# Tap Conductor Temperature

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#### erickench

##### Senior Member
Okay, here's a doozie of a question for you guys. Let suppose you had a 500 MCM feeder rated at 380A at 75'C and you wanted to run a tap that terminates to a 100A overcurrent device. Now the temperature for this OCPD is not given so you would have to apply either NEC 110.14(C)(1)(a) or (b) which states that equipment terminations rated at 100A or less shall have the connecting conductors rated 60'C otherwise it's 75'C. What temperature would you use to size the tap? 60'C or 75'C? I mean the OCPD is rated at 100A but the feeder is rated at 75'C.

#### iMuse97

##### Senior Member
since you don't have the temp rating of the 100A OCPD, I would say the 60*C must be used. NEC 110.14(C)(1)(a)(1) It matters not what the feeder is rated at, acc. to this.

#### erickench

##### Senior Member
Well Mike Holt in his book "NEC Exam Preparation" rates the tap as a 75' conductor and I was just wondering why. I just took a walk outside and thought it over carefully. Let's assume the feeder operates at full capacity 380A(or 400A), that means the operating temperature would be at 75'C. And because the tap is in direct contact with the feeder it too would experience a temperature of 75'C. That would be the only reason why the tap would have to be rated at 75'C. Being that the OCPD is not a wire/cable it would be able to handle the higher temperature.

#### iMuse97

##### Senior Member
Well Mike Holt in his book "NEC Exam Preparation" rates the tap as a 75' conductor and I was just wondering why. I just took a walk outside and thought it over carefully. Let's assume the feeder operates at full capacity 380A(or 400A), that means the operating temperature would be at 75'C. And because the tap is in direct contact with the feeder it too would experience a temperature of 75'C. That would be the only reason why the tap would have to be rated at 75'C. Being that the OCPD is not a wire/cable it would be able to handle the higher temperature.

I had not supposed, and do not think that conductors rated for a certain temp. will therefore operate at that temperature if fully loaded. I'm very sure that is not actually the case.

As I understand it, the NEC, being a minimum standard of methods and installation, bases its requirements on the engineered standards that the materials are required to meet. In the case of 114(C), the NEC is concerned with the amount of copper used in a termination of OCPDs. It further divides them into two groups, (a) & (b), and says that the smaller sized ones need the wire size required by the 60*C column to maintain an safe installation.

You supposed that it had to do with the larger size wire that the tap is taken from, but it has nothing to do with that.

#### Smart \$

##### Esteemed Member
IMO, you'd not be permitted to do it as proposed, by the consensus of interpretation on this forum. The unrated 100A ocpd would become the lowest temperature rated device on the circuit.
Temperature Limitations. The temperature rating associated
with the ampacity of a conductor shall be selected
and coordinated so as not to exceed the lowest temperature
rating of any connected termination, conductor, or device.

#### erickench

##### Senior Member
You present a good case about the mechanical problems involved in connecting wire to equipment. But in an insulated conductor. as the current rises the operating temperature also rises. For a 500 MCM cable rated at 75'C as the current approaches it's rated ampacity the operating temperature moves closer and closer to 75'C which is the maximum that the insulation can withstand. In most installations 500MCM cable is protected with a 400A OCPD therefore the operating temperature is slightly above 75'C which may cause the insulation to burn a little bit. But the NEC will allow this. In the case of a tap if the feeder is operating at 380A then the metal-to-metal contact will cause the tap conductor temperature to rise above 60'C to 75'C. But I do see where you're coming from about the wire being to big for the terminal.

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#### yired29

##### Senior Member
IMO, you'd not be permitted to do it as proposed, by the consensus of interpretation on this forum. The unrated 100A ocpd would become the lowest temperature rated device on the circuit.

If the tap conductors ampacity were sized so as not to exceed 60 degree. Would that not qualify?

Could we treat the tap as a seperate complete connection?

75 degree terminantion at the tap with wire having an ampacity not to exceed 60 degree.

We would need a 75 degree term at the tap because the 500 kCMIL could reach 75 degree.

As long as the tap conductors dont exceed 60 degree from the load on the 100 amp breaker would this work?

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#### erickench

##### Senior Member
As long as the tap conductors dont exceed 60 degree from the load on the 100 amp breaker would this work?

The metal of the tap conductor is in direct contact with the metal of the 500MCM. If the feeder current goes to 380A both metals would heat up to 75'C. This could cause a fault if the insulation of the tap burns off. Now it becomes a question on what should be the weakest link, either the tap or the equipment terminals.

#### iMuse97

##### Senior Member
But I do see where you're coming from about the wire being to big for the terminal.

You missed the point: the copper in the 60*C rated terminals must have enough mass to draw off heat that may be generated.

#### Smart \$

##### Esteemed Member
If the tap conductors ampacity were sized so as not to exceed 60 degree. Would that not qualify?

Could we treat the tap as a seperate complete connection?

75 degree terminantion at the tap with wire having an ampacity not to exceed 60 degree.

We would need a 75 degree term at the tap because the 500 kCMIL could reach 75 degree.

As long as the tap conductors dont exceed 60 degree from the load on the 100 amp breaker would this work?

