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Microwave/Convection Fire Hazard??

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techntrek View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote techntrek Quote  Post ReplyReply Direct Link To This Post Posted: 27 Sep 2009 at 4:36pm

At 110 volts the breaker should still be OK, but if the campground voltage drops below 100 volts that would bring the amperage high enough to overload it.

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gmandual View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote gmandual Quote  Post ReplyReply Direct Link To This Post Posted: 27 Sep 2009 at 5:21pm
EDIT:  9/29  This post was based on guesses of microwave performance.  See my later post here that has actual specs from manufacturer http://www.rpodnation.com/forum_posts.asp?TID=614&PID=5100#5100
 
Microwave is rated at 1450W at 120V 60hz.     So at 120v its going to pull around 12 to 13 amps.     If the voltage drops below 120v say to 110v due to resistance in power lines and distance from power source, then the wattage consumed by the microwave goes down as the voltage drop.  
 
Most circuit breakers trip with in a tolerance range of the rating for the circuit breaker.    The more times a breaker trips, the wider its tolerance tends to get and the more likely it is to trip again.    Many circuit breakers in houses, because of their age, are at about a 15% or more tolerance of their rating.  Which means they could trip  -15% of their rating.    That would put it real close to the max current used by the microwave.    From the looks of the number the manufacturer is just trying to limit complaints by "requiring" a 20amp dedicated circuit.   As they don't want to get calls about a faulty microwave always tripping a breaker.
 
A drop to 100v at the camp ground won't increase the amperage used by the microwave.   In fact the drop in voltage is just the E = IR equation balancing itself out for the current draw and resistance of the wires.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote HuronSailor Quote  Post ReplyReply Direct Link To This Post Posted: 28 Sep 2009 at 5:36am
Originally posted by gmandual gmandual wrote:

Most circuit breakers trip with in a tolerance range of the rating for the circuit breaker.   
 
That's what I was wondering, but I wasn't sure what the tolerance figures might be. My only past experience with a similar problems was when I did some fairly extensive rewiring on my last boat. The final straw that made me take on that project was that I had been having trouble with a particular AC circuit tripping the breaker. There was no way the load on that breaker should have tripped it, but it was an old breaker (15+ years old) that had come with the boat.  I bought a new, identical breaker, installed it during the rewiring project, and never had another problem.
 
The breakers are only about $12-15 at Home Depot, 10 minutes from the house. I guess if I have a problem with the microwave tripping out I'll replace the breaker first, because it will cost me at least $15 to drive back and forth to my dealer to let them try to "duplicate the problem."
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techntrek View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote techntrek Quote  Post ReplyReply Direct Link To This Post Posted: 28 Sep 2009 at 6:57am
Originally posted by gmandual gmandual wrote:

...If the voltage drops below 120v say to 110v due to resistance in power lines and distance from power source, then the wattage consumed by the microwave goes down as the voltage drop.  
 
Most circuit breakers trip with in a tolerance range of the rating for the circuit breaker.    The more times a breaker trips, the wider its tolerance tends to get and the more likely it is to trip again.    Many circuit breakers in houses, because of their age, are at about a 15% or more tolerance of their rating...
 
A drop to 100v at the camp ground won't increase the amperage used by the microwave.   In fact the drop in voltage is just the E = IR equation balancing itself out for the current draw and resistance of the wires.
 
First item above is incorrect, the wattage used by an appliance remains constant.  As a brownout occurs (voltage drops), the amperage must increase to maintain the total power used.  A 1450 watt appliance will draw 12 amps @ 120 volts, 13.2 amps @ 110 volts, and 14.5 amps @ 100 volts.  Might not even run correctly at 100 volts since that is well below the normal voltage tolerance range for most appliances.  An incandescent light bulb would run but would be noticably dim.  Compute using P=IE, power is always constant.
 
As for the second item, breakers are rated with their constant-current carrying ability, but under code they should only be normally loaded with 80% of that amound.  A 15 amp breaker should not trip with a full 15 amp load (and will have a long delay even at a few amps above that, and should not trip even at several times the amperage during the microwave's startup surge).  80% of 15 = 12 amps so the 15 amp breaker is correctly sized for the microwave.  Even after multiple trips the breaker shouldn't have a problem handling the load.
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gmandual View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote gmandual Quote  Post ReplyReply Direct Link To This Post Posted: 28 Sep 2009 at 7:35am
My statement about the microwave is correct, as the microwave is a highvoltage transformer that feeds the magnetron and is a current limited inductive load.    It will only draw its maximum current and provide the rated power output of a microwave at its rated voltage   The whole setup is highly current limited to keep putting out more power then the microwave is rated (for safety reasons), so if the voltage drops its impossible for the current to increase across the coil.    Since the microwave's are current limited then with P = IE when your voltage drops, your power output of the microwave must drop.   
 
