#### wwhitney

##### Senior Member
I think if I buy a 1600A fused disconnect switch with a UL listing for 1600A my calculation for how much load the terminations in the switch can carry continuously would be the rating of the switch per article 404 X 1/125% or (1600 X 80%) = 1280 Amps.
I bet the terminations could carry 1600A continuously, although we'd have to check the listing standard to answer this question.

But the terminations aren't the limiting factor as far as what maximum continuous current you put through the fused disconnect. If the switch is not 100% rated, and you put 1200A fuses in it, the NEC limits the continuous load through the fused switch to 1200A / 125%.

Cheers, Wayne

#### tortuga

##### Code Historian
I bet the terminations could carry 1600A continuously, although we'd have to check the listing standard to answer this question.

But the terminations aren't the limiting factor as far as what maximum continuous current you put through the fused disconnect. If the switch is not 100% rated, and you put 1200A fuses in it,
I think it would only need to be 100% rated if carries 100% of its marked rating (1600A with 1600A fuses).
the NEC limits the continuous load through the fused switch to 1200A / 125%.
Code reference?

#### wwhitney

##### Senior Member
Code reference?
215.3 for feeders, 210.20(A) for branch circuits.

Cheers, Wayne

#### tortuga

##### Code Historian
Its a service disconnect, Its listed for 1600A more than 125% of the rating of the 1200A service, 230.79.
If it was a feeder I'd still argue 215.3 Ex 1 it's rated for 100% of a continuous 1200A load, a 1600A disconnect can carry 1280 amps continuously.

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#### electrofelon

##### Senior Member
I think if I buy a 1600A fused disconnect switch with a UL listing for 1600A my calculation for how much load the terminations in the switch can carry continuously would be the rating of the switch per article 404 X 1/125% or (1600 X 80%) = 1280 Amps.
I agree with the logic. It would be silly to conclude that although the 1600A switch with 1600 fuses can do 1280 continuous, if we put in 1200A fuses something is different and the assembly won't work at 1200 amps continuous. But.... Unfortunately I don't know if I can get there as this being allowed by the code language.

Consider an analogous case for a circuit breaker. Say we have a 100% rated circuit breaker assembly which is a circuit breaker in an enclosure. Now say we take that circuit breaker and put it in a larger enclosure say maybe for the next frame size up. As silly as it is, if the circuit breaker in that larger enclosure are not a listed combination, I don't see that you can use it at 100%

#### tortuga

##### Code Historian
I see what you guys are saying.

#### electrofelon

##### Senior Member
So what does it take to get a 100% rated fused assembly? Im surprised fused safety switches are not 100% rated - particularly the 800- 1,200 amp ones, I mean those suckers are big with tons of space.

#### jim dungar

##### Moderator
Staff member
So what does it take to get a 100% rated fused assembly? Im surprised fused safety switches are not 100% rated - particularly the 800- 1,200 amp ones, I mean those suckers are big with tons of space.
Almost all bolted pressure switches are 100% rated by design, but enclosure manufacturers may not provide enough ventilation.

#### wwhitney

##### Senior Member
Its a service disconnect,
The switch is the service disconnect. The fuse is the service overcurrent device. It is also the overcurrent device protecting the downstream feeder.
Its listed for 1600A
In your example, the switch is rated at 1600A. The fuse is rated at 1200A.

The overcurrent device (fuse) can only be used at 100% of its rating if "the assembly" (215.3 Exception 1) is rated at 100%. So the switch will either be listed for 100% rated usage (and is therefore larger, more ventilated) or not. The ampere rating of the switch is immaterial, as long as it is at least the rating of the overcurrent device, 1200A.

It is tempting to say "wait, if we put a 1600A fuse in the 1600A switch, everything is rated for 1280A continuously, so it can dissipate the heat thereby generated, so how is there a question as to whether it can dissipate the heat from 1200A continuously?" To which the answer is that a 1200A fuse is different from a 1600A fuse, and the 1200A fuse may generate more heat at 1200A than a 1600A fuse would at 1200A. I would expect that the 1200A fuse has a smaller fusible element that will melt sooner, so the fusible element has a higher resistance, so it generates more heat at any given amperage compared to a 1600A fuse.

