Branch Circuit Sizing

[erickench]

I am looking at an "Electrical Inspection Manual". It state's that the 60 degree column is to be used for determining the size of a branch circuit conductor when using the 125% continuous plus 100% noncontinuous rule before temperature or any other adjustment factors are applied. Is using the 60 degree column a common practice?

 

[augie47]

IMHO, the "adjustment" factor is applied using the conductor temperature insulation.
If you have 90 deg insulation, the 60 deg column only comes into account for termination as noted in 110.14 and for NM and SE cables (interior) where the Code requires they be terminated using their 60 deg rating.

 

[charlie b]

That sounds like it is addressing the most common branch circuits a residential inspector might encounter, which is to say circuit rated under 100 amps. I think that book's author has mis-interpreted 110.14(C)(1). If the terminations are rated for 75C, and most will be, then you can use the 75C column.

 

[Volta]

That sounds like it is addressing the most common branch circuits a residential inspector might encounter, which is to say circuit rated under 100 amps. I think that book's author has mis-interpreted 110.14(C)(1). If the terminations are rated for 75C, and most will be, then you can use the 75C column.

Except, as Augie stated, for NM, SE, UF, and TW.

 

[OTT2]

I am looking at an "Electrical Inspection Manual". It state's that the 60 degree column is to be used for determining the size of a branch circuit conductor when using the 125% continuous plus 100% noncontinuous rule before temperature or any other adjustment factors are applied. Is using the 60 degree column a common practice?

I would say that using the 60 degree column to begin ampacity adjustment is not common.

110.14 (C) allows you to use higher temperature ratings where the termination device and the conductor insulation allow it (have ratings higher than 60 degrees).

 

[erickench]

This is the book written by Williams and Sargent. There are two methods used. One is where you would use the %125 continuous plus %100 noncontinuous. The other is where you would use the calculated results from NEC 220 and apply the temperature correction and adjustment factors. You're supposed to take the larger of the two sizes determined. I was curious as to why in the first case he use's the 60 degree column and in the second case he use's the 75 degree column.

 

[raider1]

Can you post the text of the question and the methods used?

I know Noel Williams quite well and and I am sure that there is a reason for the use of the 60 degree column in the first method.

Chris

 

[erickench]

Sure raider 1. I'll copy it word-by-word.

Given:
Load A is a continuous electric discharge lighting load. The load on each ungrounded conductor of the multiwire branch circuit is 15 amperes. Because of the ballasts used, the load is considered nonlinear (neutral is current- carrying). The lampholders are not heavy duty.

Load B is a noncontinuous multioutlet receptacle load for portable appliances and tools. Each ungrounded conductor of the multiwire branch circuit supplies 10 20-ampere, 125 volt duplex receptacles. The neutral conductor carries only the unbalanced current of the conductor.

Find:
What is the rating of the overcurrent devices and what are the minimum conductor sizes?

Load A Solution:
Minimum rating of overcurrent protection protection and minimum ampacity of conductors for the continuous load [210.19.210.20]:

125 percent X 15 amperes = 18.75 - minimum 20-ampere overcurrent device and minimum 18.75-ampere ampacity.

Based on 60'C terminals the minimum conductor size is 14 American Wire Gauge (AWG) from the 60'C column [110.14(C)]. Generally, however, 14 AWG must be protected at 15 amperes, so the minimum size is 12 AWG to coordinate the conductor size with the minimum overcurrent device rating [240.4(D)].

Maximum rating of overcurrent protection: Maximum 20 amperes, based on load restrictions on higher-rated circuits [210.23].

Actual calculated load from article 220: 15-ampere load (continuous/noncontinuous adjustment is not part of Article 220 calculation).

Minimum conductor size for load:

Temperature correction factor for 75'C insulation in a 115'F ambient is 0.75.

Adjustment factor for seven current carrying-carrying conductors in the raceway is 0.70.

