percentage.

Choosing a Trip Class

Class 10, 20 and 30 overloads trip within that many seconds at six times their rating. The right class depends on how long the motor takes to reach speed, and how long it can safely stall.

Worked Example: A Slow-Starting Fan

A large fan takes 14 seconds to reach speed on across-the-line starting. It draws about six times its FLA during the start.

1. A Class 10 overload trips within 10 seconds at that current. It will trip before the fan reaches speed.
2. A Class 20 overload allows up to 20 seconds, so the start can finish.
3. Check the motor's limit. The maker's safe stall time must be longer than the time the relay allows, or the relay no longer protects a locked rotor.

Raising the class is correct when the motor is designed for a long start. It is wrong when the start got longer because a bearing is failing. Measure first.

Submersible pumps and other motors that heat quickly usually call for Class 10. Follow the motor maker's data.

When Overloads Trip on a Good Motor

Raising the setting past the code limit is never the fix. NEC 430.32(C) allows up to 140 or 130 percent of FLA, depending on the motor, only if the motor will not start or carry its load at the normal setting. Find out why first.

When 250 Percent Will Not Hold

If the motor cannot start with the normal device, the exceptions in 430.52 let you go higher.

Two points are easy to miss:

• The first calculation may round up to the next standard size. The exception limit may not be exceeded, so you round down.
• You need a real reason. "It tripped once" is not enough, because the first question is whether something is wrong with the motor or load.

Worked Example: A Breaker That Trips on Start

A 30 HP, 460-volt motor has a table FLC of 40 amps. Its normal inverse-time breaker is 40 x 2.50 = 100 amps. It trips during starting with the load confirmed free and the motor tested good.

1. The exception limit is 40 x 4.00 = 160 amps.
2. You may not exceed 160 amps, so the largest standard size is 150 amps.
3. Before you change it, look at the trip curve. Also check the conductors and the starter's listed maximum device. A larger breaker may void a combination rating.

Instantaneous-Trip Breakers

An instantaneous-trip breaker, also called a motor circuit protector, has no thermal element. It works only as part of a listed combination with an overload relay. Table 430.52 sets a limit, commonly 800 percent, and higher limits apply to energy-efficient Design B motors, which have higher inrush. On an energy-efficient motor, a setting that held on the old motor may trip on starting. Set it from the listing, not from a guess.

Wye-Delta Circuits

A wye-delta starter has six leads to the motor. In the run position, the windings are in delta, so current in the leads to the motor is winding current. That is line current divided by 1.732, or about 58 percent.

NEC 430.22(C) reflects this. Line-side conductors are based on full motor current. The conductors between the controller and the motor are based on 58 percent of FLC.

Worked Example: A 100 HP Wye-Delta Motor

A 100 HP, 460-volt motor has a table FLC of 124 amps.

1. Line-side conductors: 124 x 1.25 = 155 amps. 2/0 copper, good for 175 amps at 75 degrees C.
2. Controller to motor: 124 x 0.58 = 72 amps. Many calculations then apply 125 percent, for 90 amps, which is 3 AWG copper. Confirm how your inspector reads this.

The overload needs the same care. If it sits in the winding circuit, it sees about 58 percent of line current, so it is set for that current. If it sits on the line side, it sees full current. Check the starter's wiring and follow the maker's instructions. A relay set for line current, but placed in the winding circuit, gives almost no protection.

Short-Circuit Current Rating

Sizing protection for the motor is not the end. The whole assembly must survive the fault current at that point.

Short-circuit current rating (SCCR) is the fault current a piece of equipment can safely withstand. An industrial control panel carries a marked SCCR, and NEC 409.22 does not allow one to be installed where the available fault current is higher.

A panel's SCCR is set by its weakest part. A 65,000-amp breaker does not raise a starter's rating by itself. Only a tested combination does that.

Worked Example: The Weakest Part

A panel is going in where the study shows 28,000 amps available. Its label reads SCCR 10,000 amps.

1. The label is lower than the available fault current, so the panel cannot be installed as it is.
2. Options include a tested combination with current-limiting fuses, an upgraded panel, or a change to the supply.
3. A different breaker on its own does not fix it unless the maker's data shows a tested combination.

Components with no tested rating carry a low default value, commonly 5,000 amps for smaller starters. That is why an old starter in a new, stronger system can be a problem.

Type 1 and Type 2 Coordination

These describe what happens to a starter after a short circuit, under IEC 60947-4-1 and UL 60947-4-1.

Type 2 costs more, and it gets a critical line back up faster. It comes from tested combinations in manufacturer tables, so you cannot design it by math.

Control Transformers

A control power transformer needs protection too. NEC 430.72(C) gives several options. One is a primary device of no more than 500 percent of primary current when that current is less than 2 amps.

Worked Example: A Small Control Transformer

A 150 VA, 480 to 120 volt transformer has a primary current of 150 / 480 = 0.31 amp.

1. 0.31 x 5 = 1.56 amps maximum.
2. So a primary fuse of 1.5 amps or less is within this rule.

Makers publish fuse tables that take inrush into account, so follow them. Secondary fuses protect the control wiring. Conductor limits are in NEC 430.72(B), and they depend on whether the wires stay in the enclosure.

Feeders With Several Motors

A quick reminder: feeder conductors use 125 percent of the largest motor plus the others. Feeder protection uses the largest branch device plus the other FLCs, rounded down.

Worked Example: Adding a Motor to a Feeder

A feeder serves 30 HP (40 A) and 15 HP (21 A) motors, with a 100-amp branch breaker on the larger motor. You need to add a 20 HP (27 A) motor.

1. Largest motor is still 40 A, so conductors: (40 x 1.25) + 21 + 27 = 98 amps.
2. Feeder device: 100 + 21 + 27 = 148 amps, which rounds down to 125 amps.

The existing feeder must meet these numbers, or the new motor needs a new feeder. That check is why a lead does the math before a helper starts pulling wire.

Common Mistakes

Check Your Understanding

1. A new plant electrician sees a 90-amp breaker on wire good for 45 amps, and wants to swap it for a 45-amp breaker. What do you say?

Check the math first. The breaker clears short circuits and ground faults, and the overload protects the wire from overload, so a large gap can be correct. A 45-amp breaker would likely trip on every start.

2. After one nuisance trip, a supervisor asks you to raise a branch device to its highest allowed size today. What comes first?

Find out whether something is wrong with the motor or the load. One trip is not a reason, and a larger device may also void a combination rating.

3. A motor that ran for years now labors at startup, and someone wants a higher trip class to stop the nuisance. What do you do?

Find out why the start got longer. A failing bearing or a changed load is likely, and a higher class would only hide it and leave the motor less protected.

4. An old starter with no marked rating is being reused where the study shows 18,000 amps available. What do you check?

The maker's data for a tested combination at that fault current. Without one, expect a low default value, and plan to replace it.

5. A wye-delta motor has a table FLC of 90 amps. What current are the conductors between the controller and the motor based on?

90 x 0.58 = about 52 amps, before any further calculation your inspector requires. The line-side conductors still use the full 90 amps.

Key Takeaways

• The overload stops slow heat, and the breaker or fuse clears faults, so their numbers are far apart.
• Choose a trip class from the start time and the motor's safe stall time.
• Exception limits may not be exceeded, so round down.
• In a wye-delta, leads and windings carry about 58 percent of line current.
• A panel's SCCR is set by its weakest part or a tested combination.
• Type 2 coordination keeps a starter usable after a fault, and it comes from tested tables.