Choosing Circuit Protection

May, 23 2006

Blue Sea Systems' engineers identify new factors to consider when selecting high amperage circuit protection.

Circuit protection devices are installed in circuits primarily to protect the wire. Choosing the correct circuit protection device and sizing it correctly for your boat is very likely more difficult than you think. To select the correct circuit protection device, the rating of the circuit protection device is matched to the rating of the wire it protects. To guide this match between wire rating and proper circuit protection rating, standards tables are available such as the American Boat and Yacht Council (ABYC) tables of amperage capacity for wire size and wire insulation rating. Table IV of ABYC E-11 for DC systems is frequently referenced, in particular, for 105°C rated wire (see Blue Sea Systems' catalog or click here). Choosing circuit protection would seem to be an easy matter—use this table to match the nominal value of a fuse or circuit breaker to the appropriate wire rating. However, it's not that simple.

When matching circuit protection to the wire it protects, two facts contribute to the complexity of this task:

  • Fuse Blow Point, Circuit Breaker Trip Value. The amperage at which fuses actually blow, and circuit breakers actually trip, is considerably higher than their nominal ratings. SEA, Maxi, ATO and AGC fuses, and most circuit breakers, blow or trip at about 130% of their rating. ANL fuses blow from 140% to as high as 266% of their rating. (See ANL Fuse Blow Point table below.)
  • Current Heating. Wire and circuit protection devices heat up dramatically when they carry 100% of their rated value for several minutes or more. At about 150% of its current-carrying rating, wire generates enough heat to melt its insulation.

ANL Fuse Blow Point

Nominal Rating (Amps)Avg. Blow Point (Amps) @ 500 Sec.Blow Point (%)

The way in which circuit protection is chosen depends on whether the protection is for a short circuit¹, or for an overload².

Short Circuit Protection

If a wire feeds a predetermined load that will not change, the main concern is short circuit protection. Circuit wire is sized so that it safely carries the normal current loads for the application it serves. If there is a short, the high current in the circuit lasts for a short period of time before the fuse blows, and therefore generally does not damage the wire. Precise sizing of short circuit protection is not critical. A fuse or circuit breaker rated at an amperage value equal to the wire rating, or even up to 150% of the wire rating, is sufficient because the over-current condition lasts only a short time³. SEA, Maxi, ATO ,AGC, and ANL fuses are good choices for short circuit protection.

Overload Protection

However, if a load can increase into a sustained overload condition, approximately 110-150% of the wire rating, the wires and the protective device in the circuit may heat up. This overload condition might occur with motor loads, wiring to receptacles for plug-in appliances, or panel feeders where more than an intended number of loads can be turned on simultaneously. Selecting a fuse or circuit breaker for overload protection is more complex than choosing one for short circuit protection.

Choosing Circuit Protection for Overload Protection

In general, wire should carry current continuously at no more than 80% of its rating. Recall that the blow point or trip value of most fuses and circuit breakers (except for ANL and those based on European standards) are consistently about 130% of their nominal value. Therefore, to provide overload protection for wires, choose a circuit protection device, such as SEA, Maxi, ATO, AGC, that is rated at 80% of the recommendation in ABYC E-11 Table IV.

Furthermore, wire ratings are based on wire in open areas with good air circulation. Be even more cautious if wires are surrounded by thermal insulation such as loom, conduit or constricting structures. In these conditions, wire can carry even less current before heating up excessively. Look for more information about these effects in a future technical brief.

An Example: Choosing Suitable Circuit Protection

According to ABYC tables, 4 gauge AWG wire will safely carry 160A. Furthermore, this wire will safely tolerate 185A, approximately 115% of its rated value. Since most fuses and circuit breakers have a blow point or trip value that is 130% of their nominal rating, choose a fuse that is rated at 80% of 160A. A 125A nominal rating SEA fuse is a good choice-it has a blow point of 130% of 125A=162A.

Sizing Circuit Protection to the Load

A fuse or circuit breaker should not continuously carry more than 80% of its rating to avoid overheating of the circuit protector itself. In this example we have chosen a 125A fuse to protect the #4 gauge AWG wire. The fuse should not carry more than 100A continuously. Therefore, consider the load requirement in this circuit. If the load requirement is greater than 100A, choose a larger wire, and a larger fuse suitable for the load requirement and that will protect the wire.

Because ANL fuses behave differently than all other circuit protection devices, that is, their blow point value ranges from 140 to 266% of nominal value, the 80% rule doesn't work. It is necessary to use a different procedure to select a suitable ANL fuse. To choose a suitable ANL fuse, refer to the ANL Fuse Blow Point table above. According to the table, a 100A ANL fuse has a blow point of 175A. Therefore, a 100A ANL provides suitable protection for the 4 gauge AWG circuit in this example.

  1. A short circuit is an overload condition in which current flows from the source and returns without circulating through the intended load. Short circuits can be caused by mis-connection or by insulation failure. They can result in extremely high currents, limited only by the source capacity and resistance of the wiring.
  2. Operation of equipment in excess of normal, full-load rating, or of a conductor in excess of rated ampacity that, when it persists for a sufficient length of time, would cause damage or dangerous overheating. (source: National Electrical Code Handbook, 2002)
  3. ABYC Non-motor Loads - The rating of overcurrent protection devices used to protect a load other than a DC motor shall not exceed 150 percent of the ampacity of its supply conductor. (See Table IV.)
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