- Location
- Massachusetts

All i can show you is Table 240.92 B

You have taught me something new, so I thank you.

Now who gets to decide what is and what is not a 'supervised installation'?

You are using an out of date browser. It may not display this or other websites correctly.

You should upgrade or use an alternative browser.

You should upgrade or use an alternative browser.

- Thread starter richwaskowitz
- Start date

- Status
- Not open for further replies.

- Location
- Massachusetts

All i can show you is Table 240.92 B

You have taught me something new, so I thank you.

Now who gets to decide what is and what is not a 'supervised installation'?

- Location
- Eatonton, Georgia

You have taught me something new, so I thank you.

Now who gets to decide what is and what is not a 'supervised installation'?

I hope I didnt get people a little p o I was learning most of this as we went along. I would assume for engineers supervision the responsibility of taps used would have to fall back on the engineer . If there are electrical plans a super on job could prob inforce the plans or if no plans the engineer might have to over see or at least pass on the orders . I dont really know what t should be in in 240.92 B when it came out 2.67 sec and a breaker trips in a fraction sec over the rated amps that seemed ok . Maybe you can find that out. There is another formula I found on the web that insulated copper will stand 1 amp for 5 sec for every 42 cir circular mil. without damage to insulation. have fun with the math , you should get a #10 will carry 247 amp for 5 sec. but they make no mention of what is required seconds a tap must carry a load.

- Location
- Illinois

- Occupation
- retired electrician

As far as using 240.92, it is my opinion, that that section can only be used in supervised industrial occupancies. It is not a general provision that an engineer can use in every occupancy.

- Location
- Eatonton, Georgia

Thanks Don

As far as using 240.92, it is my opinion, that that section can only be used in supervised industrial occupancies. It is not a general provision that an engineer can use in every occupancy.

Thank you very much Table 2 on page 4 is what I tried so hard to find , I think all members should study this to learn what an amazing amount of current a conductor will stand without damage long enough to trip a breaker. Notice table 2 shows even a # 12 would trip the 400 amp , it shows 489 amp for the longest breaker they list at 1/30 cycle. I believe the 1/10 rating of a tap is an approximation leaving plenty of safty margin. I believe the code allows engineers as well as electricians to use the formula in 240.92 b . It says nothing about it must be supervised , the problem is its not worth to challenge an AhJ on it with out an engineer to be responsible. I learned quite a lot since the post started.

- Location
- Illinois

- Occupation
- retired electrician

The rules in Part VIII of Article 240 apply only to "supervised industrial installations" and more specifically only to circuits that supply manufacturing or process control equipment. 240.92 is in Part VIII.... I believe the code allows engineers as well as electricians to use the formula in 240.92 b . It says nothing about it must be supervised , the problem is its not worth to challenge an AhJ on it with out an engineer to be responsible. I learned quite a lot since the post started.

240.90 General

Overcurrent protection in areas of supervised industrial installations shall comply with all of the other applicable provisions of this article, except as provided in Part VIII.The provisions of Part VIII shall be permitted only to apply to those portions of the electrical system in the supervised industrial installation used exclusively for manufacturing or process control activities.

- Location
- Eatonton, Georgia

Thanks again

The rules in Part VIII of Article 240 apply only to "supervised industrial installations" and more specifically only to circuits that supply manufacturing or process control equipment. 240.92 is in Part VIII.

Yea that wording is a little hard for me, dont you think for " supervised industrial installations" they are just refering to a a supervised facility where the installation was done and not refering to when the tap was actually installed ? in other words the area must be supervised where the overcurrent device is located , the code has several places where there are exceptions for supervised installations where the area must be supervised after the installation . Im not familiar with what process control equip would include but manufacturing covers quite a bit of facilities.

I forgot to mention T1 in the formula is 60 c because the max load and breaker is 30 amp. I beleive this install of # 10 was shown on the original electrical plans, if the decision is left to an electrician in the field (including myself) they will go with a # 8 , no ones wants to bother with all this. We are discussing if its a violation or not . I say an electrical engineer will show it is not a violation as long as the disconnect is 30 and not 40. In the formula I = 400a . dont forget it has to be squared and so does the circular mil of 10380. I you dont have calculator with log10 their are charts on the internet , if you get

.1335389 you did it right log10 150+234/60+234 =.1335389

I'm not following why you used 400 amps for "I". "I" is the short-circuit current. The 400 amps is the ampere rating of the fuse for overload conditions, not the short-circuit current. Additionally, I disagree T1 should be 60 degrees C. T1 should be the ambient temperature. For purposes of the following formulas, I'll use an ambient temperature of 35 degrees C (95 degrees F).

