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Old 12-04-2009, 10:18 AM   #16
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Quote:
Originally Posted by Jim Port View Post
I am going to disagree. The labels state "Minimum Circuit Ampacity" and "Maximum Overcurrent Protection". One tells you the required wire size, the other is the breaker size. The larger breaker size is sized to allow the startup inrush. The running amps are below the circuit ampacity.
First tells you a min circuit protection and min wire size.
Second one does not tell you that you can put a 50 amp breaker on a 10 guage wire. If it is used for more then a motor only load.

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Old 12-04-2009, 10:32 AM   #17
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Are you saying that 440.4(B) does not apply?
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Old 12-04-2009, 10:34 AM   #18
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Originally Posted by beenthere View Post
First tells you a min circuit protection and min wire size.
Second one does not tell you that you can put a 50 amp breaker on a 10 guage wire. If it is used for more then a motor only load.
Isn't the nameplate rating for the entire outdoor unit? So the max OCPD should be sized for all of the components in the unit?
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Old 12-04-2009, 10:43 AM   #19
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Not if the heat pump contains a crank case heater.

Max circuit protection doesn't mean put the biggest breaker in, and use the min amp circuit wire size.
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Old 12-04-2009, 10:47 AM   #20
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The nameplate ratings take into account the compressor and all related loads, so the minimum circuit ampacity and maximum OCPD listed can be used to select the wire and breaker... in this case 10ga and 50A. The article that applies is 440, not 430.
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Old 12-04-2009, 10:50 AM   #21
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The nameplate ratings take into account the compressor and all related loads, so the minimum circuit ampacity and maximum OCPD listed can be used to select the wire and breaker... in this case 10ga and 50A. The article that applies is 440, not 430.
This has been my take on this issue. A heat pump falls under 440.
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Old 12-04-2009, 10:55 AM   #22
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Ask your electrical inspector, if a heat pump out door unit. Is allowed to be fed by a 10 guage wire, connected to a 50 amp breaker. If it also contains an electrical heater in the same circuit. That has no other means of circuit protection.
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Old 12-04-2009, 11:42 AM   #23
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Wow
Sorry for causing this.
The unit is the outdoor part of a heat pump.
The name plate has the following data.

total amps 24
compressor amps rla 21.1 lra 96
fan amps fla 2.9

MCA 29.7 IP code IP 14

Max Overcuurent protection Amps +:50

The length of run will be 50 feet.
The unit has a crankcase heater and the only thing it says about it is to have the power hooked up for 24 hours before starting unit.


Hope this helps.


I guess the normal operation would only require MCA of 29.7 (24 amps total plus a buffer)therefore allowing a #10 wire (30 amps), but during compressor start it may pull more amps so a 50 amp cb is required?

Last edited by clell; 12-04-2009 at 11:47 AM. Reason: added info
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Old 12-04-2009, 03:13 PM   #24
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The compressor will draw far more amps then 50 when it starts.
Basically, it will roughly draw locked rotor amps every time it starts.
HVAR breakers are made for this type of over current draw during compressor start.

The 50 max circuit protection is for install conditions that have demands outside of normal installs. Such as long line sets. High lift, etc.
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Old 12-04-2009, 03:52 PM   #25
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Quote:
Originally Posted by clell View Post
Wow
Sorry for causing this.
The unit is the outdoor part of a heat pump.
The name plate has the following data.
Not a problem, you didn't cause anything
There can always be a difference of opinion on exact installation
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Old 12-04-2009, 03:55 PM   #26
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If it was a problem. A Mod would have stopped it.

And for the record. I have a lot of respect for those that are telling me I'm wrong.

I just don't believe them on this one
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Old 12-04-2009, 07:11 PM   #27
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Quote:
Originally Posted by beenthere View Post
Max circuit protection doesn't mean put the biggest breaker in, and use the min amp circuit wire size.
It doesn't not mean that either.
Art. 440 clearly allows this common method of circuitry.

The other are absolutely correct on this.

