AC wiring and breaker sizing

[Admin]

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,

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

[walker]

What type rating wire are you using on your installs?

[Admin]

We use NMD90 wire, but follow the 60C column in Table 2 of the OESC 26th Edition for the wiring sizing.

[walker]

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.

[Admin]

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

[walker]

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?

[Admin]

Just an update.  It's frustrating getting different info from the ESA but here's the latest.  They are saying the 80% rule does not apply to using smaller breakers than the Max Breaker size listed on the rating plate.  We can size both wiring and the breaker according to the Minimum Circuit Ampacity.  For example the Goodman GSX13-36 has a MCA of 18.8A and a Max Breaker Size of 30A.  According to the ESA and Table 2 we can use a 20A breaker and 12/2 wire.

Fundamentally the nameplate of the equipment is where the confusion lays.

For the 13.2A example the Compressor is 9.7A and the fan is 1.1A, 9.7 X 125%= 12.125 + 1.1= 13.2A which can be fed with a #14 conductor as it is compliant with 28-108. This is also where the minimum circuit ampacity is derived from.

However the 'load' is 9.7 + 1.1 = 10.8A. 8-104 does not permit a continuous load to exceed 80% of a breaker which is not marked for continuous duty. If a 15A breaker was used, it would be compliant to have a continuous connected load of 12A, 10.8A is within that range. With this being considered, there is no need to apply the same factor again. To elaborate 80% of 13.2 is 10.56A which is below the actual maximum load therefore the factor has already been included in the nameplate.

Section 28 defines the maximum size of overcurrent protection, It is silent on the minimum size of overcurrent protection. 8-104,which is not amended by section 28, would be the only restriction preventing the overcurrent size being smaller than the nameplate marking (13.2A for this example). The manufacturer has provided some direction to this for installers with the Max overcurrent size on the equipment which is aligned with 28-206.

Hopefully that clarifies the installers questions. While I selected one example, the same rationale would apply to the other two also.

Best regards,
Keith Bartlett | Technical Advisor, Eastern Region Electrical Safety
Authority
Cell: 613-913-5413
keith.bartlett@electricalsafety.on.ca