FUSE SIZING GUIDE PRACTICAL APPLICATION INFORMATION This guide is a general recommendation and does not include the many variables that can exist for specific situations such as special local codes, unusual temperature or other operating conditions, N.E.C. demand factors, conductor derating, etc. Recommended U.L. Current Limiting Fuse Classes & EDISON Fusegear Symbols:* Time-Delay Type Class L –LCL –600VAC or less: 601-6000A Class RK1 –LENRK –250VAC or less: 6/10-600A –LESRK –600VAC or less: 1/2-600A Class RK5 –ECNR –250VAC or less: 1/10-600A –ECSR –600VAC or less: 1/10-600A Class J –JDL –600VAC or less: 1-600A Class CC –HCTR –600VAC or less: 1/4-10A Fast-Acting Type (Non/time-delay) Class T –TJN –300VAC or less: 1-800A –TJS –600VAC or less: 1-800A Class L –LCU –600VAC or less: 601-6000A Class RK1 –NCLR –250VAC or less: 1-600A –SCLR –600VAC or less: 1-600A Class J –JFL –600VAC or less: 1-600A Class CC –HCLR –600VAC or less: 1/10-30A *The fuse classes shown are U.L. Listed as “current limiting” with 200,000 RMS symmetrical amperes interrupting rating. Classes J and L are not interchangeable with fuses having lower l.R.. Class R fuses require Class R rejection fuse clips to prevent interchangeability with Classes H and K fuses with lower interrupting rating. (N.E.C. 110-9 and 240-60b.) Figure 1 1 Main Service Conductor Cable 4 Branch Circuit Fuse d) 150% of the nameplate current Limiters (N.E.C. 240, 450-6): Size, No Motor Load rating of the largest continuous-duty a) Select by cable size and mounting (N.E.C. 240-3, 220-3):* motor. Add to “a)” above. terminal configurations required. a) 100% of non-continuous load. e) 125% of the nameplate current *Do not exceed conductor ampacity. rating of other continuous duty motors. 2 Main Service Circuit Recommended fuses: LENRK, ECNR, Fuses–Mixed Loads: NCLR, JDL, LCU, or LCL. Add to “a)” above. a) Size fuses same as item 6. 5 Branch Circuit Fuse Size, No f) Recommended fuses: LENRK/LESRK, 3 Transformer Circuit Fuses Motor Load JDL, ECNR/ ECSR, LCU, LCL. (N.E.C. 450-3b, 240-3, 240-21, (N.E.C. 240-3, -220-3):* *Do not exceed conductor ampacity. 384-16d, 430-72b Ex. 3, 4 & (c) a) 125% of continuous load. Fuse may be 7 Feeder Circuit Fuse Size, 100% as required):* sized 100% when used with a continuous Motor Load (N.E.C. 240-3, 220- a) PRIMARY FUSES: Size fuses not over rated switch. Recom-mended fuses 10b, 430-24):* 125%. As exceptions exist, refer to the same as 4. a) Determine non-continuous appropriate N.E.C. paragraphs. *Do not exceed conductor ampacity. motorload (N.E.C. 430-22a Ex. 1). Recommended fuses: LESRK, ECSR, JDL, 6 Feeder Circuit Fuse Size, b) Determine load of A/C or LCL+.* Mixed Load refrigeration equipment (N.E.C. 440-3, b) SECONDARY FUSES (Sum of (N.E.C. 220-10b, 240-3, 430-22a -5, -12). Add to “a)” above. following): 125% of the continuous load Ex. 1, 430-24):* c)150% of nameplate current rating of plus 100% of non-continuous load. Fuse a) 100% of non-continuous, non-motor the largest continuous duty motor. Add size not to exceed 125% of transformer load plus 125% of continuous, non- to “a)” above. secondary rated amps. RECOMMENDED motor load d)125% of the other continuous-duty FUSES: LENRK, ECNR, NCLR, JDL or b) Determine non-continuous motor load motors. Add to “a)” above. LCU. (N.E.C. 430-22a EX. Add to “a)” above. *Fuse size must not exceed ampacity of e)Recommended fuses: LENRK/LESRK, conductors. Where selectivity is desired, c) Determine A/C or refrigeration load. JDL, ECNR/ ECSR or LCL. refer to EDISON selectivity methods. (N.E.C. 440-3b). Add to “a)” above. *Do not exceed conductor ampacity. Z-106 PRACTICAL APPLICATION INFORMATION Levels of Government regulations and ordinances, from Federal to local, usually base acceptable safe performance and application of fuses and circuit breakers on the National Electrical Code. The NEC commonly refers proven safe performance to the Standards of 8 Branch Circuit Fuse Size, Individual Motor Load, Safety of Underwriters Laboratories, Inc. The NEC is sponsored by With Fuse Overload Protection (No Starter the National Fire Protection Association. Overload Relays): (N.E.C. 430-32, 430-36): Therefore, fuses manufactured to meet the same standards of a) Motors with 1.15 Service Factor or temperature rise not U.L. testing and “Listing” for specific types are physically and over 40 Degrees C., size fuses at not more than 125% of the electrically interchangeable, based on the standards, regardless motor nameplate current rating. of brand. b) Best protection is obtained by measuring motor running Fuses can be designed and manufactured to higher standards than current and sizing fuses at 125% of measured the minimum standards required