1. Unit Ampacity for Wire Sizing
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2. Common complaint: Background:
Current -20 models need a 20 amp breaker and the “old” (pre-A90) models didn’t
Background:
This conclusion was based on confusion related to differences in rating plate amps and unit ampacity used for wire sizing.

3. What and Why What is “Rating Plate Amps”?
What is “Unit Ampacity For Wire Sizing”?
Why are they different?
How do I properly size a fuse or circuit breaker?
Why do I have to use a 20 amp breaker?

4. Rating Plate Amps Shows minimum electrical loads
Does not include accessory loads
Does not add 1.25 factor to largest load
Required for wire sizing
Additional factors could be overlooked in the field
Could results in undersized field wiring

5. From *9MVX specifications:

6. Unit Ampacity for Wire Sizing
Shows all electrical loads tied to the furnace
Including accessory loads
Adds 1.25 factor to largest load
Required for wire sizing per NEC
Provides correct wire size to be used
Wire is then matched to the correct breaker
Eliminates field calculations !!

7. From (F/G)9MVT:

8. Unit Ampacity for Wire Sizing
Based on Three items:
Unit load
Motor safety factor
Accessories/additional loads

9. Unit Ampacity for Wire Sizing
1 – Unit load
The ampacity of the circuit board, transformers, ignitor, gas valve, etc are all totaled and adjusted to a 115 Volt rating. This gives us the worst-case amp rating for the control circuits and components

10. Unit Ampacity for Wire Sizing
2 - Motors
The highest rated motor worst case ampacity is multiplied by 1.25* for a safety margin. All other motor values are taken at their respective highest load amp rating. These values are all totaled for a total motor ampacity

11. Unit Ampacity for Wire Sizing
3 – Accessories
Any accessories that could be hooked to unit must be considered. We can not predict what model/brands could possibly hooked up, so we use the max current rating at the connection point on the control board. These values are all totaled to come up with a total accessories amp rating.

12. *9MVX0100-20 Rating Plate Amps Unit Ampacity Trans & Circuit Board
0.34
Gas Valve
0.76
Inducer Motor
0.7
Blower FLA
12.8
Total
14.6
Can I use a 14 ga wire? NO
Unit Ampacity
Trans & Circuit Board
0.34
Gas Valve
0.76
Inducer Motor
0.7
Blower FLA x 1.25 16
EAC
0.12
Humidifier 1
Total
18.92
MCA of required 12 ga. wire

13. Sizing a Fuse/Circuit Breaker (OCPD)
OCPD is Over Current Protective Device
Another name for a fuse or circuit breaker
OCPD’s protect the circuit, not the equipment
OCPD’s are sized for two types of electrical loads
100% of the non-continuous load amp draw
125% of continuous loads amp draw
Continuous load is 3 hours (or more) of operation
A furnace can run more than 3 hours
A furnace is considered a continuous load

14. For continuous loads: OCPD rating is reduced by 80%
80% is the reciprocal of 125%
1 / 1.25 = .80 or 80%
For a 15 amp breaker
15 amps x .80 = 12 amps
For a 20 amp breaker
20 amps x .80 = 16 amps

15. For *9MVX0100-20 Rating plate amps = 14.6
15 amp OCPD x .80 = 12.0 amps max
Furnace requires a 20 amp circuit breaker
Minimum wire size for 20 amp breaker
12 ga.

16. Questions