Branch Circuit

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stickboy1375 said:
I guess I was thinking in terms of air restriction, not sure I'm correct now. :) more load on the motor...


I was going off my furnace motor, which monitors amperage and will increase speed as the filter gets dirty to self correct for proper air flow.
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You thinking wrong in both cases. Plugged furnace filter does restrict air flow, your furnace may compensate by increasing motor speed but will still take same amount of work to move same amount of air. . Actual load on motor is determined by amount of air the blower is moving. More CFM = more load.

If you restrict airflow and maintain same motor speed you will have less load. On the typical household hair dryer you will not restrict flow very much before high limit shuts the unit off. I've had to clean wifes hair dryer a few times for this very reason. Resistance of element remains constant no matter how much air is flowing.
 
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kwired said:
You thinking wrong in both cases. Plugged furnace filter does restrict air flow, your furnace may compensate by increasing motor speed but will still take same amount of work to move same amount of air. . Actual load on motor is determined by amount of air the blower is moving. More CFM = more load.

If you restrict airflow and maintain same motor speed you will have less load.
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thanks. I was just shooting from the hip. :) But, if the filter is clogged and the blower needs to run faster, meaning work harder, won't that increase the amperage? Must be having a brain fart this afternoon. :)
 
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stickboy1375 said:
I guess I was thinking in terms of air restriction, not sure I'm correct now. :) more load on the motor...


I was going off my furnace motor, which monitors amperage and will increase speed as the filter gets dirty to self correct for proper air flow.
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you have that backwards.

any centrifugal blower or water pump will un-load as it is restricted, this is because it is moving less mass that is why a motor will speed up, although I don't think a capacitor motor will speed up much, universal motors such as used in vacuums and hair dryers will speed up quit a bit.
The end result is less current.
 
hurk27 said:
you have that backwards.

any centrifugal blower or water pump will un-load as it is restricted, this is because it is moving less mass that is why a motor will speed up, although I don't think a capacitor motor will speed up much, universal motors such as used in vacuums and hair dryers will speed up quit a bit.
The end result is less current.
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Thank-you. :)
 
hurk27 said:
you have that backwards.

any centrifugal blower or water pump will un-load as it is restricted, this is because it is moving less mass that is why a motor will speed up, although I don't think a capacitor motor will speed up much, universal motors such as used in vacuums and hair dryers will speed up quit a bit.
The end result is less current.
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They speed up because there is less load. When talking load I mean the load on the fan. It is the fan that is loaded by the amount of air it is moving. It just demands more or less power from the motor driving it depending on how much air it is moving.
 
stickboy your furnace fan likely is speeding up because there is less load on it and not because it is compensating because of restriction. If it doesn't have a sensor to detect CFM it will have no way to tell when it needs speeding up or how much to speed up - and we are talking variable speed blowers for this kind of feature, a single or two speed motor is only going to attempt to run one speed or the other, and will vary some depending on the actual volume of air being moved.
 
hurk27 said:
you have that backwards.

any centrifugal blower or water pump will un-load as it is restricted, this is because it is moving less mass that is why a motor will speed up, although I don't think a capacitor motor will speed up much, universal motors such as used in vacuums and hair dryers will speed up quit a bit.
The end result is less current.
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My guess is a PSC motor (which is common furnace blower motor) would have a change in speed. A PSC motor is not a high torque motor. Decreasing the load on it will probably result in less slip and result in higher speed.
 
As long as you guys brought furnace fan motors up what comes to my mind is the current these little rascals draw. The have to get hotter than a pistol. It's no wonder they find it a convenient cooling place when they put them inside the squirrel cage. Shut the furnace off and let the fan motor run which should provide some heating by itself.
 
templdl said:
As long as you guys brought furnace fan motors up what comes to my mind is the current these little rascals draw. The have to get hotter than a pistol. It's no wonder they find it a convenient cooling place when they put them inside the squirrel cage. Shut the furnace off and let the fan motor run which should provide some heating by itself.
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Since they are in the airstream they can design them to be loaded harder than if they were not in the air stream.

If you do not put in air stream motor would need to be heavier and probably need its own cooling fan - it would still likely give up a similar amount of heat. Just about any motor that runs at or near its rated load for any extended period of time experiences a pretty noticeable temperature rise.

Even what is considered a low temperature design motor can be within operating range and be too hot to touch.
 
Hold on there .....................was that 80% for continuos duty? Meaning if I have receptacle or lighting loads that aren't continuos, on a 20 amp circuit I could load it up to 20 amps....yes or no?

I've always stopped at 12 and 16 (for 15,20)....... but I thought along the way that that was permissible ......... I'll go back and read deeper in this post and look through the good book, but please clarify. Thank you.
 
ritelec said:
Hold on there .....................was that 80% for continuos duty? Meaning if I have receptacle or lighting loads that aren't continuos, on a 20 amp circuit I could load it up to 20 amps....yes or no?

I've always stopped at 12 and 16 (for 15,20)....... but I thought along the way that that was permissible ......... I'll go back and read deeper in this post and look through the good book, but please clarify. Thank you.
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In residential, The only thing I consider continuous is an electric water heater, and electric heat.
 
stickboy1375 said:
In residential, The only thing I consider continuous is an electric water heater, and electric heat.
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Thanks stickboy,

So the electric water heater circuit and electric heat circuit, after there 125% would be placed on a circuit with that value at no more than 80% of that breaker...But all other lighting outlets and receptacles outlets could be calculated and wired to 100% of the breaker rating.........Correct?
 
ritelec said:
Thanks stickboy,

So the electric water heater circuit and electric heat circuit, after there 125% would be placed on a circuit with that value at no more than 80% of that breaker...But all other lighting outlets and receptacles outlets could be calculated and wired to 100% of the breaker rating.........Correct?
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Breakers are rated 100% but not 100% when there is a continuous load. Now there are breakers rated 100% for continuous load but that is not the standard ones we normally buy.
 
ritelec said:
Thanks stickboy,

So the electric water heater circuit and electric heat circuit, after there 125% would be placed on a circuit with that value at no more than 80% of that breaker...But all other lighting outlets and receptacles outlets could be calculated and wired to 100% of the breaker rating.........Correct?
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I always use 100%, its just crazy not to in residential...
 
stickboy1375 said:
Thanks Dennis, this made more sense to what he was asking.
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Yes thank you dennis and stick boy....


I understand protecting the breaker at 80% of it's rating Unless the breaker is rated for 100%. My question was in concern to laying out the circuit relative to the 80 or 100%... So lay out the circuit for 100 %,
unless it's a continuos duty load then lay it out for 80 % unless it's a 100 % rated breaker then you could stick something else on it maybe to get to the 100%.....ha ha ha ha...

Thanks guys .......

I actually did a 600 amp feeder to some switchgear with a main breaker........upon mentioning about the 80% thing and the way their engineers designed the job, well they gave me the ok to switch out the internals of the breaker with
100% rated internals. The 80% internals are now a $600 paper weight in my shed.

Thanks again guys.
 
ritelec said:
Hold on there .....................was that 80% for continuos duty? Meaning if I have receptacle or lighting loads that aren't continuos, on a 20 amp circuit I could load it up to 20 amps....yes or no?

I've always stopped at 12 and 16 (for 15,20)....... but I thought along the way that that was permissible ......... I'll go back and read deeper in this post and look through the good book, but please clarify. Thank you.
Click to expand...

I design like you do, it's safe. But it's also more stringent than code requires.
 
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