MINIMUM CIRCUIT AMPACITY (MCA) vs MAXIMUM OVERCURRENT PROTECTION (MOCP)
Introduction
Oftentimes in the field of electrical construction, we rely on others to provide information in order for us to properly size conductors and/or overcurrent protection for a piece of equipment. That is where Minimum Circuit Ampacity (MCA) and Maximum Overcurrent Protection (MOCP) come into play. In North America, MCA and MOCP are provided by equipment manufacturers so that the equipment can be safely connected to the building’s electrical systems.
In the 2020 NEC, article 440.4(B) tells us that any type of refrigeration, heating, ventilation, or air conditioning equipment must have both the MCA and MOCP rating visibly marked on the nameplate. Below is a snapshot of a typical nameplate you might see on a piece of equipment. Notice that not only are the MCA andr MOCP indicated, but also the type of Overcurrent Protection Device (OCPD) required – this can either be a fuse or HVAC circuit breaker (circled in red).
Some of the biggest challenges with MCA and MOCP are not knowing how to properly use these values in the field. A common mistake is sizing conductors based on the overcurrent protection device. In most situations we would match the ampacity of the conductors with that of the circuit breaker or fuse serving it. For example, if we were to select a 50 amp circuit breaker for a branch circuit, we would choose a 6-AWG copper conductor, since that has an ampacity of 50 amps. We will review how to properly size conductors based on ampacity shortly. When applying this same method for sizing conductors for HVAC equipment, it can lead to oversizing of conductors. The most obvious place this can show up would be when an existing HVAC unit is being changed out for a slightly larger unit. Many times, both the conductors and OCPD are upsized together without looking at the MCA value, when in reality the existing conductors could meet the MCA value of the new piece of equipment. A simple change of the circuit breaker or fuses serving the equipment is all that would be required.
Another common mistake is not checking which type of overcurrent protection device is to be used. In the above picture, you can see that both fuse and circuit breaker are acceptable means of overcurrent protection. If the nameplate only indicates a fuse, and not a circuit breaker, a fuse must be used. We often see the name plate indicate “Fuse Only” or “Max Fuse Size” which would indicate that a fuse must be used. If a fuse is not specifically called for or indicated on the nameplate, a circuit breaker is acceptable.
Minimum Circuit Ampacity (MCA)
Minimum Circuit Ampacity, or MCA, is exactly as it sounds. It is the minimum size a conductor must be to handle the amount of current (amperes) being drawn by the equipment. This is not the actual current that will flow through the equipment; that value may vary. This is the value the manufacturer gives us to size the circuit conductors safely, without risking overheating or damage to the conductors or insulation during normal operation.
When we see an MCA value on a piece of equipment, where does that come from? Below we will review the process of calculating the MCA value.
For HVAC equipment with any type of compressor motor, we want to look for the RLA value or Rated Loaded Amps/Rated Load Current. As per 2020 NEC 440.2,: the rated load current for a hermetic refrigerant motor-compressor is the current resulting when the motor-compressor is operated at the rated load, rated voltage and rated frequency of the equipment it serves.
Start by looking for the largest RLA value and multiplying it by 125 percent. All other RLA or FLA (Full Load Amps) are added together and then multiplied by 100 percent. We then add the two values together to get the MCA value.
Here is a quick formula to show how it works: 125 percent x largest RLA value + 100 percent x all other loads (RLA or FLA)
Example 1
A 20-ton rooftop HVAC unit has the following values shown on its nameplate. Find the MCA:
Compressor 1: 22 RLA
Compressor 2: 22 RLA
Fan 1: 2.6 FLA
Fan 2: 3.2 FLA
Step 1: Multiply largest RLA value by 125 percent:
22 RLA
x
125 percent
=
27.5 amps
Step 2: Add up all other RLA or FLA values and multiply by 100 percent:
22 RLA
+
2.6 FLA
+
3.2 FLA
x
100 percent
=
27.8 amps
Step 3: Add value from steps 1 and 2 to get MCA value:
27.5 amps
+
27.8 amps
=
55.3 amps, or 55 amps if we round down. We size our conductors for an ampacity of 55 amps.
GENERAL NOTE: For equipment with single motor loads such as fans and pumps, always use full load current in NEC 430 tables. Conductors shall be sized based on 125 percent of full load current in the NEC 430 tables.
