80% rule?

[Darkwood]

Many seasoned sparks will have come across a mcb that has burnt out internally leaving a disc of brown heat damaged plastic on the sides of the mcb but no other obvious issues...most assume faulty breaker unaware it may be down to burn out due to no grouping factors been applied, and guess what they just replace like for like nd set the whole senario up again.

 

[Gardner]

Non at all taken. I am an industrial spark myself. It was just the last time gardener was on the forum he was wiring up caravans/trailers and i was just a little curious.

I know, but if such a thing were wired under the NEC any circuit loaded to the max for over 3 hours would need to be de-rated to 80%. Some items like fixed electric baseboard heaters and water heaters less than 120 gallons are considered continuous irregardless so in a case like this I can not put more that 12 amps on a 15 amp circuit, 16 on a 20 and 24 amps on a 30amp circuit (same for larger sizes)

While the NEC doesn't give reasoning behind the 80% rule its said to be for the exact reason stated in the forum that heat build up in panel boards can lower the breaker's trip curve.

 

[Gardner]

Many seasoned sparks will have come across a mcb that has burnt out internally leaving a disc of brown heat damaged plastic on the sides of the mcb but no other obvious issues...most assume faulty breaker unaware it may be down to burn out due to no grouping factors been applied, and guess what they just replace like for like nd set the whole senario up again.

That makes sense. Diversity prevents heat build up, but where a commercial premises exists multiple circuit loaded to the limit without considering any adjustments can lead to problems. However this would be considered a design issue rather than a code issue.

 

[Darkwood]

If you think about it steps have already been taken to resolve this issue before your design to make it easier to comply even if ignorant to problems....

13amp plug or fused 13amp amp spur covered by a 16mp mcb
Kw common rating of elements are 1, 2 and 3 so approx 4, 8 and 12 amps (6, 10 and 16 amp mcb's)

Obviously subject to UK voltage and not USA

Like I said it depends how you view some regulations, the one that require you should follow manufacturers guidelines would suggest you have to apply spacing for the above mentioned scenarios or a derating factor but its the case that most mcb's are supplied loose and only with the info stamped on them so as you haven't been supplied the guidelines for derating or spacing are you not in breach of the regulations anymore... this sadly is where the BS7671 falls down and also the knowledge of the last generation on sparkies who never have even discussed this in lessons at college.

 

[HandySparks]

Thanks!

So its not so much a code requirement as a requirement by manufactures?

Indeed. Worth taking the manufacturer's advice into account, as part of the normal design process. You don't want circuits tripping due to hot circuit breakers if it's your signature on the cert for design.