Sizing breaker for RTU

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Mustang125

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Master Electrician, Project Manager and Estimator
When reading a nameplate on a roof top unit and it says
min. Circuit amp 23
max fuse size 35
max breaker size 35

could I use a 30amp breaker? Normally I'd see max breaker size and use that, but it's come to my attention that I might get away with sizing my own breaker based off of min. Circuit amps.

now how about when it says
min. Circuit amps 28
max fuse size 35
max breaker size 35

could I use a 30 in that situation or no because it's more than 80% breaker. When researching the meaning of min. Circuit amps, it sounds like that has the 125% and all this other crap built into there math and that's how they come up with that number so does the 80% breaker rule apply?
 
You could use a 30A breaker in both cases, most use a max size to possibly avoid any nuisance trip.

MCA has all the multipliers/continuous use factors/etc built in. Nothing to add.


could I use a 30 in that situation or no because it's more than 80% breaker. When researching the meaning of min. Circuit amps, it sounds like that has the 125% and all this other crap built into there math and that's how they come up with that number so does the 80% breaker rule apply?
Click to expand...

There is no 80% rule. It is just using another way of calculating the conductor size using 125% as a multiplier for continuous loads.
 
Spencer125 said:
When reading a nameplate on a roof top unit and it says
min. Circuit amp 23
max fuse size 35
max breaker size 35

could I use a 30amp breaker? Normally I'd see max breaker size and use that, but it's come to my attention that I might get away with sizing my own breaker based off of min. Circuit amps.

now how about when it says
min. Circuit amps 28
max fuse size 35
max breaker size 35

could I use a 30 in that situation or no because it's more than 80% breaker. When researching the meaning of min. Circuit amps, it sounds like that has the 125% and all this other crap built into there math and that's how they come up with that number so does the 80% breaker rule apply?
Click to expand...


Your not saving anything by sizing your own breaker.
Size the wire for the Circuit amp, slap in the max 35 amp breaker and let it rock and roll.

JAP>
 
jumper said:
There is no 80% rule. It is just using another way of calculating the conductor size using 125% as a multiplier for continuous loads.
Click to expand...


This raises a whole orher question for me haha, so if you take a lighting load and multiply it by 125% for being continuous and that number comes out to 19.99 amps, you can put that on a 20 amp breaker? The only time you use the 80% rule is when you didn't do any math, like a microwave draws 16amps that's good on a 20 but if it draws 19 wouldn't you have to up it?

Also JAP, wouldn't you save money in this situation based on being able to use # 10 and a 30amp breaker? Cuz if you go with the max 35 you have to up the wire to 8 because you can't put #10 on a 35 right
 
Spencer125 said:
This raises a whole orher question for me haha, so if you take a lighting load and multiply it by 125% for being continuous and that number comes out to 19.99 amps, you can put that on a 20 amp breaker? The only time you use the 80% rule is when you didn't do any math, like a microwave draws 16amps that's good on a 20 but if it draws 19 wouldn't you have to up it?

Also JAP, wouldn't you save money in this situation based on being able to use # 10 and a 30amp breaker? Cuz if you go with the max 35 you have to up the wire to 8 because you can't put #10 on a 35 right
Click to expand...
Spencer, you are leaving out the special case that applies to motors and motor-containing appliances which specify on their nameplate both an MCA and an MOCPD.
Because the low level overload protection for the wires is provided by the overcurrent protection of the motor(s), you only need to size the wires for the MCA (without any 1.25 multiplier) but can protect those wires against short circuits and ground faults (not low level overloads) by a breaker or fuse valued at the MOCPD. There is no "next size up" allowance at this point though, AFAIK.
 
GoldDigger said:
Spencer, you are leaving out the special case that applies to motors and motor-containing appliances which specify on their nameplate both an MCA and an MOCPD.
Because the low level overload protection for the wires is provided by the overcurrent protection of the motor(s), you only need to size the wires for the MCA (without any 1.25 multiplier) but can protect those wires against short circuits and ground faults (not low level overloads) by a breaker or fuse valued at the MOCPD. There is no "next size up" allowance at this point though, AFAIK.
Click to expand...
In short, the wire ampacity* must only equal or exceed the MCA value, regardless of circuit breaker rating.

*inclusive of the terminal temperature limitation.
 
Spencer125 said:
Cuz if you go with the max 35 you have to up the wire to 8 because you can't put #10 on a 35 right
Click to expand...

To further what GoldDigger and Smart$ said, would could use #12 wire and a 35A breaker for this unit.

min. Circuit amp 23
max fuse size 35
max breaker size 35
 
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