Contractor Talk - Professional Construction and Remodeling Forum banner

1 - 15 of 15 Posts

·
Banned
Joined
·
14,078 Posts
Discussion Starter #1

·
Registered
Joined
·
10,471 Posts
Amps is amps. The rating is supposed to be on the motor, if capacitor start it should be a start/run rating. I go by the start rating and add a few amps for a safety factor. That will be your switch rating.
 

·
DGR,IABD
Joined
·
9,683 Posts
The one you picked is for a 1/4 hp motor. I'm not sure what size your whole house fan is, but I thought most of them were 1/3 horse.

The reason that the watt or amp rating of the switch appears to be higher than the horsepower rating is because of starting inrush.

Note that the resistive rating of the switch is a full 15 amps or 1800 watts, because there is no inrush with a pure resistive load. You have 1800 watts load the second you turn on the switch. Compare that with the swtich's 1/4 horsepower rating. That's only around 5 amps while running, but may be around 40-45 amps for a second when it is first switched on. This kills switch contacts. If you used a bigger motor on that time switch, it would still work.... for a while... a lot shorter while than it would have otherwise with its rated motor load or a non motor load. That's why a switch's horsepower rating is always much lower than it's amp or watt rating. It's all about the starting inrush associated with motors.

If your fan motor is bigger than 1/4 horse, you can either select a different type of timer with a greater horsepower rating, or use that timer to switch a small relay that will energize your whole house fan.

------------------

I just leafed through some catalogs. I see that most of the direct drive whole house fans are 1/8 and 1/4 horse, and most of the belt drive are 1/3 horse. Guess you'll have to crawl up into the attic and clean off the data plate on the motor.
 

·
Registered
Joined
·
10,471 Posts
md, are you certain about that 5A run to 45A start? What in the heck are you starting? I just completed installing 3 A/C units, 1700 BTU. Start is 17A and run is 11A, this is marine air and also kicks in the circ. pump which is a constant 3.5A draw. I find a 50-75% start pretty common but have never run into a 900% situation.
 

·
DGR,IABD
Joined
·
9,683 Posts
Teetorbilt said:
md, are you certain about that 5A run to 45A start? What in the heck are you starting? I just completed installing 3 A/C units, 1700 BTU. Start is 17A and run is 11A, this is marine air and also kicks in the circ. pump which is a constant 3.5A draw. I find a 50-75% start pretty common but have never run into a 900% situation.
I am absolutely, 100% positive. Motors will have an inrush current in the 600-800% range, typically. My example was off the cuff, but quite close. Mind you, this is only for a few cycles, and it only measureable on a scope but you can catch it on a meter if the motor is connected to a compressor (which will delay the startup time). Even though this short term inrush is only measureable for a few cycles, it takes its toll on switch and relay contacts in a big way.

Your 17A start and 11A run that you quoted is made up. I'm calling your bluff here. There is nothing on any data plate of any a/c unit that lists start and run current draws. If you're saying 17A start, that might be what you managed to catch on a meter.

Even though fuses and breakers have some time delay with respect to short term overloads, you often size fuses and breakers 50-75% larger than the motor run current to avoid nuisance tripping. This might be what you're thinking about, Teetor.

I'm not making up new science here. This is well documented information with respect to motors. Electricity 101. Read all about it: http://www.nikkaiswitches.com/electrical.asp Some types of loads have a 1000% inrush.
 

·
DGR,IABD
Joined
·
9,683 Posts
Teetorbilt said:
I was going by some old specs.
C'mon!! Respect your elders, but jeeze. You made those numbers up before. They weren't "old specs". This ain't my first rodeo.

You and I both know that marine electrical equipment is specially designed to be notably easy starting and very energy efficient.

The typical motor still has an inrush of about 600-800 percent. Some a little lower, some a little more, depending on application. Even your application was near 300%, which ain't nothing like 50-75%.

Ever notice when you're house's a/c or the refrigerator kicks on you can sometimes notice that the lights wink or dim for a split second? That's inrush current at work.

I'm sorrry that I'm being put in a position where I have to argue with you, Teetor, since I generally respect and agree with what you have to say. BUT.. you're way out in left field on this one.

Yes, every problem does look like an nail when you only have a hammer. Likewise, every real world problem doesn not have a marine equivalent.
 

·
Registered
Joined
·
10,471 Posts
I'll be the first to admit that the finer details of eelectricity are not my forte, that's why I have a licensed electrician on staff. Personally, I respect and hate the stuff, also have some burn scars to validate my personal opinion.

Next time that I get out to the boat, I'll photo the plate. I do so much of this that I forget a lot of it. My own and that unit I remember, I sold a crap load of them.
 

·
Banned
Joined
·
14,078 Posts
Discussion Starter #9
Thanks so far...

Here is the info off the motor:

Emerson
Model # ka55hxkwn-8586
Volt 120 HZ 60 Amp 5.7ph 1
HP 1/3B

So MD is it correct to assume based on what you said about inrush that this switch isn't designed to run any motor? Or just not this size of a motor?
 

·
DGR,IABD
Joined
·
9,683 Posts
That switch is rated for a 1/4 horsepower motor. Your motor is 1/3 hp, (which is larger than 1/4) so that switch will not suit.
 

·
Banned
Joined
·
14,078 Posts
Discussion Starter #11
How about this one?



Here is the information on it.

http://www.intermatic.com/?action=prod&pid=9086

By the way what does the 'B' after the horsepower rating mean on my motor HP 1/3B

And what does the 'ph 1' after the amp rating mean?
Amp 5.7ph 1
 

·
Registered
Joined
·
24 Posts
I beg to differ

MD your calculations are correct, however you also know that almost every piece of switching equipment whether it a simple 15 snap switch or a 400A contactor is built with tolerances that exceed the rating.

