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#34996 03/02/04 04:43 PM
Joined: Dec 2003
Posts: 6
Junior Member
Can someone please tell me how to correctly determine proper conductor size for HVAC equipment if you know the total running amps.

I need to know how to do this without any help from equipment nameplate info.

Example....Total running amps of condensing unit 16 amps. What size wire is required, and how did you determine?

Also a question about voltage drop....
A unit with a voltage rating of 208/230v...does this mean this unit can operate within a voltage range of 208-230v?
And if so.....
Is there an allowable voltage drop of +/- 10%
on motors?
Does this mean that a unit with a voltage of 187.2 - 253v (+/- 10%)????

Thanks for any help.....

#34997 03/02/04 05:17 PM
Joined: Dec 2003
Posts: 751
440.32 requires that the conductors be rated for 125% of the branch circuit selection current or the rated-load current of the motor/compressor.
Your example: 125% of 16 amps equals 20 amps. I would run #12 copper.

Voltage drop? No, that is the range of the voltages the motor will run at. These voltages are "nominal". The actual measured voltages will vary from these slightly. Yes this motor will operate at any voltage between these two. Motors will draw more amperage at lower voltages, but keep it within reason.
The NEC suggests we limit the voltage drop to 3% from the nameplate voltage for reasonable efficiency of operation. Some motors and some motor applications could handle a VD of 10%. As the voltage falls, the motor will heat up more and more. It is a trade off, early motor failure or full voltages. Which is more expensive?


#34998 03/02/04 05:20 PM
Joined: Jan 2003
Posts: 4,391
Is that 16 amp figure from taking an amp reading with a meter?

From an NEC perspective you must use the nameplate info for determining the wire size.

At least the HP and Voltage so you could use the tables in the back of 430.

208/230 would be one or the other voltage.

208-230 would be any voltage between them.

Yes on 10% but that is the extreme, you would do well to try to keep the voltage within 3%.

[This message has been edited by iwire (edited 03-02-2004).]

Bob Badger
Construction & Maintenance Electrician
#34999 03/02/04 05:44 PM
Joined: Dec 2003
Posts: 6
Junior Member
Thanks very much fellas.......

I realize that the equipment manufacturer usually provides this info on nameplates but, I needed to know how to properly do this without all the info.....just total running amps....
Incase all the info. wasn't provided.

I did get the 16 amps off a nameplate for an example.

The 125% rating for wire size and the 3% voltage drop was what I was after.


If total amps is 10.5 (10.5 x 1.25 = 13)
I would use # 14 which is good for 20 amps but use a 15 amp breaker???

If total amps is 18.1 (18.1 x 1.25 = 22.6)
I would use # 10 even though # 12 would do but, # 12 must use a 20 amp breaker.

Am I on the right track??

Thanks again.

#35000 03/02/04 06:26 PM
Joined: Jan 2003
Posts: 4,391
You really need to go over article 430, it is not that simple.

Here in no particular order are all things that need sizing, #2 is required to be sized from the nameplate rating.

1)Motor Circuit Conductors

2)Motor and Branch-Circuit Overload Protection

3)Motor Branch-Circuit Short-Circuit and Ground-Fault Protection

4)Disconnecting Means

The circuit breaker will not really have a relationship with the wire size, you may have 14 AWG on a 40 amp breaker.

Article 430 is one the articles listed in 240.4(G) so 240.4(D) does not apply to 10, 12, and 14 AWG.

Bob Badger
Construction & Maintenance Electrician
#35001 03/02/04 06:36 PM
Joined: Oct 2001
Posts: 135
You asked the same question on another site, and got the same answers.

It may require a little more depth of knowledge of the NEC than you currently

There is not a 'one rule fits all' answer.

#35002 03/02/04 08:28 PM
Joined: Mar 2004
Posts: 11
I agree completely with wolfdog. Check out the new expanded code book that simplifies some of its answers. I would'nt touch anything before i had it all fiured out. For safety, oversize your wire a little if the name plate is not specific, if possible

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