The manufacturer said

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david

Senior Member
Location
Pennsylvania
"Please quit telling people to oversize ocpd on AC branch and feeder circuits"


I am not objecting to the closing of the since it was not my responsibility to make that decision.

I am not sure why this thread was closed,

The OP wrong and also passionate. He made a statement that no one is doing it that way. Well I too am from PA. and all the HVAC guys that I know are doing it that way

Also just as a note the plans that I review from electrical engineers from W-Mart to hospitals have had conductors sized from the nameplate Min circuit ampacity with voltage drop calculated into the sizing of these conductors.

Back in 1990 when I started inspecting this information wasn?t readily available in the form of a nameplate with the max over-current protection and min circuit ampacity calculated by the manufacture. I think giving careful consideration to the code sections would have had the same result though.

A final note when I?m told that someone talked to the manufacture and the manufactured said this or that it is usually someone talking to their supplier and the manufactured is most often not aware of the conversation.

Edit:
Actually am not sure it was 1990, not exactly sure what year they started posting that on the name plate. Started reading the code back in the late 70?s been in the trade to long to remember when things like this changed
 
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MasterTheNEC

CEO and President of Electrical Code Academy, Inc.
Location
McKinney, Texas
Occupation
CEO
A final note when I?m told that someone talked to the manufacture and the manufactured said this or that it is usually someone talking to their supplier and the manufactured is most often not aware of the conversation.

I am not sure what thread you are referring too but I 100% agree with the above statement....:angel:
 

Mgraw

Senior Member
Location
Opelousas, Louisiana
Occupation
Electrician
This is from ICP, a HVAC manufacturer.

"Frequently asked questions:

?What is MCA?
Minimum Circuit Amps (MCA) is a calculated value that specifies the minimum main power wire size. More specifically, MCA is the highest steady-state electrical current that the rooftop unit should see when operating correctly.

?What is MOCP?
Maximum Over-Current Protection (MOCP) is a calculated value that determines the maximum size of the over-current protection device (fuse or breaker). There are different MOCP equations depending on your application.

For a more comprehensive understanding, you should study the NEC and UL procedures."
 

jap

Senior Member
Occupation
Electrician
I guess I'll be the odd man out, but that's nothing new.
I've always done it as the OP explained, although I'm not as passionate about it.

I know of very few times that I've actually sized the wire from the Panel to the outdoor unit at its Minimum Circuit Ampacity and then put in an overcurrent device that was rated at the Maximum overcurrent protection.

I would never argue the point that everybody else was wrong for doing it that way because they are not.

Generally we look at the Maximum Overcurrent protection rating on the Unit install that breaker and then size the wire to that overcurrent device.
Never had an issue with customers complaining about the cost, and never really had to think about it being a dis service to them.
It's not like we do it to be underhanded in any way. That's just the way we do it.

JAP>
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
let me ask you this.

if you are using a CB for the short circuit protection on a motor circuit, the code allows you to size it at 250% of the motor FLC.

lets say you had a 15Hp/480V motor (21 FLA). Code would allow you to use a CB of 52.5 amps (or whatever the next std size is).

The code only requires conductor ampacity of 125% of FLC (in this case #10).

would you size the conductors to the rating of the CB being used (if you went with 250%)? or to the FLC as the code permits.
 

jap

Senior Member
Occupation
Electrician
let me ask you this.

if you are using a CB for the short circuit protection on a motor circuit, the code allows you to size it at 250% of the motor FLC.

lets say you had a 15Hp/480V motor (21 FLA). Code would allow you to use a CB of 52.5 amps (or whatever the next std size is).

The code only requires conductor ampacity of 125% of FLC (in this case #10).

would you size the conductors to the rating of the CB being used (if you went with 250%)? or to the FLC as the code permits.

No,
Your confused.
We were talking about an air conditioning unit..... remember?

JAP>
 

jap

Senior Member
Occupation
Electrician
I rarely see a Maximum Overcurrent protection 250% above what the minimum circuit ampacity is on the nameplate of an AC Unit.


JAP>
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
I rarely see a Maximum Overcurrent protection 250% above what the minimum circuit ampacity is on the nameplate of an AC Unit.


JAP>
And if the nameplate does not also include an LRA number, you really have no idea what the starting surge really is. There may even be a soft start feature built into the internal controller.
 

jap

Senior Member
Occupation
Electrician
And if the nameplate does not also include an LRA number, you really have no idea what the starting surge really is. There may even be a soft start feature built into the internal controller.