Breakers can be rated at 80% or 100% constent load current.    My reference book at work lists the codes on the front that tells you what type you have.   Will have to check it against whats in the rig.   (Won't confuse everyone by even mentioning the 125% rule) :)
All home breakers will function at their rating of the breaker, its just depends on for how long.   For a constant load you are considering something around 3 hours or so at the rated current.  constant load rating for the breaker.   The reason being that most all traditional breakers trip based on heat, and in 3 hours of constant rated load  you build up enough heat to trip the breaker.    All breakers should (in theory)  be able to handle intermittent loads up to their 100% rating with out tripping, as the heat doesn't get enough time to build up.   A microwave is not normally on for more then 10-20 minutes so its actually considered to be an intermittent load.  
The 15% I speak of is that in the real world you have heat build up in the breaker box, this constant heat from other breakers and lack of ventilation (plus normal wear) can cause breakers to trip as much at 15% under their rating for both constant and intermittent loads.   So even intermittant loads under the 15amp breaker rating can trip a breaker, when in theory it shouldn't.    The ratings given to breakers are done under laboratory conditions, and in the real world things behave a bit differently.   Breakers also have a rating on how fast they will trip if you go over the max rating on the breaker.  They usually use slower trip breakers where you have things that can have a high start up current like motors in refrigerators or clothes driers.

If my memory is correct  then the microwave is it just on the edge of the code.   A permanently installed appliance is not to exceed 80 percent of the branch circuit amp rating.    And since its on its own breaker, its within code if the published numbers by the manufacturer are correct.   Will have to double check the building code books, to make sure that hasn't changed.   Been a while since I have had to keep up to the date.
After work will pull out the microwave and look at the back.   On the back of the microwave, per code, they will have a sticker that lists the maximum amperage draw of the appliance.    Its stupid (but not uncommon) that they didn't put that in the manual.
 
EDIT:  9/29  This post was based on guesses of microwave performance and specifications.  See my later post here that has actual specs from manufacturer and classification of device under NEC http://www.rpodnation.com/forum_posts.asp?TID=614&PID=5100#5100
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Post Options Post Options   Thanks (0) Thanks(0)   Quote HuronSailor Quote  Post ReplyReply Direct Link To This Post Posted: 28 Sep 2009 at 8:32am
Originally posted by HuronSailor HuronSailor wrote:

Page 4 states that the unit requires a 20A circuit, yet on page 7 it lists power consumption as follows: 1450W (microwave), 1350 (convection), 1350 ( top & bottom grill), 850 (top grill).
 
 
This info is in the manual.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote techntrek Quote  Post ReplyReply Direct Link To This Post Posted: 28 Sep 2009 at 8:47am

I have a background in electronics, and disagree gmandual.  You can not change the wattage of an appliance.  If you drop the input voltage physics/mathematics must balance out the equation so amperage increases.  P=IE.  Yes, microwaves have a current limiting circuit but that limits the current to the nameplate amperage at the nameplate voltage.  If you drop the voltage on a light bulb or microwave the effective work done (the light output or heating output) will drop, but the inputs will balance out to maintain the same wattage used.  Same wattage input, lower measured wattage output.  I have a 6kw UPS at home which can vary its voltage output.  I can hook a microwave up to a Kill-A-Watt and show you the amperage used at various voltages...

You also said above "In fact the drop in voltage is just the E = IR equation balancing itself out for the current draw and resistance of the wires."  Not sure what you meant by this because the resistance of wires has nothing to do with the operation of an appliance - 1000 feet of 12 gauge wire only has 1.59 ohms, very small.  This number only affects the electrician when they choose the wire size to put in the wall to limit voltage drop from the panel to the outlet (to not less than 3%), it never enters into any equation related to the operation of the appliance plugged into the outlet.
 
Edit: I just checked and a dedicated 20 amp circuit breaker is required by code for a built-in microwave.  Not sure why because a countertop microwave can be plugged into a non-dedicated countertop circuit (also 20 amps per code) with other countertop appliances on the circuit...
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David and Danette View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote David and Danette Quote  Post ReplyReply Direct Link To This Post Posted: 28 Sep 2009 at 9:15am
    I am reading all the posts on the microwave and circuit breaker. Has there been a final answer as to the size breaker, it sounds like a 20 amp. As I said before it is great to have all this experience and knowledge from the Rpod owners. Thankyou for all the information.  
       David and Danette
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gmandual View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote gmandual Quote  Post ReplyReply Direct Link To This Post Posted: 28 Sep 2009 at 9:32am
I think you are confusing the difference between limiting and regulating.  A whole range of factors change the wattage that a device consumes.    Many AC appliances are usually current limited or voltage limited.    This is much different then being current or voltage regulated.    Limited means it will never go higher, but may go lower.   Regulated means it stays at that level.   