With a short-circuit value of 400 amps, the #10 wire is good for 2.320 seconds. However, looking at the time-current curve for a 400 amp Bussmann KTN-R Class RK1 fast-acting fuse, a 400 amp short-circuit current does not intercept the time-current curve for the fuse, meaning it will never trip under a 400 amp short-circuit. The wire would burn up and the fuse would never trip. Bad, for obvious reasons.

If you use a more realistic short-circuit value of say, 4000 amps, the #10 wire would be good for 0.031 seconds while the 400 amp Bussmann KTN-R Class RK1 fast-acting fuse would trip in about 0.050 seconds. Therefore, the wire will burn up before the fuse will trip.

However, if you use a 4000 amp short-circuit value coupled with a #8 wire, the wire would be good for 0.078 seconds while the Bussmann KTN-R Class RK1 fast-acting fuse would trip in about 0.050 seconds. Therefore, the fuse will trip before the wire will burn up and all is good.

The whole point of this dissertation is the OCPD needs to clear the fault before the wire heats up beyond what the results of the calculations in 240.92(B) indicate, otherwise the wire will burn up before the OCPD clears the fault. This is completely dependent upon the time-current curve of the upstream OCPD.

I am also in the camp that the ampacities in the 90 degree C column of 310.16 can not be used, except for calculating deratings, if the terminations on either end of the feeder tap are rated at anything lower than 90 degrees C. If a 90 degree C wire is landed on a 75 degree terminal, the ampacity of that wire is effectively limited to the 75 degree C column of 310.16. It is my understanding that this is for the wire to effectively act as a heat sink, of sorts, for the termination. Per 240.21(B)(4), the tap conductors would be required to have an ampacity of 40 amps and per the 75 degree C column of 310.16, the tap conductors would be required to be #8.

- Location
- Illinois

- Occupation
- retired electrician

It means that you can't use the provisions of Part VIII in a commercial. residential or institutional occupancy. It can only be used in a manufacturing facility and only for manufacturing or process control equipment within that facility. The rule assumes that a supervised industrial occupancy has on site engineering and maintenance people.Yea that wording is a little hard for me, dont you think for " supervised industrial installations" they are just refering to a a supervised facility where the installation was done and not refering to when the tap was actually installed ? in other words the area must be supervised where the overcurrent device is located , the code has several places where there are exceptions for supervised installations where the area must be supervised after the installation . Im not familiar with what process control equip would include but manufacturing covers quite a bit of facilities.

The term needs to be defined as it is used in multiple code articles, but the proposals to define it were rejected.

- Location
- Eatonton, Georgia

I'm not following why you used 400 amps for "I". "I" is the short-circuit current. The 400 amps is the ampere rating of the fuse for overload conditions, not the short-circuit current. Additionally, I disagree T1 should be 60 degrees C. T1 should be the ambient temperature. For purposes of the following formulas, I'll use an ambient temperature of 35 degrees C (95 degrees F).

With a short-circuit value of 400 amps, the #10 wire is good for 2.320 seconds. However, looking at the time-current curve for a 400 amp Bussmann KTN-R Class RK1 fast-acting fuse, a 400 amp short-circuit current does not intercept the time-current curve for the fuse, meaning it will never trip under a 400 amp short-circuit. The wire would burn up and the fuse would never trip. Bad, for obvious reasons.

If you use a more realistic short-circuit value of say, 4000 amps, the #10 wire would be good for 0.031 seconds while the 400 amp Bussmann KTN-R Class RK1 fast-acting fuse would trip in about 0.050 seconds. Therefore, the wire will burn up before the fuse will trip.

However, if you use a 4000 amp short-circuit value coupled with a #8 wire, the wire would be good for 0.078 seconds while the Bussmann KTN-R Class RK1 fast-acting fuse would trip in about 0.050 seconds. Therefore, the fuse will trip before the wire will burn up and all is good.

The whole point of this dissertation is the OCPD needs to clear the fault before the wire heats up beyond what the results of the calculations in 240.92(B) indicate, otherwise the wire will burn up before the OCPD clears the fault. This is completely dependent upon the time-current curve of the upstream OCPD.

