Column-by-Column: Single-Phase Fuse Sizing

In our column-by-column series over the last few weeks we’ve been looking at Table 13 of the Franklin Electric AIM Manual on Single-Phase Motor Specifications. Opposite this page is Table 14, Single-Phase Fuse Sizing. In this post, we’ll examine the six columns in this table listed for each submersible motor.

As a first step, it’s important to understand that fuses and circuit breakers are not overloads. Overloads, which are found either in the motor or the control box, protect the motor. Fuses and circuit breakers, on the other hand, protect the electrical system. That is, they protect the wiring in the circuit and trip or blow to interrupt excessive current. This prevents against wire damage from overheating or even fire.

Although fuses and circuit breakers have the same function they operate differently. A fuse is a type of low resistance resistor that acts as a sacrificial device to provide over-current protection. When the current gets beyond a certain threshold a small link inside the fuse gets hot enough that it melts, thereby “blowing the fuse” and causing power to be cut from the system.

Fuses are offered as standard or dual element / time delay fuses. A standard fuse is a fast response fuse, which means it will trip instantly any time the amperage exceeds the fuse rating. In any system the standard fuse offers the least amount of protection, as it allows the current to run higher before tripping. One can think of a standard fuse as providing catastrophic protection.As the name implies, dual element / time delay fuses combine two elements into one package. One element operates like a standard fuse. However, a second element reacts at a lower current but is far slower to react. This time delay element provides better system protection and allows the momentary start-up current to pass through. Although not an overload per se, this arrangement offers some secondary overload protection to the motor.

A circuit breaker is a mechanical over-current protection device, using an electromagnet to literally flip a switch off and cut power. Being electromechanical, circuit breakers can be reset whereas fuses are sacrificial and must be replaced.

With that as background, let’s take a closer look at Table 14.

The first thing you may notice is that there are two broad categories, Maximum per NEC and Typical Submersible. The column Maximum Per NEC represents maximum fuse and circuit breaker size requirements as recommended by the US National Electric Code. The NEC offers a broad, general rating system that applies to all motor types. Amperages in these columns are calculated by the NEC using factors including Locked Rotor Amps and Maximum Load Amps. The amps recorded here are the highest amount of amps recommended for a fuse or circuit breaker.

Moving to the right we find the column labeled Typical Submersible, specifying the same three categories below but at lower amperages. These Typical Submersible sizes are Franklin Electric-recommended, calculated specifically for typical Franklin Electric submersible motor performance by engineers utilizing decades of experience in the field.

Turning our attention to the three columns below, we see values for the three types of over-current protection we previously discussed: Standard Fuse, Dual Element Time Delay Fuse, and Circuit Breaker.

The idea behind the recommended over-current protection amps is to get the lowest amp fuse or circuit breaker that will allow the brief start-up amps to pass through without tripping and still provide some protection for the motor.

For example, the Maximum Per NEC Standard Fuse on a 4-inch, 2-wire, 115 V motor is 35 amps. This means that a 35 amp standard fuse is the smallest able to withstand the brief 64.4 start-up amps (or Locked Rotor Amps) of the motor without nuisance-tripping. However, it also means that this motor, with a maximum load of 12.0 amps, has the potential to run at up to 35 amps before tripping the fuse, possibly overloading the system and offering little motor protection.

The Maximum Per NEC Dual Element Fuse recommended for this motor is 20 amps while the recommended circuit breaker size is 30 amps.

Franklin’s Typical Submersible columns recommend a standard 30 amp fuse, a 15 amp dual element fuse, or a 30 amp circuit breaker. Based on experience, these sizes work, protect the wire, and offer the motor some secondary overload protection. If the amps were any smaller, our system could nuisance-trip at start-up.

Electrical codes require that fuse or circuit breaker protection be provided as part of the installation, and it’s critical that these components be sized correctly. Hopefully, this post has shed some light on how each submersible motor listed can have six different fuse or circuit breaker sizes listed and where these numbers come from.

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