8.3 Amp limit on 20 amp branch circuit for magnetic loads?

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sparkycoog

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Texas
Sorry to be vague, but after having some issues with 20 amp fuses on a dimming system burning out and contacting the manufacturer (which will remain anonymous as will I) they limit their 20 amp line side to 8.3 amps for magnetic low voltage loads (such as MR-16 fixtures, transformers for low voltage lights, etc). The same system can handle the full 16 amps of a continuous load for incandescent loads. This was news to me. I always thought that it would just be 80% of the 16 amps and the maximum is 12.6 amps (and I had gone even below that) and it still blew out the fuse. They now have the 8.3 amps written in the documentation but it wasn't there before when this system was installed so when I asked about it, I was told that this was always in the NEC and it hasn't changed regardless of the label.

Can someone please let me know where the 8.3 amps came from? Is this actually general knowledge NEC code that I was not aware of? I have to say, I feel rather stupid at the moment.
 
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jim dungar

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sparkycoog said:
Sorry to be vague, but after having some issues with 20 amp fuses on a dimming system burning out and contacting the manufacturer (which will remain anonymous as will I) they limit their 20 amp line side to 8.3 amps for magnetic low voltage loads (such as MR-16 fixtures, transformers for low voltage lights, etc). The same system can handle the full 16 amps of a continuous load for incandescent loads. This was news to me. I always thought that it would just be 80% of the 16 amps and the maximum is 12.6 amps (and I had gone even below that) and it still blew out the fuse. They now have the 8.3 amps written in the documentation but it wasn't there before when this system was installed so when I asked about it, I was told that this was always in the NEC and it hasn't changed regardless of the label.

Can someone please let me know where the 8.3 amps came from? Is this actually general knowledge NEC code that I was not aware of? I have to say, I feel rather stupid at the moment.
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It is a manufacturer's design issue not an NEC one.
You are experiencing a problem with the inrush current of your transformer and of the cold lamp filament. A different protective device may have time-current characteristics which could accommodate more than 8.3A.
 
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sparkycoog

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Location
Texas
jim dungar said:
It is a manufacturer's design issue not an NEC one.
You are experiencing a problem with the inrush current of your transformer and of the cold lamp filament. A different protective device may have time-current characteristics which could accommodate more than 8.3A.
Click to expand...

Thanks for your input. It makes me feel a little less stupid :) I politely asked them to point out the area of the NEC code that it's in so I have a feeling an engineer is going to give me some formulas (which actually would be awesome) and pass it off as stuff that electricians need to know already (which wouldn't be so cool).
 
mbrooke

mbrooke

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What type of breaker is it. Square D by chance?


Breaker makers offer breakers specifically for tungsten. HID and high inrush loads.
 
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sparkycoog

Member
Location
Texas
mbrooke said:
What type of breaker is it. Square D by chance?


Breaker makers offer breakers specifically for tungsten. HID and high inrush loads.
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It's a Square D Q breaker but it's not the breaker that's tripping. I remember we had some issues with the heavier loads with 1000 watt 24volt transformers causing the tripping but after I programmed the lights to have a ramp time of 5 seconds, never had the issue with those. The problem is with the actual dimming system's built in fuses which are slow burn 20amp and it happens after they've been on for long periods of time. We're going to change a few lights on the same breaker to LED to lessen the load though LED's have an inrush as well.
 
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sparkycoog

Member
Location
Texas
jim dungar said:
Now it sounds like it is a heat transfer problem.
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Honestly, it's just a perfect storm. The room where the system is kept is not ventilated and they have all their AV gear in there as well; They decided to use higher wattage bulbs than we strongly recommended; and the lights are on basically all day at full blast. None of this information was given at the time of the installation of course. Fortunately they are understanding and are going to use LED bulbs in trouble areas where possible.
 
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