Adjusted ampacity = Table ampacity X 0.75 X 0.70

Minimum adjusted ampacity = 15 amperes from Article 220.

Therefore the Table ampacity X 0.75 X 0.70 must be at least equal to 15 amperes, and

Minimum table ampacity (75'C column before correction or adjustment) =
15 amperes/0.70 X 0.75 = 15 amperes/0.525 = 28.57 amperes.

Minimum conductor size based on conditions of use: 10 AWG THWN [310.15 and Table 310.16].

Overcurrent device: 20 amperes to comply with both the minimum and the maximum sizes determined above.

Conductor size: No. 10 AWG THWN, the largest of the minimum sizes determined above.


Load B Solution:
Load calculated in accordance with Article 220:

Load = 10 X 180 VA per outlet = 1800 VA [220.14(I)]

1800 VA/120 volts = 15-ampere noncontinuous load.

Minimum rating of overcurrent protection and minimum ampacity of conductors for the noncontinuous load:

100 percent X 15 A = 15 - minimum 15-ampere overcurrent device and minimum 15-ampere ampacity [210.19, 210.20].

Based on 60'C terminals, the minimum conductor size is 14 AWG from the 60'C column [110.14(C)].

Minimum overcurrent device ratings and conductor sizes for type of load:

Only 20-ampere overcurrent device may be used for 20-ampere receptacles
[210.21(B)(3)]. (This is both the maximum and minimum rating of the overcurrent device in this example.)

Because 14 AWG must generally be protected at 15 amperes, the minimum conductor size is 12 AWG to coordinate the conductor size with the 20-ampere overcurrent device rating [240.4 and 2404(D)].

Minimum conductor size for load:

THHN conductors are 90'C conductors in dry or damp locations. However, since they are associated with 75'C conductors in the same raceway, they are limited to 75'C operating temperature [310.10].

Temperature correction factor for 75'C insulation in a 115'F ambient is 0.75.

Adjustment factor for seven current-carrying conductors in the raceway is 0.70.

Adjusted ampacity = Table ampacity X 0.75 X 0.70.

Minimum adjusted ampacity = 15 amperes from Article 220.

Therefore, the Table ampacity X 0.70 X 0.75 must be at least equal to 15 amperes, and

Minimum table ampacity (75' column before adjustment or correction) =
15 amperes/0.70 X 0.75 = 15 amperes/0.525 = 28.57 amperes.

Minimum conductor size based on conditions of use: 10 AWG THHN [310.15 and Table 310.16].

Overcurrent device: 20 amperes to comply with all the minimum and maximum sizes determined above.

Conductor size: 10 AWG THHN, the largest of the minimum sizes determined above.

 

[erickench]

IMHO, the "adjustment" factor is applied using the conductor temperature insulation.
If you have 90 deg insulation, the 60 deg column only comes into account for termination as noted in 110.14 and for NM and SE cables (interior) where the Code requires they be terminated using their 60 deg rating.

Augie, I suppose it could be a non-metallic cable though the problem or even the diagram that come's with it does'nt indicate this. NEC 334.80 confirms what you say about using the 60 degree column. But it also state's that the 90 degree column could be used for derating if the final derated ampacity does not exceed that for a 60 degree rated conductor. Maybe it does exceed it so the author uses the 75 degree column instead?

 

[erickench]

Excuse me but I'm thinking out loud.:grin: It can't be because the temperature factor in the 90 degree column is higher than the 75 degree column. That means the final minimum table ampacity would be smaller.:roll:

 

[Dennis Alwon]

The author states

Based on 60'C terminals the minimum conductor size is 14 American Wire Gauge (AWG) from the 60'C column [110.14(C)]. Generally, however, 14 AWG must be protected at 15 amperes, so the minimum size is 12 AWG to coordinate the conductor size with the minimum overcurrent device rating [240.4(D)].

