fuses rated 80%

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malachi constant

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I know a standard circuit breaker is 80% rated - e.g. if you put an 18A load on a 20A breaker it isn't going to work. My co-worker (also a PE) tells me standard fuses are also 80% rated. Is he correct?

Practical example: I have always fused panels and panel feeders assuming a 100%-rated fuse. If I had a panel with 85A of calculated lights and receptacles, I felt comfortable with a 100A fuse. In this example, do I have to use a fuse rated 85A*1.25 -> 110A? And this would only leave the Owner with 110A*0.8 = 88A -> 88A-85A = 3A calculated capacity? Yikes!

FWIW, the fuses I am referring to are Bussmann LPN-RK/LPS-RK and similar.

Thoughts?
 
A standard circuit breaker is not 80% rated. A 20A breaker is rated for 20A. An 18A load on a 20A breaker is just fine as long as the load is not continuous. In the event a continuous load is expected, the breaker rating must be increased by 25%.
 
Unless specifically labled otherwise, all UL Listed ENCLOSED overcurrent protective devices are effectively "80% rated". This corresponds with the NEC inregards to sizing conductors and OCPD for continuous loads at 125% of full load.
 
Jim I agree with what you are saying I just feel they way you put it sounds like 18 amps non-continuous on a 20 amp OCP is going to be a problem.

We can load a feeder to 100% non-continuous with standard run of the mill OCPDs.

JMO, Bob
 
FWIW, I have been told by multiple fuse manufacturers that when designing a fused circuit, that the continuous load must be no more than 80% of the fuse rating. The reason is that between 80-110% of the fuse rating, there is insufficient current to cause the fuse to open, but there is plenty of heat available to 'cook the fuse'. And so it cooks until it eventually fails. No code reference here, just feedback from the OEM's. My interest in this was for supplementary OCPD useage in our equipment. The 125% multiplier mentioned by Jim Dungar appears in many places in our beloved ;) book (motors, transformers, etc) provides a more formal statement in specific situations.
 
Both fuses and breakers are tested will 100% loads, but they are tested in open air, not in an enclosure. The 80% limit is based on heat from other OCPDs in a common enclosure.
Don
 
To answer your question - yes, fusible switches are also 80% rated. You would have to use a bolted-pressure switch to get a 100% rated switch, and those are only available in 800A and larger sizes as far as I know.
 
don_resqcapt19 said:
The 80% limit is based on heat from other OCPDs in a common enclosure.
Don
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I don't think that's quite right Don, because even single enclosed unit CB's are 80% rated unless noted otherwise.
 
From a SquareD document.
UL requires that circuit breakers must be able to carry their continuous current rating indefinitely at 40?C in free air in order to achieve a UL Listing. The NEC recognizes that devices applied in end-use equipment can be affected by heat build up during normal operating conditions. For this reason, NEC 220-10(b), below, requires that circuit breakers be selected based on the characteristics of the load (particularly, the portion of the load which will be on continuously for three hours or more at a time).
Click to expand...
Don
 
don_resqcapt19 said:
From a SquareD document.
Don
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Not sure who you were replying to Don but in my comment I meant the the derating is based on heat buildup from the OCP itself, not other OCPs.
 
Let's not overcomplicate things here. On the other hand, as electricians we sometimes tend to over-simplify. The fact is that a fuse or circuit breaker will hold 100% of its rated current at its rated ambient tempertature indefinitely. It will probably hold 125% of its rated current for a long, long time. It is only if it is continuously loaded (3-hours or more) that the load should be limited to 80%. This has often been "simplified" to the "fact" that no breaker or fuse should ever be loaded to more than 80% of its capacity. Then we get statements like "a 20-amp breaker loaded to 18-amps won't work". The fact is that it will work fine. It will probably never trip with an 18-amp load. The whole purpose of a "low impedence ground-fault return path" is so that, in the event of a ground fault or short circuit, a breaker will see many times its rated current and thus trip quickly. Until a breaker sees around 200% of its rated ampacity, it will take many minutes to trip. Look at the trip curve for any breaker.
 
Thanks for the responses. I don't fully understand though. Please clarify as to which is correct:
a. An enclosed fuse/breaker is 100% rated for non-continuos loads, but 80% rated for continuous loads.
b. All enclosed fuses/breakers are effectively 80% rated.

I use "effectively" in the 2nd option meaning "for all practical purposes, even though they aren't necessarily actually 80% rated, you should treat them that way."
 
Sparkie and Don,

You are both right, the heat and temperature rise is based on the device itself, and other devices in the assembly. Reminder: The assembly and devices have to be rated for 100%, not just the breakers.

This is the difference, especially between switchgear (LV and MV) and other equipment such as panelboards and switchboards. The switchgear is specifically designed and tested to operate with all the breakers operating at 100% of rated current, at an ambient temperature of 40 deg C. (No appreciable solar affect). Above 40 deg C, the switchgear has to be derated. The derating is left up to the manufacturer, whereby the maximum ambient needs to be provided when purchasing.

Fuses have much the same limitations in that unless specifically approved for 100% operation, the increased temperature will affect the clearing time (longer) and continuous current carrying capability(lower).

Malachi, Breakers and fuses are rated for 100% of there nameplate rating, they are loaded to some lesser value (determined by calculation) in order to compensate for the heat gain and temperature rise, unless specifically rated for 100% operation in the assembly.
 
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Malachi,
I think statement "A" is closer to what I understand to be true. Remember that ambient temperature enters into the calculation, but in general a breaker may be loaded to 100% of its ampacity as long as the load is non-continuous. Only if the load is continuous would you need to limit the loading of the breaker or fuse to 80%. If you find a breaker or fuse (and the panel for the breaker fuse) that is rated for use at 100% for continous loads, the derating is not needed. See 210.19(A)(1) in the NEC for the exact wording. Also see article 100 for the definition of a continuous load.
 
malachi constant said:
a. An enclosed fuse/breaker is 100% rated for non-continuos loads, but 80% rated for continuous loads.
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For most CBs - yes. That's what you'll get if you don't say otherwise. But you CAN buy CBs in larger ratings (400A+) that are rated for 100% loading, continuous or non-continuous. Look at some catalogs.


malachi constant said:
b. All enclosed fuses/breakers are effectively 80% rated.
Click to expand...

No, most are. Again, you can buy 100% rated in larger sizes.
 
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