I was just trying to make the point that usually the Max overcurrent protection as compared to the Min Circuit amps on the nameplate of an AC unit is not as drastic as Petesonra's
fancy explanation of motor short circuit protection 101.

JAP>
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
No you didn't.
JAP>
Actually he did.
let me ask you this.

if you are using a CB for the short circuit protection on a motor circuit, the code allows you to size it at 250% of the motor FLC.

lets say you had a 15Hp/480V motor (21 FLA). Code would allow you to use a CB of 52.5 amps (or whatever the next std size is).

The code only requires conductor ampacity of 125% of FLC (in this case #10).

would you size the conductors to the rating of the CB being used (if you went with 250%)? or to the FLC as the code permits.
 

jap

Senior Member
Occupation
Electrician
Actually he did.

Well you all must be seeing some words that I dont since no where in that paragraph did hear him ask

" would you consider a similar situation common with motors".


Unfortunately, none of the motors I hook up come with a sticker that recommends a Maximum Overcurrent device like Air Conditioning Units do.


That's where I get to use the valueable information Petrasonra shared with me earlier on how to figure it up.


JAP>
 
I think there are also two important non code things to consider in this discussion:


1. "All the guys that I know are doing it that way." The fact is, in my experience, most electricians and inspectors dont really know the codes very well at all, let alone HVAC guys.
2. " the manufacturer said so" The fact is many manufacturers are also real bad with codes and theory (particularly grounding). I see it all the time. It was funny, I was having a discussion on a RE forum a few months ago where I was disagreeing with someone on an issue who was saying "XXX solar manufacturer agrees with my position." I stated that I generally dont listed to what manufacturers say about grounding or codes. A few days later, I come across an article in homepower by Mr X from XXX solar who was saying that earthing is important because it keeps the earth and metal parts at the same potential during a fault thus eliminating/reducing shock hazards.

Oh, ok and one code thing. Note that the wire/OCPD sizes we are most use to are sized to the 75 degree rating when the conductor itself is good for 90, AND in general applications have a further derating from 240.4(D).
 

ActionDave

Chief Moderator
Staff member
Location
Durango, CO, 10 h 20 min from the winged horses.
Occupation
Licensed Electrician
Well you all must be seeing some words that I dont since no where in that paragraph did hear him ask

" would you consider a similar situation common with motors".


Unfortunately, none of the motors I hook up come with a sticker that recommends a Maximum Overcurrent device like Air Conditioning Units do.


That's where I get to use the valueable information Petrasonra shared with me earlier on how to figure it up.


JAP>
Motors 1 hp and up you are allowed to put smaller wire on a bigger breaker. A/C manufactures just do all the maths for you, the principal are the same.
 

david

Senior Member
Location
Pennsylvania
In the closed thread please stop telling people, ?Velram? is a bit more passionate then you are. I an not sure anything the members here can say that will reassure him that when you have a circuit sized to the min circuit ampacity and provide overcurrent protection in accordance with the name plate data max overcurrent protection we are creating an un-necessary potential risk for fire.

My experience doing electrical inspecting of HVAC installations after checking the units name plate for min circuit ampacity and max overcurrent, I have had the circuit over current protection changed about 30 different times.

The request for the change had nothing to do with air conditioning units. On electric furnace installations shipped from the manufacture with two circuit design the units have two 60 amp two pole breakers. But the manufacture has no idea what heating units are going to be added to the unit on site. A lot of times the unit after installation would need 50 & 30 amp or 60 & 50 amp.

We do look at the name plate data and if the installation circuit has a min ampacity and max overcurrent it passes inspection. If there truly are manufactures saying that when the unit is supplied by a circuit based on the min circuit ampacity you cannot use the max overcurrent protection indicated on the name plate data we would like to talk to that manufacture.
 

Mgraw

Senior Member
Location
Opelousas, Louisiana
Occupation
Electrician
The request for the change had nothing to do with air conditioning units. On electric furnace installations shipped from the manufacture with two circuit design the units have two 60 amp two pole breakers. But the manufacture has no idea what heating units are going to be added to the unit on site. A lot of times the unit after installation would need 50 & 30 amp or 60 & 50 amp.

If the electric furnace has breakers installed in the unit this usually comes as a heater kit from the factory. The breakers supplied with the heater kit are sized to the heaters installed and any other load in the unit.
 
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