I need to run some conduit and wire to a rooftop 3 phase RTU. The plans call for a maximum circuit ampacity of 98 amps with a 100 amp breaker. I want to run 1" EMT with three #3 THHN copper conductors and a #8 copper ground. These wire sizes are determined by the 75 degree C column of the ampacity table. An engineer has told me that since the wire size is smaller than 1/0, I must use the 60 degree C column when determining conductor size because of the temperature rating of the lugs. Can anyone shed some light on this? What is he talking about? Thanks in advance.
Temperature conductor rating
- Thread starter jazzman
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- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
I disagree with the engineers but I would ask if you took 310.15(B)(3)(C) in the 2011 into consideration
- Location
- New Jersey
- Occupation
- Journeyman Electrician
What is the minimum circuit ampacity? And isn't 100 amps greater than the maximum of 98 amps?
Sorry. I meant minimum.
Typically on this type equipment there is an MCA (minimum circuit ampacity) and a MOCP (maximum overcurrent protection, sometimes spec'd as circuit breaker or fuse). While a 100A breaker is most likely compliant, it may not be high enough to mitigate nuisance tripping on start. The MOCP is usually one to three standard ratings higher than the MCA but hardly ever a 2A difference.
The conduit is installed below the roof and enters the RTU from below. No conduit is exposed to the elements. But thank you for your response.
I always assumed that the lugs on all new equipment were rated for 75 degree C. Am I mistaken?
I have double checked the drawings and it does state 98 MCA and MOCP 100. I wonder if the engineer was mistaken is his calculations.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
IMO that's a pretty good assumption.
These ratings are determined by the manufacturer and should be on the equipment nameplate. If you don't have the equipment on hand, they may be on the cut sheet.
If you can verify the manufacturer's MOCP is higher, RFI the engineer.
wallypiper
Member
- Location
- Atlanta, GA, USA
MCA is 125% of the nameplate FLA of the largest motor in the unit plus the FLA of all other connected loads that can run simultaneously.
MCA is the MINIMUM CIRCUIT AMPACITY. That is, the minimum rating of the conductors and other components feeding the equipment.
MOCP is the MAXIMUM OVERCURRENT PROTECTION and may be calculated by the manufacturer based on the wiring and other components in the equipment that must be protected by that device. In many pieces of equipment like a rooftop ahu, the incoming power connection is just a set of lugs connected to a power distribution block. On the load side of that block, there may be smaller wire supplying various loads inside the unit, with their sizes based on the tap rule. Or there may be a main disconnect or even a main circuit breaker or fuses. There is also a code section which defines the MOCP based on the MOCP of the largest connected load (430.52) plus the FLA of all other loads. In many cases, MOCP is just 250% (TM circuit breaker or dual element fuses) of the largest motor FLA plus other loads but the equipment manufacturer can specify a LOWER number to protect internal wiring and components or a higher number based on the various exceptions to 430.52 which allow ratings up to 1700% of the motor FLA.
Motor circuits are different. Since running overload protection is supplied separately from short circuit protection, the short circuit protection can have much higher ratings especially if it is instantaneous trip only.
Many equipment manufacturers seem to have a very poor understanding of all this. Ask many of them for the MCA on their unit and they will just tell you the HP or KW rating of the main motor and look at you with a blank stare when you tell them that isn't the right number. It will usually get you close since the main motor tends to be much larger than the other loads but it doesn't answer the question. It isn't hard to calculate the MCA if you know what all the connected loads are, but the manufacturer should do it for you since he certainly must know that. For a lot of equipment that may include some smaller motors (lubricant pumps, cooling fans etc.) and electric heaters (oil preheat perhaps) plus control transformers and other typical loads found in equipment control panels. To make matters worse, UL has different nameplate requirements that DON'T include MCA or MOCP as defined by the NEC. But they do have the FLA of the largest load and the sum of the other loads or a list of them so you can calculate the correct values.
We are system integrators and I deal with this mess all the time. A lot of the equipment we work with comes from Europe or Asia and getting this information from those guys can be virtually impossible. They don't seem to understand that we may need to get wire pulled and other things installed BEFORE your 2,000 amp Whizbang-500XR shows up on the job site.
MCA is the MINIMUM CIRCUIT AMPACITY. That is, the minimum rating of the conductors and other components feeding the equipment.
MOCP is the MAXIMUM OVERCURRENT PROTECTION and may be calculated by the manufacturer based on the wiring and other components in the equipment that must be protected by that device. In many pieces of equipment like a rooftop ahu, the incoming power connection is just a set of lugs connected to a power distribution block. On the load side of that block, there may be smaller wire supplying various loads inside the unit, with their sizes based on the tap rule. Or there may be a main disconnect or even a main circuit breaker or fuses. There is also a code section which defines the MOCP based on the MOCP of the largest connected load (430.52) plus the FLA of all other loads. In many cases, MOCP is just 250% (TM circuit breaker or dual element fuses) of the largest motor FLA plus other loads but the equipment manufacturer can specify a LOWER number to protect internal wiring and components or a higher number based on the various exceptions to 430.52 which allow ratings up to 1700% of the motor FLA.
Motor circuits are different. Since running overload protection is supplied separately from short circuit protection, the short circuit protection can have much higher ratings especially if it is instantaneous trip only.
Many equipment manufacturers seem to have a very poor understanding of all this. Ask many of them for the MCA on their unit and they will just tell you the HP or KW rating of the main motor and look at you with a blank stare when you tell them that isn't the right number. It will usually get you close since the main motor tends to be much larger than the other loads but it doesn't answer the question. It isn't hard to calculate the MCA if you know what all the connected loads are, but the manufacturer should do it for you since he certainly must know that. For a lot of equipment that may include some smaller motors (lubricant pumps, cooling fans etc.) and electric heaters (oil preheat perhaps) plus control transformers and other typical loads found in equipment control panels. To make matters worse, UL has different nameplate requirements that DON'T include MCA or MOCP as defined by the NEC. But they do have the FLA of the largest load and the sum of the other loads or a list of them so you can calculate the correct values.
We are system integrators and I deal with this mess all the time. A lot of the equipment we work with comes from Europe or Asia and getting this information from those guys can be virtually impossible. They don't seem to understand that we may need to get wire pulled and other things installed BEFORE your 2,000 amp Whizbang-500XR shows up on the job site.
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