Author Topic: AC wiring and breaker sizing  (Read 1305 times)

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AC wiring and breaker sizing
« on: May 11, 2016, 12:10:35 PM »
As of May 5, 2016 the Ontario Electrical Safety Code 26th Edition has been adopted in Ontario.  There have been some changes regarding conductor sizing.

Most AC installers would follow Table 2 of the Electrical Code, and size the conductor according to the minimum circuit ampacity listed on the AC rating plate, as per Rules 4-006(2) & 8-104.  In my case, Lennox, no longer specifies the termination temperature, so we must use the 60C column in Table 2. 

The breaker is sized by following the maximum overcurrent protection listed on the AC rating plate. 

The allowable wiring length is determined by Table D3 and Rule 8-102 by calculating the voltage drop.

For example the Lennox 13ACX-18 lists a maximum overcurrent device of 20A.  We would install a 20A breaker.  The minimum circuit ampacity is listed at 12.4A.  Using Table 2 and the 60C column, we could use 14/2 wiring.  Factoring in the voltage drop we can use 14/2 wire up to 95', then we have to use 12/2 wire.

I attached a Lennox 13ACX wiring/breaker sizing chart and the OESC Table 2 below.

OESC Rule 4-006(3) says,

Quote
Notwithstanding Subrule (2), for high-voltage equipment where conductor termination temperatures are not marked, it shall be permitted to consult the manufacturer to establish the permitted termination temperature

I am hoping Lennox will say that their termination temperature rating is 75C, like on their older units.  This way we can use the 75C column in Table 2, which would allow smaller gauge wiring.

13ACX-18 - Max Overcurrent Device = 20A / Min Circuit Ampacity = 12.4A = #14 wire
13ACX-24 - 25A / 14.7A = #14 wire
13ACX-30 - 30A / 18.7A = #12 wire
13ACX-36 - 35A / 22A = #10 wire
13ACX-42 - 40A / 28.1A = #10 wire
13ACX-48 - 50A / 31.9A = #8 wire

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Re: AC wiring and breaker sizing
« Reply #1 on: November 16, 2017, 12:36:04 PM »
What type rating wire are you using on your installs?

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Re: AC wiring and breaker sizing
« Reply #2 on: November 16, 2017, 12:51:11 PM »
We use NMD90 wire, but follow the 60C column in Table 2 of the OESC 26th Edition for the wiring sizing.

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Re: AC wiring and breaker sizing
« Reply #3 on: November 16, 2017, 12:56:50 PM »
Okay that's what I've been using too. I think it's rated for up to 90C?  I just carry 10/2 and use it on pretty much everything.

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Re: AC wiring and breaker sizing
« Reply #4 on: July 25, 2018, 03:50:31 PM »
Only if the AC rating plate lists the termination temperature of 90C, can that column be followed.  This would allow you to use even smaller wiring.

The attached pic is of an older Lennox air handler, which lists a termination temperature of 75C.  This means if our minimum ampacity was 20A or under we could use #14 wire.  The newer Lennox units don't list the termination temp so we use the 60C column, and a min ampacity of 15A or less can use #14 wire.  If the min ampacity was between 15.1A and 20A we need to use #12 wire using the 60C column.

I just tell the installers to look at the rating plate for 2 things,

Max breaker size - Install this breaker
Min Ampacity - Determines size of wiring

Table 2 @ 60C:
0A - 15A = #14 wire
15.1A - 20A = #12 wire
20.1A - 30A = #10 wire
30.1A - 40A = #8 wire
40.1A - 55A = #6 wire

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Re: AC wiring and breaker sizing
« Reply #5 on: July 25, 2018, 06:46:45 PM »
Now you don't always have to use the max breaker size am I correct,

Say the minimum ampacity is a 14.5A and the max breaker size is 25A, a 20A breaker could also be used in place of a 25A?

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Re: AC wiring and breaker sizing
« Reply #6 on: July 25, 2018, 09:20:29 PM »
I’ve heard of a 80% rule before and found this in the Electrical Code.

Quote
Rule 8-104
(6) Where a fused switch or circuit breaker is marked for continuous operation at 80% of the ampere rating of its overcurrent devices, the continuous load as determined from the calculated load shall not exceed
(a) 80% of the rating of the circuit where the ampacity of the conductors is based on Column 2, 3, or 4 of Table 2 or 4; or

I spoke to ESA and you can use a 20A breaker.  80% of 20A = 16A.  As long as this is above the minimum ampacity of 14.5A you can use the smaller breaker.  80% of a 15A breaker is 12A, so it could not be used in your example.

Here’s a good article that explains why we don’t worry about using a smaller wire with a larger breaker with AC’s, because our compressor and fan motor have their own built in overload protection and those windings would fail before our wire.

https://www.achrnews.com/articles/135676-what-most-techs-get-wrong-about-wire-sizing