I am also in the camp that the ampacities in the 90 degree C column of 310.16 can not be used, except for calculating deratings, if the terminations on either end of the feeder tap are rated at anything lower than 90 degrees C. If a 90 degree C wire is landed on a 75 degree terminal, the ampacity of that wire is effectively limited to the 75 degree C column of 310.16. It is my understanding that this is for the wire to effectively act as a heat sink, of sorts, for the termination. Per 240.21(B)(4), the tap conductors would be required to have an ampacity of 40 amps and per the 75 degree C column of 310.16, the tap conductors would be required to be #8.

You cant just pick 4000 amp , i used the 400 just to see how long the # 10 would carry it because I didnt have a time for the breaker. There is not really a such thing as as a 400 amp short circuit the amps would fly past the 400 as it climbs until something blows or trips, the reading at the top of chart 2 says use 75 degree for the conductor start temp T1 that is the max it is allowed to operate (note I dont say you can use 90 here because the circuit is not allowed to run at that) the conductor doesnt stay at the ambient temp when loaded to max. You can use any wire u want on a 75 deg terminal for short circuit rating because the temp will reach 150c at the max allowed the 90 makes no difference ,you just cant rate a #10 but for a 30 amp running load . in table 2 a # 10 at the .5 sec breaker you mentioned will rise to its rated 777amp at the point the breaker trips. 777 is what you should use not 4000 . The number 10 will take 4000 for much quicker breaker , refer to table 2 . if you put the correct numbers in the answers in formula 240.92 B will correspond to table 2. Table 2 shows a #12 will stand 489 amp at .5 sec and that will trip the 400a breaker also .

Last edited:

- Location
- Eatonton, Georgia

Im sorry I cant spend any more time on this I will leave everyone with the solution to 240.92 B

I = current flow when breaker trips

A= cir mil area of conductor = 10380 for #10

t= seconds breaker is rated to trip=(we will use a .5 sec breaker)

T1 = max allowed conductor temp for circuit load= 75c

T2= max allowed short circuit temp for conductor = 150c for thermoplastic

IxI/AxA x t =.0297 LOG10 T2+234/T1+234

IxI/10380x10380 x .5= .0297 log10 150+234/75+234

IxI/10380x10380 x.5 = .0297 log10 1.2428 (check log10 table for 1.2428 its .0934217)

IxI/10380x10380 x.5 = .0297 x .0934217

IxI= 10380 x 10380 x.5 x .0297 x .093417

I= square root of 603,729

I= 777 amp

Thanks to Don we have a chart to compare, look at table 2 page 4 of his post for #10 with a .5 second trip breaker . the chart shows the same 777 trip amps allowed when the breaker trips. The # 10 also passes the max allowed circuit rating of 30 amp for #10 so the #10 can be used for the tap for manufacturing plant with supervised installation

I = current flow when breaker trips

A= cir mil area of conductor = 10380 for #10

t= seconds breaker is rated to trip=(we will use a .5 sec breaker)

T1 = max allowed conductor temp for circuit load= 75c

T2= max allowed short circuit temp for conductor = 150c for thermoplastic

IxI/AxA x t =.0297 LOG10 T2+234/T1+234

IxI/10380x10380 x .5= .0297 log10 150+234/75+234

IxI/10380x10380 x.5 = .0297 log10 1.2428 (check log10 table for 1.2428 its .0934217)

IxI/10380x10380 x.5 = .0297 x .0934217

IxI= 10380 x 10380 x.5 x .0297 x .093417

I= square root of 603,729

I= 777 amp

Thanks to Don we have a chart to compare, look at table 2 page 4 of his post for #10 with a .5 second trip breaker . the chart shows the same 777 trip amps allowed when the breaker trips. The # 10 also passes the max allowed circuit rating of 30 amp for #10 so the #10 can be used for the tap for manufacturing plant with supervised installation

Last edited:

You cant just pick 4000 amp , i used the 400 just to see how long the # 10 would carry it because I didnt have a time for the breaker. There is not really a such thing as as a 400 amp short circuit the amps would fly past the 400 as it climbs until something blows or trips, the reading at the top of chart 2 says use 75 degree for the conductor start temp T1 that is the max it is allowed to operate (note I dont say you can use 90 here because the circuit is not allowed to run at that) the conductor doesnt stay at the ambient temp when loaded to max. You can use any wire u want on a 75 deg terminal for short circuit rating because the temp will reach 150c at the max allowed the 90 makes no difference ,you just cant rate a #10 but for a 30 amp running load . in table 2 a # 10 at the .5 sec breaker you mentioned will rise to its rated 777amp at the point the breaker trips. 777 is what you should use not 4000 . The number 10 will take 4000 for much quicker breaker , refer to table 2 . if you put the correct numbers in the answers in formula 240.92 B will correspond to table 2. Table 2 shows a #12 will stand 489 amp at .5 sec and that will trip the 400a breaker also .

