# Using thermal fuses in parallel?

If one has some 216C 10A thermal fuses, could one wire two in parallel in order to fuse a 10A heated bed on a 3d printer? The bed is 110V and 1100W btw, so in normal operation it should draw 10A. Seeing as how this is right at the top of the current range, I thought I would put two side by side just to be safe.

I can't think of any reason this would not work, theoretically the current should be pretty evenly divided between the two, no?

edit: this is not a duplicate question! Why is everyone so quick to mark things duplicate on here before even reading the whole post?!? The "duplicate" question is even asking about fuses of different ratings- my fuses are identical.

The arguments linked in the other posts all have to do with current-limiting fuses. Thermal fuses blow at a particular temperature, and as long as they are within their rated current, one does not expect them to be used to limit current.

Again- two fuses in parallel, if given 10A, will theoretically each carry 5A. Even if the fuses are terribly mismatched, due to manufacturing irregularities, and one carries 8A and the other carries 2A, this is fine because we are concerned about max temp, not max current. As long as the second fuse takes at least 1A, then the other can take 9A and they will both be under the rated 10A. Then if the temperature happens to climb to 216C, they will blow. Why would this not work?

And if the answer is still no, at what current would they be acceptable to use in parallel?

• Asked too often on here, no is the response. Commented Nov 8, 2019 at 6:57
• @SolarMike If it's asked 'too often' then you can find a duplicate question, right? Commented Nov 8, 2019 at 11:15
• @BruceAbbott I can use the search same as you and the op... Commented Nov 8, 2019 at 11:27
• – user199402
Commented Nov 8, 2019 at 16:14
• "my fuses are identical" - No, they're not. They are nominally the same, but nothing in practice is really the same. It makes no difference whether they are the same nominal value or different, the answer is the same. Commented Nov 24, 2019 at 2:41

To cool down emitions around "Is the question duplicate/or not" - i will just answer this one.

(1) Fuses in parallel:

You can place fuses in parallel, however this does not "double" the nominal current rating. Therefore, it is - besides high reliability system where a "false-fuse trip" is to be covered - useless.

(2) But why?

Fuses - and specifically thermal fuses - are not just "switches" which trigger at a given current or temperatur.

They have a resistance R(I,T,V) which is variant on T=Temprature, I=Current, and V=Production variance.

If you model two resistors in parallel, the current is equal as long as R1=R2. But, due to the V-dependence this will "never be applicable" in reality. These are "real devices", so they differ in parameters.

Therefore current sharing is not equal.

(3) My opinion:

In a device you are going to get certified, you intend to sell, or you want to use without problems: Big no no!.

However: Prototyping, waiting for "actual parts" to arrive, "I couldn't care less"... Why not? By paralleling two 10A fuses, you will maybe get a 15A nominal rating out of it, before one fuses trips and the second one trips shortly after that.

(4) In conclusion:

Just get a Inom x 2 fuse. So in your case a 20A device.

• Would the significant temp coefficient not cause the fuses to balance? I would think it would.
– Drew
Commented Nov 30, 2023 at 14:34
• @Drew To some degree for sure. 100% - dont think so. Also, the tempco varies from device to device, and from application to application. Small airflow over one fuse only, one fuse closer to a heatsink, one fuse absorbs a bigger impuls from a transient and so on. Steady state maybe. Commented Nov 30, 2023 at 14:41
• I wrote an "answer" yesterday and then deleted it (at least for now. A vital point to note here is that these are NOT current tripped fuses. They are trpped by external temperature. This makes an immense difference to answers (or should :-) ).There will be SOME contribution from internal current based heating BUT this may be small (and specs are not available for the fuses in question) and a competent fuse designer would keep it as small as possible to make the fusing temperature as coniustent as possible across the current range. Commented Dec 1, 2023 at 0:43
• Here is a datasheet for a formally specified product. In the graph on page 4 it shows a 11 degrees C temperature rise due to self heating at rate max current (15A in this case). ie trip temperature would be about 11 C lower at full current than at no current || A say 10A device operated at say 15A would (very probably) not have a self heating temperature high enough to trip but may fail for other reasons. Commented Dec 1, 2023 at 0:50
• @RussellMcMahon Sir, this indeed is a vital point, that should make a difference to the answers. I assumed, these devices with either trip on over-temperatur logic or on over current. Should have invested more than 3seconds looking at the product page. Commented Dec 1, 2023 at 1:09