Maximum Overcurrent Protection (MOCP or MOP)
Maximum Overcurrent Protection, or MOCP, is the maximum size of a fuse or circuit breaker required to provide adequate protection of the equipment during a fault condition. If we put too small of a fuse or breaker ahead of this circuit, the breaker could cause a nuisance trip when the equipment turns on. If we install too large of a fuse or breaker, the conductors could heat up to the point that the insulation around the conductors melt, but the breaker never trips. Let’s look at how MOCP is calculated.
The MOCP value is calculated as follows, which is from the 2020 NEC, article 440.22.
We want to look for the largest motor FLA or RLA value and multiply it by 175 percent. All other RLA or FLA values are added together and then multiplied by 100 percent. We then add the two values together to get the MOCP value.
Here is a quick formula to show how it works: 225 percent x largest RLA value + 100 percent x all other loads (RLA or FLA).
EXAMPLE 2:
A 100-ton rooftop HVAC unit has the following values shown on its nameplate. Find the MOCP:
Compressor 1: 50 RLA
Compressor 2: 30 RLA
Compressor 3: 20 RLA
Fan 1: 5 FLA
Fan 2: 12 FLA
Fan 3: 10 FLA
Step 1: Multiply largest RLA value by 175 percent:
50 RLA
x
175 percent
=
87.5 amps
Step 2: Add up all other RLA or FLA values and multiply by 100 percent:
30 RLA
+
20 RLA
+
5 FLA
+
12 FLA
+
10 FLA
x
100 percent
=
77 amps
Step 3: Add the total value from steps 1 and 2 to get the MOCP value:
87.5 amps
+
77 amps
=
164.5 amps. The next available circuit breaker or fuse size is 175 amps.
Always remember to check the type of overcurrent protection device required by the nameplate. If the nameplate indicates MOCP as “Maximum Fuse Size”, a fused disconnect must be used instead of a circuit breaker.
How to Size Conductors Based on MCA
Now that we are familiar with the minimum circuit ampacity, let’s use it to size conductors using NEC table 310.15(B)(16). Table 310.15(B)(16) will cover most common installations, in that it covers raceways with no more than three current carrying conductors.
When looking at Table 310.15(B)(16), we want to stick to the 75°C column. Even though common THHN/THWN type wire is rated for 90°C, the terminals for equipment such as motor controllers, variable frequency drives, circuit breakers, disconnect switches, etc are UL rated and tested for 75°C under 600V. So we must use the 75°C column.
See image below for an example of a circuit breaker with terminal ratings.
Using the MCA found in example one which is 55 amps, we decide to use copper conductors for this example. Start out by looking at the 75°C column under copper. Notice that there is no conductor size that corresponds to 55 amps. That means we’ll have to go the next size up from there, which is 6-AWG rated for 65 amps. It is always safer to size up rather than down for conductors.
Example 3
Size the branch circuit conductors and overcurrent protection device based on the nameplate below.
MCA: 660 amps
MOCP: 800 amps
Compressor 1 RLA: 270 amps
Compressor 2 RLA: 270 amps
Eight 1-½-HP fans: 6.5 amps each
Let’s check the MCA value to see if it is correct.
Remembering the formula:
125 percent
x
largest RLA value
+
100 percent
x
all other loads (RLA or FLA)
Step 1:
125 percent
x 270 amps = 337 amps
Step 2:
270 amps
+
52 amps (or 6.5 amps x eight)
=
322 amps
x
100 percent
=
322 amps
Step 3:
Add step 1 and step 2 values: 659 amps, rounding up to 660 amps.
Let’s check the MOCP value to see if it is correct.
Remembering the formula:
175 percent
x
largest RLA value
+
100 percent
x
all other loads (RLA or FLA)
Step 1:
175 percent
x
270 amps
=
472.5 amps
Step 2:
270 amps
+
52 amps (or 6.5 amps x eight)
=
322 amps
x
100 percent
=
322 amps
Step 3:
Add step 1 and step 2 values: 794.5 amps, rounding up to 800 amps OCPD.
Conclusion/Wrap up Points
- Size conductors based on the MCA nameplate value rather than the MOCP or OCPD.
- The OCPD trip rating for branch circuit feeding equipment shall match that of the MOCP value on the nameplate of the equipment. Go to the next size up that is available for the fuse or circuit breaker.
- Match the OCPD type (fuse or circuit breaker) with what is listed on the nameplate of the equipment.
- Use the 75°C column of NEC Table 310.15(B)(16) for copper or aluminum to choose the conductor size. Match the conductor ampacity with the MCA value.
- The MOCP rating will always be higher than that of the MCA rating. Sizing the branch conductors based on the MCA rather than the MOCP is safe and NEC compliant.