No offense, but being in the HVAC trade for as many years as you have been I am greatly surprised you are questioning this.
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Answers based on the 2008 & 2011 NEC.

Last edited by Speedy Petey; 12-04-2009 at 07:13 PM.
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Old 12-04-2009, 07:56 PM   #28
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A few code references;

1) Article 440 is entitled "Air Conditioning And Refrigerating Equipment".

2) 440.4 (B). Multimotor and combination-load equipment shall be provided with a visible nameplate marked with the makers name, the rating in volts, frequency and number of phases, minimum supply circuit conductor ampacity, the maximum rating of the branch-circuit short-circuit and ground-fault protective device and the short circuit rating of the motor controllers or industrial control panel.

3) Article 110.3 (B) states Listed or labeled equipment shall be installed and used in accordance with any instructions included in the listing or labeling.

4) 440.6 The size of the conductors covered in this article shall be selected from Table 310.16 through Table 310.19 or calculated in accordance with 310.15 as applicable.

5) 110.14 (1) (a) (3). (This one applies to almost all modern electrical equipment, including HVAC units.) Conductors with a higher temperature rating (than 60C,) if the equipment is listed and identified for use with such conductors. (Almost all equipment is listed for 75C.) Note, if type NM cable is used, 334.80 limits the ampacity to the 60C column in 310.16.

6) 440.21 (Part III) is entitled Branch circuit short-circuit and ground fault protection. It states that The provisions of part III specify devices intended to protect the branch-circuit conductors, control apparatus, and motors in circuits supplying hermetic refrigeration motor-compressors against overcurrent due to short circuits and ground faults. They are in addition or amendatory to the provisions of article 240. (Article 240 is entitled Overcurrent Protection.)

Based on the above, #10 wire is the smallest (there is no largest), and a 50 amp breaker is the largest (there is no smallest).

Based on the resistance table, #10 will cause a voltage drop of 24 volts per 100' at 100 amps. In other words, if the unit in question were connected with 10/2, and the cable were 100' long, the voltage drop during starting would be about 10%.

The above are actual facts, derived from the 2005 edition of the NEC. Here's my opinion;

If this were my unit, I would have absolutely no problem using 10/2 NM, and a 50 amp breaker. I've had to educate several inspectors as to the code here. Only one refused to learn, and he was given the opportunity to show cause to the state license board why he should keep his inspectors license. He lost.

All electrical equipment involving motors is designed to start with less than full voltage. If it were not, the nameplate would specify larger wire. A 10% voltage drop is completely acceptable for any equipment that is UL listed. In fact, the equipment must demonstrate satisfactory operation at voltage levels of + or - 10% in order to be UL listed. HVAC units typically have acceptable voltage ratings on the label. Usually for a 230 volt unit, it's 207 minimum and 253 maximum.

To get really technical, if the supply voltage is 240, the #10s could be about 260' long and you'd still have about 207 volts during starting.

Sorry for the long post, I just couldn't shorten it any without losing valuable information.

Rob

Last edited by micromind; 12-04-2009 at 08:00 PM.
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Old 12-04-2009, 08:01 PM   #29
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Nice post mind.

I have been shot down for it by 2 electrical code inspectors..
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Old 12-04-2009, 08:24 PM   #30
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You need to educate your inspector-critters up there in non-licensed land.

Around here, we are required to have continuing education Code classes to maintain and renew our electrical licenses. These classes are mainly taught by electrical inspection agencies and their licensed inspectors.

Derived from attending these classes, this topic has come up for discussion many times, with the same answer: The Minimum circuit ampacity label determines the wire size; the maximum overcurrent protection determines the breaker size.

The one relevant Code section that was not quoted in this thread is 240.4(D). This is the section that instructs one that #14 wire = 15 Amps max, #12 = 20 Amps, #10 = 30 Amps, etc. Most everyone remembers this requirement, and mistakenly apply this to circuits for labeled HVAC equipment.

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

Quote:
....Unless specifically permitted in 240.4(E) or (G), ....
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.

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