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Thread: Cable sizing for a stove.

  1. #21
    Diamond Member AndyD's Avatar
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    Quote Originally Posted by Canuck View Post
    Just an observation and a subsequent question. In Canada we use the wattage information for load calculations when it is on the nameplate data. Also one of the things happens a lot here is manufacturers of the tops and oven are different and the two are not designed to be connected to the same circuit as there is no approved overload devices between the units to prevent the top from drawing too much current from a breaker that would be to large for the top itself.
    The circuit breaker is sized to protect the cabling of the electrical installation from overload, not to protect the actual appliances that are plugged in or connected. If an appliance requires its own overload protection it must be built into the appliance or designed into the appliance. For example many stoves have an internal wiring arrangement that prevents all the hob elements and the oven running simultaneously.

    Quote Originally Posted by Canuck View Post
    The question is are you allowed to load a breaker and conductor to 100% in a residential application? Here the breaker is not allowed to loaded to more the 80% in residential but the conductor can be loaded to 100%. In commercial the breaker and conductor must be sized to 125% Do you do anything like that?
    Yes, you can load a circuit to 100% of its capacity. There's sufficient safety allowances in the reference tables in the regs that dictate a circuits maximum load capacity so no additional blanket derating is required.
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    Thanks for the reply Andy. Regarding the stove and top comment I probably was not clear enough. What seems to happen here on occasion is an older wall oven and separate cook top have been installed previously using a 40A feed. The oven has all of its own protective fusing in its own junction box. In these older arrangements there was also a sub feed from the oven box to the cooktop box of smaller conductors based on its draw. The cooktop also has its own fusing in a junction box. The conductors in between the units are not rated for the 40A breaker that is feeding them as well as the stove. These cooktop conductors are usually within 3 meters of each and are classed as tap conductors. When the units are replaced the homeowner buys two separate units that are now of a much higher rating (often 40A and 30A, sometimes more) that require their own individual feeds from the panel. The owner gets upset because "the old ones were fine like that so just hook them up the same". In this case to do it correctly they must have their own individual feeds as the existing conductors from the panel only have a 40A rating whereas the new loading would be well over that.

    Regarding circuit loading, are you saying then that in a commercial application a known load of lets say 30A would only require 30A conductors and a 30A overcurrent device? (Here both would have to be sized at 125% of rated current of the load) In a residential application however there is are no demand factors and the circuit can be sized to 100% of rated current.

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    Diamond Member AndyD's Avatar
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    Quote Originally Posted by Canuck View Post
    ...........Regarding circuit loading, are you saying then that in a commercial application a known load of lets say 30A would only require 30A conductors and a 30A overcurrent device? (Here both would have to be sized at 125% of rated current of the load) In a residential application however there is are no demand factors and the circuit can be sized to 100% of rated current.
    There's no electrical safety reason you couldn't supply a 30A load using a cable that's rated at 30 A and an MCB rated 30A, (Ib ≤ In ≤ Iz).

    From memory there's a reg that states a circuit should be designed in such a way to avoid unnecessary nuisance tripping so it could be argued that if Ib = In = Iz you may be in breach of this reg if tripping occurs on days where ambient temperatures are high (making the thermal tripping curve of the MCB more sensitive) for example. Also a highly inductive or capacitive load or a load with a high inrush current or very poor power factor might be a consideration that would encourage me to upsize the circuit.

    A manufacturer will state the appliance current in the perfect storm worst case scenario ie with the highest permissible supply voltage, under absolute maximum load and even then they err on the side of caution. In real life I've never taken a current reading on an appliance and found it to be more than 95% of the manufacturer stated current. If the appliance was a fairly linear load with no notable inrush or start current and was manufacturer rated at max 30A I wouldn't have an issue installing it on its own 30A circuit as long as the ambient temperatures weren't going to be above average. That would hold for both domestic and commercial applications.
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    Quote Originally Posted by AndyD View Post
    Yes, you can load a circuit to 100% of its capacity.
    You can load a circuit to way more than 100 % of its capacity...if correctly designed for the application...an old aircon unit is a good example...using a D or C1 curve breaker you can load the circuit more than 10 times its capacity for a short period of time...new aircon no load require this type of overload protection...in fact we have had to replace these type of breakers due to nuisance buzzing for some reason (i havent done any research into this issue yet) ...this is why it is important for the person supplying the breaker to understand the application...we have also had to use type 2 breakers for electro magnets for holding down tools in CNC machines...due to the type of inrush current.

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    Thanks for the discussion. Obviously there are some differences in our codes which of course is what determines what we can and cannot do in each country. I think the reasoning for the demand factors applied here are that in a residential type occupancy the actual loading on average is less and the duration of loading is also less then what may be in a commercial application. At any rate it is interesting to view the comments and questions on the forum. Hats off to you though for sure in the area of safety. Your requirements of leakage protection has been in place for a long time and over here they are only starting to enforce such things like arc fault protection on all receptacle circuits of 15 and 20A configurations. The leakage type (which we call ground fault circuit interrupters) has been a requirement around washbasins and outside for quite some time. With the new arc fault requirement as well we often install beakers that are AFCI and GFCI combined for use outdoors and the AFCI inside. This requirement only applies to receptacles not lighting.

    ians: Like in your examples, we also have situations where the breaker size is larger then the current handing of the conductors. Usually in motor or compressor situations where the counter EMF does not rise fast enough to prevent overcurrent from tripping the breaker. Our rules allow the rating of the overcurrent device to be increased up to 400% in order to allow the motor to start. This of course does not mean that you would install a breaker at 400% of FLA but rather that it the breaker can be increased up to 400% to allow the motor to start. In most case the over load devices can be set at 115% or 125% of full load amps depending on the service factor of the motor. Is "aircon" short for airconditioner? What is a type 2 breaker?

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    Quote Originally Posted by Canuck View Post
    Thanks for the discussion. Obviously there are some differences in our codes which of course is what determines what we can and cannot do in each country. I think the reasoning for the demand factors applied here are that in a residential type occupancy the actual loading on average is less and the duration of loading is also less then what may be in a commercial application. At any rate it is interesting to view the comments and questions on the forum. Hats off to you though for sure in the area of safety. Your requirements of leakage protection has been in place for a long time and over here they are only starting to enforce such things like arc fault protection on all receptacle circuits of 15 and 20A configurations. The leakage type (which we call ground fault circuit interrupters) has been a requirement around washbasins and outside for quite some time. With the new arc fault requirement as well we often install beakers that are AFCI and GFCI combined for use outdoors and the AFCI inside. This requirement only applies to receptacles not lighting.

    ians: Like in your examples, we also have situations where the breaker size is larger then the current handing of the conductors. Usually in motor or compressor situations where the counter EMF does not rise fast enough to prevent overcurrent from tripping the breaker. Our rules allow the rating of the overcurrent device to be increased up to 400% in order to allow the motor to start. This of course does not mean that you would install a breaker at 400% of FLA but rather that it the breaker can be increased up to 400% to allow the motor to start. In most case the over load devices can be set at 115% or 125% of full load amps depending on the service factor of the motor. Is "aircon" short for airconditioner? What is a type 2 breaker?
    I didnt say the breaker is rated higher than the current carrying capacity of the cable...you can use a breaker which is rated for the correct cable size...with a slower trip curve...ie curve 1 or curve 2...depending on the application...for example curve 1 motor start up (referred to as in rush current) for "old" type air conditioners pump motors etc...curve 2 for a big transformer...like a magnetic table or cnc machines which have 400-230 3 phase transformers... required to reduce the voltage so they can be connected to 400 volt supplies in SA.

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