to pass U.L. testing to current for normal motor operation. Reference be “Listed”. to “Average Time/Current Curves” is recommended. For all of the reasons above, Edison Fuses Inc. manufactures d) Recommended Fuses: LENRK/LESRK, JDL, or ECNR/ECSR . fuses not only to meet industry standards, but also to reach the 9 Branch Circuit Fuse Size, Individual Motor Load, highest quality standards found in the industry. With Starter Overload Relays (N.E.C. 430-32, 430-52): National Electrical Code a) For “back-up” N.E.C. overload, ground fault and short- The National Electrical Code (N.E.C.) is sponsored by the National circuit protection size the fuses the same as (8 a, b) above, or Fire Protection Association. This Code is adopted as the MINIMUM the next standard size larger. standard for public safety by the federal government, states, counties, cities and many private organizations. Enforcement is b) The fuse sizes in a) above may be increased as allowed by usually the responsibility of Professional Electrical Inspectors NEC references. Generally, dual element fuses should not exceed 175% of motor nameplate F.L.A. and non-U.L. defined provided with enforcement authority. time-delay fuses not more than 300%. Fuse Ratings c) Recommended fuses: LENRK/LESRK, JDL, ECNR/ ECSR or LCL. 10 Fuse Sizing for Individual Large Motors With F.L.A. Above 480 Amps or Otherwise Require Class L Fuses - (N.E.C. 430-52): Application Tips 1. Size fuses as closely as practical to the ampacity of the protected circuit components without the probability of Fuses with an A-C voltage rating may be applied at system unnecessary fuse opening from harmless, transient current voltages below the fuse voltage rating, but not at voltages above surges. This usually requires a choice between time-delay and the fuse voltage rating. The other A-C fuse ratings remain the non-time-delay fuses. same at applied voltages below the fuse voltage rating. 2. Use Class R fuse clips with Class R fuses to prevent installation A-C rated fuses should not be applied in D-C voltage circuits of fuses with less interrupting rating or current limitation. Class H unless D-C application ratings are provided by the fuse fuse reducers cannot be used with Class R fuse clips. manufacturer. Except for some special purpose fuses, D-C ratings 3. When a conductor is oversized to prevent excess voltage drop, are not usually shown on fuse labels. size the fuses for the ampacity of protected circuit components EDISON Time/Current Curves, Peak Let-Through Curves are based instead of oversizing fuses for the larger conductor. on 60 Hertz A-C data. The operating frequency (Hertz) will affect fuse characteristics in Safety various ways. Industry PUBLIC SAFETY standards that apply to overcurrent Time/Current Curves will not shift and fuse ratings will not change protection devices (OPD), fuses and circuit breakers, are intended from 1-100 Hertz in normal applications. If ferrous hardware is to apply to both types of devices. The use of these devices serves used to mount the fuses, eddy current heating could alter the public safety which includes all aspects of fuse or circuit breaker ratings. performance and dependability. Above 100 Hertz, “skin effect” could alter the fuses’ rating Any industry requirements that apply to fuses apply equally to characteristics. This effect must be analyzed on an individual circuit breakers. Industry standards do not intend any deliberate application basis. compromises for public safety. Z-107 PRACTICAL APPLICATION INFORMATION FUSE RATINGS, CONT. Other characteristics shown on fuse labels such as “interrupting rating”, “current limiting”, “time-delay”, “fast-acting” and Correction Factors for Edison Fast-Acting and Non- (1) Underwriters Laboratories, Inc. (U.L.) Class are described DelayCorrection Fuses Factors for Edison Fast-Acting and Non- Delay Fuses(1) elsewhere. Ambient Celsius Maximum Load Current Fuse labels show “certification” or “listing” logo’s when applicable. TemperaturesAmbient Celsius (%Maximum of Rating) Load Current Such logo’s may be: -20Temperatures to -40 93%(% of Rating) -19-20 to to -1 -40 90%93% • “U.L.” (U.L. “Listed”) zero-19 to -124 86%90% • “CSA” (Canadian Standards Association “certified”) 25zero to to60 24 80%86% 6125 to to 80 60 60%80% • (U.L. Recognized) 8161 to to 100 80 45%60% (1) Such logos mean that the fuse meets the performance standards Fuses installed81 to 100 in an enclosure with a base loading45% of 80% as required by the National(1) Electrical Code. Contact Edison Fuses Inc. for other conditions. of these safety organizations. Fuses installed in an enclosure with a base loading of 80% as required by the National Electrical Code.