So you can not say that a switch and enclosure is 100% rated at 1200A just because it has a 1600A rating.

Cheers, Wayne

#### wwhitney

##### Senior Member
It would be silly to conclude that although the 1600A switch with 1600 fuses can do 1280 continuous, if we put in 1200A fuses something is different and the assembly won't work at 1200 amps continuous.
Something is different, the fuses. 1200A fuses may generate more heat at 1200A than 1600A fuses would generate at 1200A.

More importantly, even if the 1200A fuses and the 1600A fuses generate the same rate of heat when used at 1200A, in a non-100% rated enclosure, that rate of heat from 1200A may still be enough to elevate the temperature in the enclosure above the design ambient temperature for the fuses (unknown, but we have no testing guarantee that this wouldn't happen). Which would shift the trip curve of any installed fuses to blow sooner. Which wouldn't matter for the 1600A fuses, as there's a 400A buffer to avoid nuisance blowing. While it would matter for the 1200A fuses.

Cheers, Wayne

#### tortuga

##### Code Historian
In your example, the switch is rated at 1600A. The fuse is rated at 1200A.

The overcurrent device (fuse) can only be used at 100% of its rating if "the assembly" (215.3 Exception 1) is rated at 100%.
The NEC is permissive, 404 does not prohibit me from using the nameplate rating of the switch X 80% to get the 100% rating of 1200A
So the switch will either be listed for 100% rated usage
That would only apply if I wanted to use the 1600A switch at 1600A with 1600A fuses.

#### wwhitney

##### Senior Member
The NEC is permissive, 404 does not prohibit me from using the nameplate rating of the switch X 80% to get the 100% rating of 1200A
The switch is not the overcurrent device, the switch's amp rating is immaterial for applying 215.3.

The fuse is the overcurrent device, and its rating matters for 215.3. There are limits on when the fuse may be used at 100% of its 1200A rating (215.3 Exception). Those limits are not related to the amp rating of the switch/enclosure. What matters is whether the assembly of the switch/enclosure and the fuse is listed together for 100% operation. That "together" listing may not be inferred simply from the 1600A rating of the switch/enclosure on its own.

Cheers, Wayne

#### wwhitney

##### Senior Member
Here's a fairly simple model of fuse thermal behavior that I just made up, with made up values too. The model is surely inaccurate overall, but should accurately illustrate the issue here, just using I2*R, and that heat transfer rates are proportional to the temperature difference.

- The fusible element blows when it reaches 150C (maybe it melts, maybe it just weakens and a spring overcomes its reduced strength)
- The fusible element is sized so that at the rated current it achieves a 90C temperature rise over its ambient temperature. The physical size of the fuse is independent of its rating, so the total heating from any size fuse carrying its rated current is the same.
- So in free air at 40C, the fusible element at rated current will reach only 130C and not blow.

- A normal enclosure and switch/busses are designed so that at rated current with the rated fuse size the internal temperature rise is 30C over the enclosure ambient. Let's say half of the heat generation comes from the fuse itself, half from the switch/busses.
- So at 40C ambient, with rated fuse and rated current, the normal enclosure interior would reach 70C, and the fuse would reach 160C, and blow prematurely.
- While if we reduce the current to 80% of rated, all the heat generation goes down via a factor of 80%2 = 64%, so all the temperature rises go down to 64% of the previous case. That means the 120C total temperature rise becomes only 77C temperature rise, and the fuse reaches only 117C, and it holds.

- A 100% rated enclosure and switch/busses are designed so that at rated current with the rated size fuse the internal temperature rise is 15C over the enclosure ambient. Let's say the switch/busses heat generation is the same as the normal enclosure, so the 100% rated enclosure is twice at good at rejecting heat, so you only get half the internal temperature rise.
- Then at 40C ambient, with rated fuse and rated current, the 100% rated enclosure would reach 55C on the interior, and the fuse would reach 145C, and hold.

