breaker size, wire gauge, and minimum circuit ampacity (MCA)

Question

I'm confused about when a circuit breaker (OCPD) must be sized for the gauge of the wire connected to it, and when it must be sized for the required minimum circuit ampacity (MCA) of the circuit being driven.

Nameplate of equipment such as HVAC specifies the MCA (minimum circuit amapacity) and MOP (maximum overprotection device). My understanding is that provided the wire is sized to meet the MCA, that the OCPD may be larger than normally used for that size wire (as long as it doesn't exceed the MOP spec). For example, I installed an HVAC compressor with MCA of 23 amps and MOP of 40 amps; I used 10awg wire and a 30amp breaker. But my understanding is that I could have used a 40amp breaker. This seems wrong to me, because a 40amp breaker could allow 40amps to flow (or 32 amps continuously), whereas 10awg NM-B can only safely carry 30 amps. I guess the rationale is that the equipment cannot draw more than 23amps, unless something goes wrong, and if it does, it'll draw more than 40amps and trip the breaker. Why you'd want to use a 40amp breaker is a mystery to me, unless that's just what you happen to have lying around.

Which is the case for me. I have a subpanel that feeds a two water heaters, one draws 4500 watts and the other 3000 watts, fed by 10awg and 12awg NM-B respectively, protected by 30 amp and 20 amp breakers respectively. The total amperage is (4500+3000)/240 = 31.25 amps; since it's considered a continuous load, this must be upsized by 25% to about 39 amps. So clearly I can feed the subpanel with 8awg NM-B. But can I protect the feeder with a 50amp breaker instead of a 40amp one ?

Clearly I should just purchase a 40 amp btreaker. But clearly I'm also confused, and I'd appreciate any light that can be shone on this.

Answer 1

There are certain exceptions, but normally, including HVAC:

• The specs on the device determine minimum and maximum circuit size. You get a breaker to match.
• The breaker size determines minimum wire size - e.g., 15A = 14 AWG, 20A = 12 AWG, 30A = 10 AWG, 40A = 8 AWG. (For larger sizes, copper and aluminum will vary as will cables vs. individual wires in conduit.)

Using the HVAC example:

• MCA = 23A. Since it is continuous, that really means 28.75A, which means 30A breaker.
• MOP = 40A.

So you have two choices:

• 30A breaker with 10 AWG or larger wire
• 40A breaker with 8 AWG or larger wire

Unless you have some sort of accessory devices on the same circuit, there is nothing gained by going with 40A/8 AWG. Really. If the breaker is tripping at 30A/10 AWG then it is likely there is something actually wrong.

When I had my heavy-up, my electrician actually looked at the HVAC equipment (something like 17A MCA) on an original 30A fused circuit and installed a 25A breaker because there was no reason to use a larger breaker. And no, it hasn't tripped even once.

Using the water heater subpanel example:

• Current needed = 31.25A x 1.25 = 39.06A
• Since each circuit has an appropriate breaker, the subpanel must be fed by a feeder capable of at least 40A = 8 AWG.
• The subpanel could have other things. But if you will only ever have the water heaters on the subpanel, 40A breaker with 8 AWG cable is fine.
• If you want to have capacity for other things in the subpanel, use larger wire/cable and a matching breaker. But you can't use a 50A breaker on 8 AWG cable.

As with many other things (particularly airplanes) the electrical code has multiple layers of protection. In this instance, by using a 40A breaker for the subpanel instead of a 50A breaker, to match the 8 AWG cable, you avoid the possible future problem of:

• Someone adds more stuff to the panel.
• They check to see that the panel can handle 50A, because the panel itself can and the feed breaker is 50A.
• They don't check the wire on the feed breaker, because almost nobody does that - after all, if it says 50A, I can use 50A.
• All loads together now draw 50A.

With a 40A breaker, the breaker will trip within a reasonable amount of time. With a 50A breaker, the breaker will likely never trip but the 8 AWG cable will set your house on fire.

The way to think about it is: Breaker protects the wire. Beyond that, it comes down to manufacturer recommendations (in the case of the HVAC), minimum circuit ampacity (usually easy to determine), continuous use derate (+25% when needed) and sometimes other factors (e.g., multiple circuits in a conduit can sometimes require larger wire size). Note that feeders for an entire service (i.e., the feed into your main panel) have different rules.

Once you know the breaker size, you can go to an ampacity chart to determine the wire size.