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#209289 03/22/13 11:28 PM
Joined: Mar 2004
Posts: 814
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BigB Offline OP
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240.4 tells us the overcurrent protection required for small conductors unless otherwise permitted in 240.4 (A) thru (G). 240.4(G) refers us to 440, Parts III & VI for refrigeration equipment. 440.22(A) allows the use of an OCPD 175% of the motor compressor rated load.

Situation is a roof top unit nameplate reads "Minimun Circuit Ampacity 26 amps" and "Maximum Fuse or HACR Circuit Breaker 40 Amps" The unit is wired with #10 copper fed from a 40 amp HACR circuit breaker.

Is the installation compliant?

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Joined: Jun 2004
Posts: 613
S
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Yes...I always go by the nameplate to size the conductors and overcurrent protection.

What part bothers you?

What is the RLA of the compressor?

Joined: Mar 2004
Posts: 814
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BigB Offline OP
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I always do the same, but right now I have 2 people challenging it, a home inspector and another EC. The only reason I want confirmation that I am correct is because I seem to remember something back in prior codes about #14, #12 & #10 limited to 15, 20 & 30 amp OCPDs respectively without exception but now I cannot find it.

Last edited by BigB; 03/23/13 01:30 AM.
Joined: Jul 2004
Posts: 9,940
Likes: 34
G
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240.4(D) and the previous "dagger notes" to 310.16 were never "without exception". We always had the exceptions now in 240.4 (A)-(G).
I understand that the supposed "hard and fast rule" about small conductors (14ga = 15a etc) is firmly ingrained in urban lore but it is simply not true.
Most notably, most dedicated motor circuits and welders have different rules. It is because of the nature of these loads.
To fully understand this, you need to know the difference between overload protection (provided in the motor or controller) and over current (short circuit) protection, provided by the branch circuit over current device.


Greg Fretwell
Joined: Jun 2004
Posts: 613
S
Member
240.3 tells us to use table 240.3 for overcurrent protection of equipment as specified by it's article in the table...

440.6(A) guides us on how to do this...

Use 440.32 or 440.33 to size branch circuit conductors...

example: (440.32): 20.8 RLA x 125% = 26amps

Then table 310.16 75 degree terminals require #10 copper

Then go to 440.22 for breaker or fuse size...

example: 20.8 amps x 175% = 36.4amps

So this manufacturer went with the 40amp Breaker because your allowed up to 225% per 440.22.

Hope this helps. Think I nailed it.

shortcircuit

Joined: Jul 2004
Posts: 9,940
Likes: 34
G
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That looks right to me but I just trust that the manufacturer used the U/L marking guide and a real engineer to make up the label.
It is good to understand the process tho ... sort of like long division wink


Greg Fretwell

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