With this model, what happens if we put a 1200A fuse in a 1600A normal enclosure and run 1200A through it? The heat generation from the fuse is unchanged from the case of a 1600A fuse carrying 1600A, as it is a fuse running at its rated current. The heat generation of the switch/busses is reduced by a factor of (12/16)2 = 9/16, but it was only half of the total heat generation.

So the new total heat generation rate compared to the old is 1/2 (the fuse) + (1/2) * (9/16) (the switch/busses) = 25/32 of the previous case. That means the interior temperature rise of the enclosure is only 25/32 of the previous case, or (25/32) * 30C = 23C. So the fuse reaches 40C external temperature + 23C enclosure temperature rise + 90C fuse temperature rise = 153C, and the fuse blows prematurely.

Cheers, Wayne

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#### tortuga

##### Code Historian
As always you make a great explanation and I concur, there could be extra heating.
It would be interesting to see if UL or the CMP have discussed this matter.
the assembly of the switch/enclosure and the fuse is listed together for 100% operation.
I perused UL 363 and did not see anything about this special combination fuse/switch assembly '100% rating'.
UL 363 13.1 says a knife switch shall carry its rated current continuously (100% rating) without any part showing a temperature rise of more than 30C.
13.2 describes a current test requiring the fuses to be omitted.
Nothing under current ratings in 33

#### jim dungar

##### Moderator
Staff member
Something is different, the fuses. 1200A fuses may generate more heat at 1200A than 1600A fuses would generate at 1200A.
Manufacturers now produce Class L fuses with lower ratings, like a 1600A body with a 1200A element.

#### electrofelon

##### Senior Member
Eaton does provide a program that will tell you the jest dissipation of fuses. The location is in this page but I couldn't find it. Be interesting to see what the heat dissipation is of a 16 amp versus a 1200 amp fuse

#### wwhitney

##### Senior Member
I perused UL 363 and did not see anything about this special combination fuse/switch assembly '100% rating'.
I'm thinking it would be in the UL listing standard for the fuse (the overcurrent device), rather than for the switch. If you investigate that, I'd be interested to know what you find.

Cheers, Wayne

#### tortuga

##### Code Historian
I'm thinking it would be in the UL listing standard for the fuse (the overcurrent device), rather than for the switch. If you investigate that, I'd be interested to know what you find.

Cheers, Wayne
I am no UL expert or anything but all fuses I know of are rated for continuous loads some even110%.
That document Fellon posted looks interesting, and Jims mention of a lower rated 1200A Class L.

#### wwhitney

##### Senior Member
I am no UL expert or anything but all fuses I know of are rated for continuous loads some even110%.
Just perused UL 248-1 (Low Voltage Fuses-General Requirements). The fuses are tested at 25C (+/- 5C), not 40C. Fuses up to 600A must hold rated current, and those above 600A must hold 110% of rated current. The test is done in still free air with the fuse horizontal. The test proceeds until the fuse temperature stabilizes, and the recorded temperature rise is the temperature rise rating of the fuse. The maximum allowable temperature rise varies by fuse class. No discussion of 100% rating within an enclosure.

Further perused UL248-10 (Low Voltage Fuses-Class L). No discussion of 100% rating within an enclosure.

Apparently further research is required on the UL standards. Maybe UL 98?

Cheers, Wayne

#### wwhitney

##### Senior Member
Maybe UL 98?
Perused just Section 9 (Markings) of UL 98 (Enclosed and Dead-Front Switches). Section 9.2.5 requires fused switches to be labeled "Continuous load current not to exceed 80 percent of the rating of fuses employed in other than motor circuits." I saw no mention of alternative labeling that allows 100% rating (for other than motor circuits) based on passing some tests referenced elsewhere in the standard.

So if we want a 100% rated fused disconnect (for other than motor circuits), it will have to be listed to a standard other than UL 98. And note that the wording in UL 98 9.2.5 refers to 80% of the rating of the fuses, not 80% of the rating of the switch, in accordance with my comments in this thread.

Cheers, Wayne