I know a tap conductor can be "engineering-level calculated" such that the temperature rating at the ends is not exceeded... but the NEC makes no provisions or exceptions for such that I'm aware of (without great deliberation among members ).

#### yired29

##### Senior Member
The metal of the tap conductor is in direct contact with the metal of the 500MCM. If the feeder current goes to 380A both metals would heat up to 75'C. This could cause a fault if the insulation of the tap burns off. Now it becomes a question on what should be the weakest link, either the tap or the equipment terminals.

We would have to use wire that has an insulation of at least 75 degree with a design no to exceed 60 degree from the 100 amp load.

I,m not sure if the 500 reached 75 degree that would make the tap conductor 75 degree at the terminantion of the 100 amp OCPD.

I know this is a stretch.

If all else fails get a 100 amp OCPD with a 75 degree term.

#### Smart \$

##### Esteemed Member
You missed the point: the copper in the 60*C rated terminals must have enough mass to draw off heat that may be generated.

But in this case, the "heat" is coming from the other end. At least that's what the code seems to want us to assume

#### erickench

##### Senior Member
You missed the point: the copper in the 60*C rated terminals must have enough mass to draw off heat that may be generated.

Let me ask you a question. What would happen if the operating temperature exceeded the temperature rating of the equipment? Would something burn or melt? Or is the rated equipment temperature just the operating temperature at rated load? I mean this is an OCPD we're talking about. It has a short circuit rating.

#### iMuse97

##### Senior Member
But in an insulated conductor. as the current rises the operating temperature also rises. For a 500 MCM cable rated at 75'C as the current approaches it's rated ampacity the operating temperature moves closer and closer to 75'C which is the maximum that the insulation can withstand. In most installations 500MCM cable is protected with a 400A OCPD therefore the operating temperature is slightly above 75'C which may cause the insulation to burn a little bit. But the NEC will allow this. In the case of a tap if the feeder is operating at 380A then the metal-to-metal contact will cause the tap conductor temperature to rise above 60'C to 75'C.

Clearly, temps rises as current increases. But, the operating temp will not typically be anywhere near 75*C on a 400A/500kcmil installation; probably closer to 50*C; warm to the touch, generally. Try touching some fully loaded conductors sometime. You can. Your hand will not burn.

Your comment about insulation burning makes no sense at all: if insulation were "burning a little bit" it would degrade: the 75*C rating of the insulation is the operating temperature which the insulation can withstand without degradation. In high ambient temperature conditions some conductors will approach this threshold, but they will not "burn" if they have been sized correctly.

#### LarryFine

##### Master Electrician Electric Contractor Richmond VA
It seems to me that Smart's quote in post #5 says it all.

#### iMuse97

##### Senior Member
Let me ask you a question. What would happen if the operating temperature exceeded the temperature rating of the equipment? Would something burn or melt? Or is the rated equipment temperature just the operating temperature at rated load? I mean this is an OCPD we're talking about. It has a short circuit rating.

It also has an overload trip mechanism and it would sense an overload condition (ie. Heat) and trip under 100A. I am assuming that the whole point of "rated temperature" is to keep the wire from operating above that temperature. If an installation is designed which allows wire to operate at the maximum, it is poorly designed.

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#### erickench

##### Senior Member
It seems to me that Smart's quote in post #5 says it all.

Okay, NEC 110.14(C) settle's the question. I'll have to contact Mike's editor and tell him about it.

#### yired29

##### Senior Member
110.14 (C) Temperature Limitations. The temperature rating associated with the ampacity of a conductor shall be selected and coordinated so as not to exceed the lowest temperature rating of any connected termination, conductor, or device. Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction, or both.

Maybe I'm still reaching but 110.14 seems to be based on the ampacity which is flowing through the conductor. None of the 500's ampacity should flow through the tap conductors.

So if we use a #1 AWG THHN for the tap conductors, based on the ampacity of the conductor it will not exceed 60 degree. We would still need a 75 degree term at the tap supply.

#### iMuse97

##### Senior Member
Okay, NEC 110.14(C) settle's the question. I'll have to contact Mike's editor and tell him about it.

I'm fine with the resolution of that question, but you've brought up a few more.

1. What are temperature ratings on lugs for?
a. Does it keep us from melting them?
b. Does it say something about the mass of the lug and the size (ie. mass) of wire it is designed for, and it's ability to dissipate the heat that is generated therein?

2. What is the typical operating temperature of wire when installed in a code compliant manner, loaded continuously to 80% of ampacity with a 35*C?

#### Smart \$

##### Esteemed Member
Maybe I'm still reaching but 110.14 seems to be based on the ampacity which is flowing through the conductor. None of the 500's ampacity should flow through the tap conductors.
Not possible. If there is current through the tap conductor, it has to be some of the current flowing through the 500.

So if we use a #1 AWG THHN for the tap conductors, based on the ampacity of the conductor it will not exceed 60 degree. We would still need a 75 degree term at the tap supply.
That would be fine if we met the ampacity of 500's at 60?C. The requirement says, "...shall be selected and coordinated so as not to exceed the lowest temperature rating of any connected termination, conductor, or device." Conditions stated exceed that value.

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