This is line with what Charlie was addressing. We often assume 75C but since no termination ratings are given we must use the 60C column based on 110.14(C). We could use the 75C if that info is known. In the real world today with the new equipment 75C is the status quo.

 

[erickench]

I think Mike Holt would disagree with you. He would say always use the 75 degree column if the temperature is not given. This is what I read in his books.

 

[Dennis Alwon]

I think Mike Holt would disagree with you. He would say always use the 75 degree column if the temperature is not given. This is what I read in his books.

Eric-- how can you use the 75C rating if their is a possibility that some terminals are 60C. I think you read Mike incorrectly unless the load is over 100 amps. In this case 75C is appropriate. 60C for 100 and bellow.

Read 110.14(C).

 

[erickench]

We're talking about the same conductor using two different methods with two different temperatures. Okay, the amps would be less than 100 A as determined from the 100% noncontinuous plus 125% continuous rule. In that case the 75 degree conductor would be used for derating purposes.

 

[Dennis Alwon]

We're talking about the same conductor using two different methods with two different temperatures. Okay, the amps would be less than 100 A as determined from the 100% noncontinuous plus 125% continuous rule. In that case the 75 degree conductor would be used for derating purposes.

Sorry I was not talking about derating but rather the final limit for the circuit. Sure if you have 90C wire you can derate from 90C but if after derating the ampacity is still within the 75C column rating you could not use the 75C column unless you knew the rating of the terminals. If none are given then you must use the 60C for the final wire size.

 

[erickench]

If the circuit rating is calculated by using the 125% continuous plus 100 % noncontinuous rule then you would be able to determine the temperature using NEC 110.14(C)(1). In this case it would be 60 degrees. Would'nt it make sense to go just one step higher to 75 degree conductors instead of two steps to 90 degree conductors?

 

[Dennis Alwon]

If the circuit rating is calculated by using the 125% continuous plus 100 % noncontinuous rule then you would be able to determine the temperature using NEC 110.14(C)(1). In this case it would be 60 degrees. Would'nt it make sense to go just one step higher to 75 degree conductors instead of two steps to 90 degree conductors?

Eric I lost you. What do you mean by go one step higher. Are talking about derating? What type of insulation are we talking about?

 

[erickench]

Look at NEC 110.14(C)(2). It state's:

Conductors with higher temperature ratings, provided the ampacity of such conductors is determined based on the 60'C (140'F) ampacity of the conductor size used.

It's unfortunate that the author did not indicate that 60' degree amapcities had to be used for this particular problem. We had to deduce that ourselves by determining that the current is gonna be less than 100 A.

 

[kwired]

two minimum values are determined for sizing any conductor.

1. Minimum size required for the termination temperature rating.

2. Minimum size required for the insulation temperature rating.


Value # 1 is only adjusted for continuous loading if necessary.

Value # 2 is adjusted for ambient temperature and number of current carrying conductors in same raceway or cable. You start derating at insulation temperature rating regardless of termination temperature rating.

Final conductor size you must use is the larger conductor of the two.

Value # 1 is usually the one you end up using for final conductor size unless Value #2 has enough deration involved that it results in a larger conductor than value #1

 

[kwired]

Look at NEC 110.14(C)(2). It state's:

Conductors with higher temperature ratings, provided the ampacity of such conductors is determined based on the 60'C (140'F) ampacity of the conductor size used.

It's unfortunate that the author did not indicate that 60' degree amapcities had to be used for this particular problem. We had to deduce that ourselves by determining that the current is gonna be less than 100 A.

And in the real world if the temperature rating is not known or marked on the equipment then you must use 60 deg for less than 100 amp circuit and 75 deg for 100+ amp circuits.

If I were taking a test of some type and temperature was not stated I must assume 60 deg. If this were to impact pass/fail of the test I should have the right to justify my calculations, but that is a new topic of discussion.

You can't be wrong from a safety view if you have a larger conductor than required (as long as the terminal will accept the size you try to land on it).