AC circuit

[220/221]

I have read the threads about runing smaller wire than "normal" to AC units (inductive loads?) but never really grasped the concept.

208V 3 phase 17.5 ton AC

Specs say 90 amp MOCP.

# 4's or can I go smaller?

[Cow]

Need to know the min. circuit ampacity to answer your question as well as the wire type.

[Flex]

I didnt think you sized the wire according to the MOCP. I definitely could be wrong.

[480sparky]

I have read the threads about runing smaller wire than "normal" to AC units (inductive loads?) but never really grasped the concept.

208V 3 phase 17.5 ton AC

Specs say 90 amp MOCP.

# 4's or can I go smaller?

Wnat does the nameplace say for minimum circuit ampacity?

[220/221]

MCA = 83 amps

thwn

[Cow]

#4 looks like to me.

[LarryFine]

I didnt think you sized the wire according to the MOCP. I definitely could be wrong.
You certainly may, but it's overkill in most installations.

[celtic]

Give 440.6(A) Excep. #1 an eyeballing.

[LarryFine]

90 amp MOCP.Breaker 90a or less (but don't).

MCA = 83 ampsWire for 83a or more.

[Flex]

You certainly may, but it's overkill in most installations.

thought so but i wasnt to sure of myself

[220/221]

Breaker 90a or less (but don't).Wire for 83a or more.

Well, yeah...but I have followed threads where they are talking about running #10 or 12 on a 50 amp breaker for ac condensers and wondered if the same twisted logic applied here.

[walkerj]

Well, yeah...but I have followed threads where they are talking about running #10 or 12 on a 50 amp breaker for ac condensers and wondered if the same twisted logic applied here.


That would apply if the MCA was, say 25 amps and MOCPD was 50 amps

[benaround]

I have read the threads about runing smaller wire than "normal" to AC units (inductive loads?) but never really grasped the concept.

?

220/221,

It's really about using a larger breaker rather than a smaller wire. The MCA is the size

of the conductor needed for normal operation, the MOCPD is the size a the OCPD needed

to start the motor without tripping. This is allowed because the motor has overload

protection and will open the circuit before the MCA conductors ampacity is reached.

The MOCPD will handle the short circuit and ground fault tasks as well as the high

amperage of the motor at start-up. Also, this is the only load on the circuit.

Hpoe this helps.

[don_resqcapt19]

MCA = 83 amps

thwn
It is rare to have the minimum circuit ampacity that close to the maximum overcurrent size for an AC unit, but as others have said the code permits you to install the wire to match the minimum circuit ampacity and connect that wire to the maximum permitted OCPD.

[Rewire]

what you will find is the ocpd is usually 125% of the mca

[Dennis Alwon]

#4 looks like to me.

I would say #4 if you use conduit and either 75C or 90C insulation. If you are wiring a home and using NM or SE cable then I would say #2 (#3 is not available in NM)

[Dennis Alwon]

what you will find is the ocpd is usually 125% of the mca

Actually the max. ocpd is usally 175% of the mca.

[Dennis Alwon]

Well, yeah...but I have followed threads where they are talking about running #10 or 12 on a 50 amp breaker for ac condensers and wondered if the same twisted logic applied here.

LOL-- it is not twisted logic. What you are refering to is art. 240.4(G) which allows us to use #12 wire at 25 amps , #14 at 29=0 amps and #10 at 35 amps. Also the a/c units have a built in overload protection which protects the branch circuit from running at a higher ampacity than intended. The OCPD that we install only protects for short circuit and ground fault.

[220/221]

Gotcha.

Thanks :)

[norcal]

LOL-- it is not twisted logic. What you are refering to is art. 240.4(G) which allows us to use #12 wire at 25 amps , #14 at 29=0 amps and #10 at 35 amps. Also the a/c units have a built in overload protection which protects the branch circuit from running at a higher ampacity than intended. The OCPD that we install only protects for short circuit and ground fault.


Don't ya mean 14 AWG @ 20A?

[Dennis Alwon]

Don't ya mean 14 AWG @ 20A?

Look at Table 310.16 and tell me what the amps are for #14 at 60C or 75C.

Then look at the note (*) at the bottom which directs us to 240.4 (D). Read the first sentence in 240.4 (D) that pertains to 240.4(G)

[Cow]

He's referring to this Dennis:

#14 at 29=0 amps .

[Dennis Alwon]

He's referring to this Dennis:

Thanks. I don't know how that happened but yes 14 @ 20 amps. Don't know where the = and 9 came from...:-?

I also read his post incorrectly thinking he was was questioning me about 14 @ 20 amps. My fault sorry.

[norcal]

He's referring to this Dennis:


Quote:
Originally Posted by Dennis Alwon View Post
#14 at 29=0 amps .


Thank you, should have made myself more clear.

[danickstr]

you thought you hit delete (for the 9) and actually hit =, then you hit 0, thinking you were back at the 2.
:smile: