Fusing for Resistance Heating

Consider a resistance-heating load comprised of 12 heaters. Each individual heater is rated for 3000 watts at 480 volts, 3 phase and all the heaters are located approximately 25 feet from the enclosure containing the switching device. Is it permissible to run a single set of appropriately sized (6 gauge) conductors, fused at 50 amps, across the 25 feet and then tie all the individual heater leads to these conductors at a power distribution block. Do I properly understand NFPA 79 article 7.2.11.2 (quoted below) to mean that additional fusing of the individual heaters is not required, despite their relatively small gauge lead wires? Thoughts? Anything else that could necessitate additional fusing?

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Quoted from the 2002 edition of NPFA 79.

7.2.11 Resistance Heating Branch-Circuit Overcurrent Protection.

7.2.11.1 If the branch circuit supplies a single non-motor operated load rated at 16.7 amperes or more, the overcurrent evice rating shall not exceed 150 percent of the load rating.

7.2.11.2 Equipment employing resistance-type heating elements rated at more than 48 amperes shall have the heating elements subdivided. Each subdivided load shall not exceed 48 amperes and shall be protected at not more than 60 amperes.

7.2.11.3 The addional overcurrent protective devices shall include all of the following:
(1) Installed within or on the machinery or provided as a separate assembly
(2) Accessible but need not be readily accessible
(3) Suitable for branch-circuit protection.

The main conductors supplying these overcurrent protective devices shall be considered branch-circuit conductors.

What happens when a heater shorts out?

colerick said:

What happens when a heater shorts out?

Click to expand...

The wire become a "fuse" and will melt/ short out. Definitely this is not a panel I would want to be around when the electrons find a grounded conductor.

Twelve fuse holders... $300.00



Twelve 480v 10A fuses... $216.00


Not having to redo a control panel and repair fire damage.... priceless.

Ability to easily troubleshoot....

PRICELESS.

So, assume you took delivery of a panel built as described. Specifically what parts of the code would you cite to get the supplier to change it?

The math maybe

I was confused, what's new, with the math or description

50A times 480v = 24000w
BUT 12 heaters @ 3000w each = 36000w/480v = 75A

Still not sure I understand it precisely but the math, after watching replies too, does not seem appropriate for sizing the feed.

There is a thought in the back of my head, memory going bad I reckon. I have worked with systems designed similar to what you are mentioning.

If my memory is right some elements are U shaped and 2 elements are conneted together in other words you have UU with phase A going to top of first element, Phase B to center of both elements and Phase C to top outside of second element.

Here is where my memory fails me BUT I think the elements were rated for matching to obtain total wattage of that ckt ie the 2 combined totalled 3Kw output; which means your ckt would have 18000W total; which would be 37.5A; which is within the requirements of NFPA 79.

The aspect of it shorting is usually controlled because the elements should or do not "touch" anything (normally), most times just burn out and the heat dissipates.

Rsdoran,

Good catch...

agarb,

Has this equipment been paid for?

Andy

I edited my reply above instead of replying, getting senile.

As I mentioned though it was common in some cases to run from a single feed and just jumper to the next connection.

My suggestion is check on the elements and see if they actually produce 3000W each or interconnected provide 3000W.

rsdoran said:

36000w/480v = 75A

Click to expand...

You also need to divide by 1.73 since it is 3 phase. This gives about 43 amps.

I believe I understand the rest of your description. Have to think about that.

Why it will be cos theta, its pure resistance. Your formula applies to inductive loads etc.

See equation 3.52 here: http://www.phptr.com/articles/article.asp?p=101617&seqNum=8&rl=1

If it would be an inductive load we would have the cos theta factor which is different than the square root of 3.

OK, I need to study this link, too. Hopefully I haven't opened-mouth, inserted foot.

agarb said:

See equation 3.52 here: http://www.phptr.com/articles/article.asp?p=101617&seqNum=8&rl=1

If it would be an inductive load we would have the cos theta factor which is different than the square root of 3.

OK, I need to study this link, too. Hopefully I haven't opened-mouth, inserted foot.

Click to expand...

I think your right if these are 3 phase heaters.

P= sqrt3 (El)*(Il) cos theta

Because it is a resistive load cos theta is 1.

But are you sure these are 3 phase heaters? or is it just single phase 480V (3 fuses or two)?

agarb said:

Consider a resistance-heating load comprised of 12 heaters. Each individual heater is rated for 3000 watts at 480 volts, 3 phase

Click to expand...

emphasis added.

allscott said:

I think your right if these are 3 phase heaters.

P= sqrt3 (El)*(Il) cos theta

Because it is a resistive load cos theta is 1.

But are you sure these are 3 phase heaters? or is it just single phase 480V (3 fuses or two)?

Click to expand...

3 fuses.

Actually they are single phase heaters rated for 480 volts. But wired in delta configuration with 4 heaters in parallel per side of the delta. Hope that doesn't confuse things more.

So:
- The 12 heaters wired in a delta configuration.
- 4 heaters in parallel per side of the delta
- 4*3kw = 12kw per side of the delta
- 12kw/480v = 25 amps of *phase* current (through the side of the delta)
- 1.73*25 amps = 43.3 amps of *line* current

Thanks, Alaric, for the added emphasis.

So back to the original question, let's assume all the sizings are correct. I can understand that individual fuses would be nice, perferred, easier to troubleshoot, etc. But does code actually require them? The only other (pertinent?) reference I can find in NFPA 79 is 7.2.4.2.1, "Conductors, other than flexible cords and fixture wires shall be protected against overcurrent in accordance with their ampacities as specified in Section 13.5, unless otherwise permitted in 7.2.4.2.2 through 7.2.4.2.6. I believe a strong arguement could be made that the heater pigtails could be considered as 'fixture wires' as they come directly from the manufacturer in this fashion.