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Old 12-04-2009, 08:34 PM   #31
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Quote:
Originally Posted by kbsparky View Post

However, if you read the first sentence of this section you find the words:



240.4(E) refers to the tap conductor rules, and 240.4(G) offers a table of other Articles of the Code to refer to when selecting overcurrent protection.

This means that the limitations of 240.4(D) do not apply to these other articles, which include conductors serving listed HVAC equipment, etc.
I'm aware of that section.

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Old 12-04-2009, 08:57 PM   #32
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It took me awhile to find this in my files.... it is the best explanation and thorough discussion of the subject at hand in this thread I have in any book or training. It is well worth a good hard read..even for seasoned HVAC guys....


http://www.iaei.org/magazine/?p=3944
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Old 12-05-2009, 05:35 AM   #33
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Thats a good article.

Except, its wrong on how the manufacturer determines min circuit amp of the condenser. They don't calc at 125% of the total load.
They only calc 125% of the compressor's RLA, and then add the RLA/FLA of the condenser fan, and the 100% load of the crank case heater, if it has one.

21.1X1.25=26.375
2.9X1.25=3.625
26.375+3.625=30

Throw in a 40 watt crank case heater and your over 30 amps.

Check your condenser at your house and you'll see.
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Old 12-05-2009, 09:43 AM   #34
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Quote:
Originally Posted by beenthere View Post
Thats a good article.

Except, its wrong on how the manufacturer determines min circuit amp of the condenser. They don't calc at 125% of the total load.
They only calc 125% of the compressor's RLA, and then add the RLA/FLA of the condenser fan, and the 100% load of the crank case heater, if it has one.

21.1 X 1.25 = 26.375
2.9 X 1.25 = 3.625
26.375 + 3.625 = 30

Throw in a 40 watt crank case heater and your over 30 amps.

Check your condenser at your house and you'll see.
Why did you claculate the fan motor at 125%? I don't know the manufacturers calculation, but when sizing other circuits you only add 125% to the largest motor and 100% for the rest.

21.1 X 1.25 = 26.375
2.9 X 1 = 2.9
26.375 + 2.9 = 29.275
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Old 12-05-2009, 10:12 AM   #35
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Because in that article. It said manufacturers calculate the total condenser load by 125%.
And that the min wire size is calculated to carry 125% of the total load.

So I pointed out that the article is wrong in what they say(manufacturer wise). And showed the math for 125% on the OPs equipment specs he listed.
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Old 12-05-2009, 11:08 AM   #36
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Quote:
The manufacturer has already calculated the conductor size to be based on the total of all of the motor loads in the combination-load equipment times 125 percent
.


Yes I agree I didn't catch that...the author would be in error. It is contradictory to his other statements though so I wuld just say like a lot of articles he just had a small oversite.

But I think the point is that the minimum circuit ampacity on a heat pump or any combination load HVAC takes into account all loads including the heater.

Or are you saying that the heater is not considered...I don't follow because you keep wanting to add the heater to the minimum circuit ampacity or am I misunderstanding you? I've never seen information on the nameplate for crankcase heater ampacity or wattage.

I've ran the branch circuits to a bunch of a/c condensors and heat pumps and hung the disconnects. I have never been shot down for not adding in a crankcase heater to the mca nor in any continuing education classes have I ever been trained to do that on modern HVAC equipment stating a max fuse/bker, bcsc, or mca.... so unless there is something really obscure that most instructors of HVAC equipment don't know nor the NEC I would say this is not necessary. Just size your conductors to the mca if you don't have other factors to consider like distance. If ya want to run 8 awg that's your business as a homeowner but as a contractor you probably would not...unless you needed to upsize for a reason other than what the homeowner might install some day.... Might as well run a neutral too...

It would still be possible to use 10 awg even if the mca was over 30 amps. Depends on the wiring method.
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Old 12-05-2009, 03:18 PM   #37
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Crank case heaters are not a motor load. Nor do they have an overload protector. Like a fan motor or compressor motor does.

The inspector that shot me down. Had worked for an HVAC company. Or he wouldn't have known about the heater.

Lots of inspectors don't know if one is on the compressor.
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Old 12-05-2009, 03:24 PM   #38
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As I understand things and it really seems to be... clear as mud... the minimum circuit ampacity takes into account all loads of the unit, so I'm guessing that if you ran the calculations on the motors you would come up a tad bit less than the mca if there was a heater involved....but that is a guess.
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Old 12-05-2009, 03:40 PM   #39
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Min circuit amp is more then what the total load draw should be. Since it is 125% of the compressor.

Motors have their own overload protection.
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Old 12-05-2009, 04:09 PM   #40
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What I'm trying to say is that if you calculate the mca from nameplate data and take 125% of the compressor rla plus the fan motor fla and if a heater was involved you would come out less than the nameplate mca if the mca is taking into account the heater.

Example:

Compressor Rla = 17 amps

Fan motor Fla = 2 amps

Heater = 2 amps

MCA 25.25

If the mca takes that heater into account then the calculation for mca would be

1.25(17) + 2 + 2 = 25.25

If it didn't it would be 23.25 and you would know that a heater would be involved because the motor loads total less than the nameplate mca.
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Old 12-05-2009, 04:26 PM   #41
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Most inspectors just look at the Min amp rating. Not the data used to come up with it.
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Old 12-05-2009, 07:29 PM   #42
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My point is that you do not worry about obscure loads. The requirement is for the manufacturer to include all loads necessary to size the branch circuit conductors in the MCA on the nameplate. You don't go looking in the electrical schematic for the wattage of the heater and add it to the MCA.

Imo you could have asked for AHJ intervention in that inspectors ruling and he woud have quickly been asked to remove the red tag. It is amazing to me about what I hear with some areas inspectors knowledge. Maybe you should wipe his read tag you know where and mail it to him, he should appreciate it since he is doing the same with the electrical code.
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Old 12-05-2009, 07:40 PM   #43
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Nope. I save those kind of things for real important issues.

Such as once when I had them out to tell me everything I needed to do to a swap out to meet code.
And on final inspection. they decided they needed fire dampers.

Had to remind them that they never mentioned that in their other inspections.

AND, they just passed a unit in the same type building beside the one I was doing. And it had no fire dampers.


I see no reason to run min size wire, and install max size breaker. When I can run the next size larger wire to begin with.
Only thing running smallest size wire does, is save a couple bucks on the install.

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