HVAC - conductor vs. breaker size

[eagle]

Superintendant needs a little clarity here from Electrical side ...

Situation : Seeing a 35 amp rated AC condenser with a # 10 conductor . Occasionally seeing #10 on a 40 amp breaker. Sub says NEC provisions allow for this excess for start up draw.

Question 1:
When HVAC condensing units have a maximum breaker rating of 35 amps stamped on the unit..
can the breaker exceed the conductor rating.. if so by how much ? Isn't the # 10 only good for the 30 amp ?

Question 2:
Where can I find the applicable code in the NEC 02' handbook that provides this.

Final Question:
Where can I find reference to maximum length on "Liquidtight" whips ( from disconnnect to unit ) serving condensers. Locally, muni's have been enforcing 6' max. They are running about 12' Is this accurate ?

 

[bphgravity]

Re: HVAC - conductor vs. breaker size

Section 240.4(D) does apply to the requirements in Article 440.

Use the nameplate ratings for sizing the conductors and the OCD. The minimum circuit ampacity is used for the circuit conductors, and the maximum overcurrent protection listing is for the OCD.

If you don't have that, use 440.22 for the OCD sizing, 440.32 for the branch circuit conductors, and 440.12 for the disco.

Article 356 covers LFNC installations.

 

[eagle]

Re: HVAC - conductor vs. breaker size

bphgravity ..

Just needing to make sure ...

SO your saying that if the listed nameplate rating of breaker minimum is 30 amps ...then the conductor should be # 10 & no less ? ..... and if the listed maximum is 40 amps the OCP can be 40 and no more ?

Your also saying that if the nameplate is missing then we go to 440.2, 32 & 12 that allow for percentage bumps because now we only have motor load rating to work with ?

Am I following this correctly ?

On the LFNC ...
356.10 appears to say that if it's LFNC - B it can be installed longer than 6' if secured per 356.30 but if it's LFNC it seems to get refered right back to 356.10 as long as it's secured. It also does not appear to say exactly how long. WHat the heck is the difference between LFNC & LFNC -B. Maybe this is why everyone gave up locally and just installs at 6' max. to skip the head scratching...

 

[a.wayne3@verizon.net]

Re: HVAC - conductor vs. breaker size

Wire for min. and fuse for max.on a condensor,LFMC is still conduit install like conduit but a 6 ft free whip to the unit is ok.

 

[luke warmwater]

Re: HVAC - conductor vs. breaker size

eagle,
click HERE for an older but very informative thread on this topic.
Todd

 

[bphgravity]

Re: HVAC - conductor vs. breaker size

Originally posted by eagle:
SO your saying that if the listed nameplate rating of breaker minimum is 30 amps ...then the conductor should be # 10 & no less ? ..... and if the listed maximum is 40 amps the OCP can be 40 and no more ?

Keep the conductor and OCD as separate issues for this application. A minimum breaker size will allow the unit to run, but could be subject to nuisance tripping from starting loads. The maximum OCD size allows for this. The conductor size is based an operational current capacity. The unit is required to have overload relays to prevent overloading of the conductors. The OCD we provide is actually for short-cirucit and ground-fault protection, not overloading.

356.10 appears to say that if it's LFNC - B it can be installed longer than 6' if secured per 356.30 but if it's LFNC it seems to get refered right back to 356.10 as long as it's secured. It also does not appear to say exactly how long. WHat the heck is the difference between LFNC & LFNC -B. Maybe this is why everyone gave up locally and just installs at 6' max. to skip the head scratching...

Read 356.2 Definitions for the specs on type B. If your using the gray stuff, then it is type B. 356.10(5) allows longer lengths per method #1 allowed in 356.30.

 

[iwire]

Re: HVAC - conductor vs. breaker size

Originally posted by eagle:
Isn't the # 10 only good for the 30 amp ?

No 10 AWG copper is rated 30 to 40 amps depending on the temperature limitation you have to follow.

If you are talking NM cable you must use the 60 C rating, if it is a pipe and wire job there is a good chance you could use the 75 C rating (35 amps).

A unit wired with NM that has the following info:

Min. circuit ampacity 30 Amps

Max overcurrent protection 50 Amps

Can be served with 10 AWG on a 50 amp breaker.

Just a note about the max overcurrent rating.

If it says Maximum Fuse Size 50 amps you will have to use fuses.

As the others have said it is acceptable to size the wire to the minimum and the breaker to the maximum rating.

The minimum wire size allowed may not be large enough for longer runs but that is a design issue.

 

[eagle]

Re: HVAC - conductor vs. breaker size

Thanks to all for the speedy answers and link. I think we're all set now.

Much appreciated

 

[wrestless]

Re: HVAC - conductor vs. breaker size

Eagle,
If you look on the nameplate of unit you will see RLA ( Rated Load Current) This current
is already multiplied by the minimum which is 125%
for minimum circuit ampacity so size conductors to this using 60 degree column. Then for O.C.P.
you can multiply the RLA by 225% No greater than.
Art 440.22 (a)

 

[iwire]

Re: HVAC - conductor vs. breaker size

The two markings you need to look for that relate to this thread are:

1)Minimum supply circuit conductor ampacity.

2)Maximum rating of the branch-circuit short-circuit and ground-fault protective device.

This is all you need for wire and breaker or fuse selection.