80% rule?

[Gardner]

Do the regs have any de-rating requirements on loads that run over 3 hours? Are any considered continuous vs none continuous? Normally any circuit that will see full current for over 3 hours must be loaded to no more that 80% of the breaker handle rating.

 

[andyb]

Do the regs have any de-rating requirements on loads that run over 3 hours? Are any considered continuous vs none continuous? Normally any circuit that will see full current for over 3 hours must be loaded to no more that 80% of the breaker handle rating.

Have to say this doesn't ring any bells with me, I'm fairly sure that it's not in the reg's. Maybe a guide somewhere?

 

[Darkwood]

Possible the OP has read a tech' spec for a board without spacers between breakers, there is a derating factor that is sometimes applied, also applies to modular contactors etc.

 

[Gardner]

Possible the OP has read a tech' spec for a board without spacers between breakers, there is a derating factor that is sometimes applied, also applies to modular contactors etc.

So if breakers are hunched together in a board the circuit needs de-rating?

 

[davesparks]

Are you in America by any chance? It's normally only Americans who use the term 'handle rating'

 

[Gardner]

Are you in America by any chance? It's normally only Americans who use the term 'handle rating'

My bad, MCB rating

 

[Darkwood]

So if breakers are hunched together in a board the circuit needs de-rating?

Yes it may but dependent on a few factors...

Size of enclosure
Ambient temperature
% loading of mcb's in respect to the rating

Most mcb's are calibrated at 30c so any temperature lower or higher will alter the service current it can deliver e.g.

A 63amp B or C curve Merlin C60H at 30c has a max service current of the mcb rating, the same mcb at 20c has a max service of 66.2amps.... if you say had a ambient enclosure temp of 50c then the same mcb would be only good for max service current of 54.2amps.

This is another one of those areas that yrs of dumming down the education system has seen disappear... this is another reason why domestic trained and any modern trained Electrician should really thing about doing advanced design courses before going onto the larger projects.

This is not usually a concern at domestic level as the demand rarely goes close to the service current of the devices for any real length of period.

PS Noted you are USA so you need to seek advice within your own region this is advice under UK regulations and standards so may differ.

 

[Gardner]

Yes it may but dependent on a few factors...

Size of enclosure
Ambient temperature
% loading of mcb's in respect to the rating

Most mcb's are calibrated at 30c so any temperature lower or higher will alter the service current it can deliver e.g.

A 63amp B or C curve Merlin C60H at 30c has a max service current of the mcb rating, the same mcb at 20c has a max service of 66.2amps.... if you say had a ambient enclosure temp of 50c then the same mcb would be only good for max service current of 54.2amps.

This is another one of those areas that yrs of dumming down the education system has seen disappear... this is another reason why domestic trained and any modern trained Electrician should really thing about doing advanced design courses before going onto the larger projects.

This is not usually a concern at domestic level as the demand rarely goes close to the service current of the devices for any real length of period.

PS Noted you are USA so you need to seek advice within your own region this is advice under UK regulations and standards so may differ.

I am working with BS7671 in this project.

So the way regs are written, I could legally have a 24/7 lighting circuit tally up 19.95 amps with 2.5mm2 CU and apply a 20amp MCB? As long as the enclosure ambient temperature does not exceed the MCB listed temperature rating, I am code complaint?

Of couse a panel board with 42 such circuits could go over 30*C requiring each circuit to be de-rated?

 

[Darkwood]

I am working with BS7671 in this project.

So the way regs are written, I could legally have a 24/7 lighting circuit tally up 19.95 amps with 2.5mm2 CU and apply a 20amp MCB? As long as the enclosure ambient temperature does not exceed the MCB listed temperature rating, I am code complaint?

Mcb's are designed to operate at their rated load at the specific ambient temp' to which they were clibrated (this will be a variable which is dependent on brand and type of device).

A 20amp mcb will happily run at 20amps indefinately if there is no restriction on heat dissapation, as this mcb is running at max mcb rating it will then be subject to any grouping factors, ambient temp' adjustments and any other factors that could restrict or aid the dissapation of heat... these particular factors are usually sourced from the manufactures tech data sheets and absent from the BS7671.

This is more so an advanced criteria and manufacturers rely largely on the majority of boards fitted been subject to diversity and loads running very long periods around max service load of the device are uncommon especially grouped together ..this tends to be more to do with control systems, boards dedicated to lighting banks etc again not the usual field of your standard Electrician...

So in short the BS7671 has no specific regulations regarding grouping of mcb's, contactors etc are its too big a variable but it does specify manufacturers guidelines are to be followed which is basically a blanket regulation saying do your homework regarding the gear you install.


I assume all the other calculations have been taken into account when asking this question if applicable?.. like inductive loads, as these will affect the mcb in other ways like nuisance tripping especially with grouped lighting switching together, again manufacturers tech support will specifiy a certain Rating and Type of mcb and how many of a specific lamp style it can switch on together.

 

[HandySparks]

Worth reading the technical info from the manufacturer of the circuit breakers. For example, see this post:

http://www.electriciansforums.co.uk...rum/89101-air-gap-bettwen-mcb.html#post925592

 

[rapparee]

20 amp lighting circuits. What sort of lights are you installing.

 

[davesparks]

20 amp lighting circuits. What sort of lights are you installing.

5KW ones!

 

[Gardner]

Worth reading the technical info from the manufacturer of the circuit breakers. For example, see this post:

http://www.electriciansforums.co.uk...rum/89101-air-gap-bettwen-mcb.html#post925592

Thanks!

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

20 amp lighting circuits. What sort of lights are you installing.

Off the head example, but Id imagine 20amp lighting circuits are the norm for commercial spaces and parking lot lighting circuits?

 

[Darkwood]

Don't you love it when the domestic realm get to peak into the the large Commercial and Industrial sectors and are shocked at the loading for say a lighting circuit lol.. no offense meant rapparee

 

[rapparee]

Don't you love it when the domestic realm get to peak into the the large Commercial and Industrial sectors and are shocked at the loading for say a lighting circuit lol.. no offense meant rapparee

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.

 

[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.