If a microwave was wattage regulated it would require voltage and current regulation to maintain the wattage output, but neither is the case.   It is only current limited under current safety standards.   This means at the microwaves rating 1450w at 120v, it will only ever draw a max current of X.  (X is the driving question behind this thread)   It may draw less, as it is an inductive load in series with a resistive load.  So a drop in voltage can also mean a drop in current drawn.

It may draw less, but should never draw more.    Microwaves are current limited to keep the coils in the transformers from melting and arcing.    If the voltage goes up, microwaves can actually put out higher wattage then rated, if the input voltage goes down they put out less power.  (will find some links to the IEEE Articles on this)   But the current across the high voltage transformer low side will never exceed the current limitation circuit on the microwave.   This is a big problem for the EU, as they have standardized on 230v power but the UK was always 240 (and still is.)   Which means a 850watt microwave is actually closer to a 900watt microwave when used in UK vs other parts of the EU.   Right from one of the old text books I used to use:

"With many household appliances this has little or no effect, but the microwave oven is susceptible to changes in the electrical supply, in a similar way to early Black & White televisions, which experienced picture decay during low voltage periods. With microwaves the output power generally increases with voltage rise and decreases with voltage decline, but this is dependent on the components used and the oven manufacturers declared classification."

I work in the power industry, have a MS in EE, one point was a card carrying electrician, and up to recently taught this stuff at the local university, so been been doing this for awhile.  :)  The resistance of the wiring has everything to do with the operation of appliance, which is why you get brown outs.   The flaw in your logic is that you are attributing the resistance of the wiring to only the wiring in the house.   The resistance in the circuit is the resistance in the entire circuit which in reality is all the way back to a power plant.   In most RV parks, the wiring is well....non exactly great.   So you often experience much higher resistances from "point of power"

The reasons "brown outs" occur is because as people draw more current through power lines (that have a fixed resistances)  the voltage has to drop to maintain the current draw. If you are in a location and you don't see the nominal 120volts, this means that the amount of current being drawn over the infrastructure you are connected to and the resistance in the wires you connect to, has caused available potential to drop.

But that digresses quite a bit from the main topic.   There are number stated in the manual for the microwave.   But in an electrical inspection for code compliance, those numbers don't mean much as manuals are rarely accurate or read by inspectors.     There is a sticker on the back of the microwave, that will be the true indication of what is safe/required for the microwave.  Will check on that this evening.

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techntrek View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote techntrek Quote  Post ReplyReply Direct Link To This Post Posted: 28 Sep 2009 at 9:52am
I was editing my last post as you were writing yours, and as I now say in that post, I agree that the output work will increase or decrease depending on input voltage, but that does not change the inputs which still must balance out.  A light bulb will output less light (less work done) but will still draw 60 watts whether its running on 120 volts or 100 volts.  A microwave will output less heat (less work done) but will still draw 1450 watts whether its running on 120 volts or 100 volts. 
 
I will try to fit in some experimenting in my garage tonight to have some concrete numbers.  I'm just getting over the flu (yeah, that one) and my wife has it now so it may not happen quite yet.
 
Brown outs occur because as more current is drawn (above the rated maximum) the power lines heat up which changes their resistance to a higher resistance.  They do not have a fixed resistance as you stated.  Current is now the constant, so in I=E/R, resistance increases so voltage must drop.  You can also get a voltage drop as the generator itself is overloaded (while the wires are not) but you usually only see that on small gensets or in third world countries; "modern" countries prefer to do rolling blackouts before brownouts occur.
 
Using a term from accounting, the resistance of the wires up to the point of the outlet is a "sunk cost".  The EE that is designing the circuits for a stereo receiver does not care what those resistances are - they have already been taken into account from the generating site to the customer's outlet by the power company and the electrician that installed the home's wiring (by selecting wire size and transformers).  The EE only cares about the internal resistance of the stereo so it doesn't exceed 12 amps when plugged into a 120 volt outlet.  He isn't including power line resistance into any of his equations.
 
David and Danette - sorry for the digression, yes a 20 amp circuit breaker should be in all of our 'pods to meet code.
 
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