Yeah the swich says 1/3 HP rating but I wouldn't think twice on it. the difference between a 1/4 HP and 1/3 HP is what....140 watts or so?

Certainly not enough to cause concern even with the inrush current which for all practical purposes can be considered the equivalent of a momentary short circuit.

If that switch can handle an 1800watt resistive load, it certainly can handle a tiny 1/3 HP motor.

The switch will survive.

You probably think I'm a crazy man that throws caution to the wind at any given moment, but it's not so. I'm careful and well aware of the condition that Mike is talking about.

BTW Mike, the "Tap" on digital intermatic timer that you show there does have a fast action internal switch that operates with a battery so it does have the rating that you are looking for however you have to program the on/off times for each day of the week. But it's simple and allows for multiple on/off times throughout the week.

Oh yeah, The "B" most likely refers to the temperature rating of the motor. Class B being the most common. It allows for a continuous operating temperature of 130deg Celcius. (260+degrees F) without affecting the motors operability.
The "1ph" refers simply to the motor being a single-phase powered motor.


Now I'll wait for the lambasting.
 

·
DGR,IABD
Joined
·
9,683 Posts
Chicago said:
Now I'll wait for the lambasting.
No lambasting from me. You're a professional, and that's one of the choices you make. I'm not comfortable exceeding manufacturers horsepower ratings as labeled on their switches. There are two reasons for this.

One, the UL has not tested the switches for greater than the indicated horsepower. This will put some liability on me if something should happen down the road.
UL General Guide said:
SNAP SWITCHES (WJQR)
While many of these snap switches will operate successfully
on circuits that have some reactance in general, an inductive load should
not exceed one-half the amp rating of the switch at the voltage involved.
However, some of these snap switches are marked with additional horsepower
ratings at one or more voltages which indicate that a switch which
is so marked has been tested for the control of a motor of the horsepower
and voltage rating indicated.
Since the UL General Guide is not a permissive document as the NEC is, you cannot exceed the horsepower rating of a switch that is so rated.

Secondly, the NEC has a few rules.
nec said:
430.110 Ampere Rating and Interrupting Capacity.
(A) General. The disconnecting means for motor circuits
rated 600 volts, nominal, or less shall have an ampere rating
not less than 115 percent of the full-load current rating
of the motor. [ROP 11–79]
Exception: A listed nonfused motor-circuit switch having
a horsepower rating not less than the motor horsepower
shall be permitted to have an ampere rating less than
115 percent of the full-load current rating of the motor.
Notice that the original switch Mike selected did not have a straight ampere rating. It had ratings for various load types. You can't apply the 115% rule to this switch, because it has no pain ampere rating. You could apply that 115% rule to an ordinary general use snap switch that did not otherwise have a horsepower marking.

Yes, if you installed the 1/4hp switch on a 1/3 horsepower motor, it would probably be just fine for a great while. I am resolved, however, that it would be a violation to do so and it's life (as compared with a higher horsepower rated switch in the same installation) would be shortened. Would I install it? Probably so, but I couldn't pretend that it was permissible or proper.
 

·
Registered
Joined
·
24 Posts
Well said

mdshunk said:
Yes, if you installed the 1/4hp switch on a 1/3 horsepower motor, it would probably be just fine for a great while. I am resolved, however, that it would be a violation to do so and it's life (as compared with a higher horsepower rated switch in the same installation) would be shortened. Would I install it? Probably so, but I couldn't pretend that it was permissible or proper.
True a violation it is and therefore I will add that...ahem...I would only do this in my own home. Did I just say that? Sure, it's true,

When it comes to testing and applying ratings, I do understand that they (UL for instance) will almost always push the limits of a devices ablilities before assigning a rating. Did the switch perform flawlessly at 1/4HP and burst into flames at 1/3HP? Probably not. But they posted a limitation where they were certain was considerably under dangerous conditions given the likelyhood that those limits may be exceeded to some degree.
But yes limits are limits. Don't jaywalk, don't go 5 miles over the speed limit, don't swim right after eating, and whatever you do. DO NOT apply a 1/4hp rated switch to a 1/3hp motor. Unless it's my home. In which case, I will do it myself. I must be nuts.
 

·
Registered
Joined
·
1 Posts
Beware of Intermitc timer

After 20 years of flawless service an Intermitic whole-house fan timer quit working. Up to that time changing settings was relative easy, one-page instruction easy to follow not too many steps involved for 4 fresh-air cycles daily.
I purchased the new and "improved" Intermatic" timer identified as a direct replacement. One page directions now in 4 languages, and very (very) small print font. The instruction biased to "dusk & dawn" settings with multiple daily cycles deep within instructions. Called tech service, got a guy who was just training (woman's voice in background) who led me through the 130+ tiny button presses necessary to get 4 off-on cycles per day. This was easier because I had not installed the unit yet and could hold it in my hand in good light in my office with the speaker phone on. Maybe half hour because its' easy to miss a step and have to repeat several moves.
But, the unit quit for lack of a noise suppressor which I had to order and pay for seperatly So I got the part, reinstalled the unit but now had to again do the settings. I had to put on my headset and headlamp to make the tech service call and perform the 130+ moves on the tiny buttons in shadowed area of the bathroom.
It was ridiculous. Another example of too much technology and not enough ergonomics. Now just 7 months later this unit has stopped working and I'm looking for a manual timer with the little pull out tabs.
That's how I ended up on this forum and have wanted to tell this tale to someone who might value from it. Matching motor size to timer is easy compared to setting this type unit up.
 
1 - 15 of 15 Posts
Top