HVAC School - For Techs, By Techs HVAC/R Conductor, Breaker, and Fuse Selection
In this podcast episode, Phil Barr joins Bryan to explain sizing for wires and breakers in HVAC/R work. You will be able to select breakers, conductors, and fuses properly and without confusion. Phil Barr is the leader of the electrical division at our business.
HVAC/R equipment may have hermetically sealed motors. Unlike squirrel-cage motors, hermetically sealed motors have an outer shell that makes it impossible to access the inner components. Semi-hermetic equipment, such as some compressors, look like hermetic equipment but can open up. Wire sizing varies between hermetically and non-hermetically sealed motors, and the NEC explains the wire sizing requirements, but YOU need to know the context for those requirements.
Once you know your equipment type, check the nameplate with a rating, such as MCA, RLC, branch circuit selection, etc. The manufacturer will establish that rating, and you will use it to look up the correct wire sizing requirements. Wire insulation and conductor type also dictate the sizing and installation requirements. Conductor length and voltage drop also affect wire sizing.
Fuses or circuit breakers prevent shorts. Shorts are undesigned paths with little to no resistance, so fuses and circuit breakers protect equipment and buildings from overcurrent due to shorts, NOT thermal overload. So, you use MOCP as a guideline for sizing your breakers. Thermal overload protection keeps conductors from melting under overload conditions.
If you want a breaker that is under the MOCP value but it exceeds the MCA and the terminations are rated correctly, you can typically use a breaker between the MCA and MOCP. However, you will still want to follow manufacturer recommendations and check with your AHJ.
Phil and Bryan also discuss:
- MCA (minimum circuit ampacity)
- "Undersized" conductors in new constructions
- Reducing voltage drop
- MOCP and related terms
- Inrush current
- Adjustment factors