I understand you can't just pick 4000 amps as the short-circuit current, no more than you can arbitrarily pick 400 amps. The point is "I" is the short-circuit current and that short-circuit current value needs to be calculated. After plugging that value into the formula in 240.92(B), the resultant "t" needs and compared against the clearing time derived from the trip curve of the OCPD to ensure the OCPD will clear the fault before the temperature of the conductor exceeds "T2."

- Location
- Eatonton, Georgia

I understand you can't just pick 4000 amps as the short-circuit current, no more than you can arbitrarily pick 400 amps. The point is "I" is the short-circuit current and that short-circuit current value needs to be calculated. After plugging that value into the formula in 240.92(B), the resultant "t" needs and compared against the clearing time derived from the trip curve of the OCPD to ensure the OCPD will clear the fault before the temperature of the conductor exceeds "T2."

I was doing something else using 400 because I didnt know what t value breaker was being used. 240.92 B can be used that way to see the time before a conductor goes past its 150 max. When u get an answer like over 2 seconds it will definately trip a 400a .5 t breaker. I dont see much wrong with what you said above except I dont know how you calcalated I with out knowing t because I is completely dependant on the t value. Thats why I plug in every thing but I and solve. But you can solve for any thing in the formula if you know all the others, for instance you could plug in everything but T2 and the answer would come out 150 c if you did it correctly. All I can say is if the method you use is correct everything will match table2 pg4 in the post above, that is how the table was made . why not just use table 2 to select the proper tap size , breaker time rating , and see the amps at the trip time.

- Location
- Massachusetts

As long as the breaker is closed the full fault current is on those conductors. That makes the breaker size irreverent.

If the breaker takes 2 seconds to open the conductor will get much hotter 8000 amps of fault current than with 400 amps of fault current for the same amount of time.

But I could be wrong, I am really out of my element.

- Location
- Eatonton, Georgia

thanks iwire

As long as the breaker is closed the full fault current is on those conductors. That makes the breaker size irreverent.

If the breaker takes 2 seconds to open the conductor will get much hotter 8000 amps of fault current than with 400 amps of fault current for the same amount of time.

But I could be wrong, I am really out of my element.

You are correct, and I should never have used the 400 amp it got every one confused . I should have chosen the worse case breaker as a .5 sec and solved the formula like above post. I have just found if you use the breaker rateing for the max short circuit rating , the time will go past the time rating for the breaker if it conductor is large enough . Notice on table2 400 a is past the time scale for #12 and#10 and both will pass for tap but for #14 on a .5 breaker 400 does not pass. It is because 400 is a absolute min point and it has to be exceeded to trip the breaker. It works but is a stupid way for me to use the formula and should be disreguarded. Things are complicated enough already.

- Location
- NE Nebraska

You are correct, and I should never have used the 400 amp it got every one confused . I should have chosen the worse case breaker as a .5 sec and solved the formula like above post. I have just found if you use the breaker rateing for the max short circuit rating , the time will go past the time rating for the breaker if it conductor is large enough . Notice on table2 400 a is past the time scale for #12 and#10 and both will pass for tap but for #14 on a .5 breaker 400 does not pass. It is because 400 is a absolute min point and it has to be exceeded to trip the breaker. It works but is a stupid way for me to use the formula and should be disreguarded. Things are complicated enough already.

Slightly more than 400 amps on a 400 amp breaker is not fault current it is overload current. The topic of discussion is tap conductors. They need to be sized 1. for the load to be carried, and 2. large enough to be able to handle any fault current that may be imposed on them. The larger of 1 or 2 is the minimum size required. If you have 400 amps on a 30 amp tap conductor the overcurrent device on the load side of the tap conductor is the device that should be opening and not the feeder overcurrent device.

- Location
- Eatonton, Georgia

Thanks iwire

As long as the breaker is closed the full fault current is on those conductors. That makes the breaker size irreverent.

If the breaker takes 2 seconds to open the conductor will get much hotter 8000 amps of fault current than with 400 amps of fault current for the same amount of time.

But I could be wrong, I am really out of my element.

Im not exactly in my element either and realizing now some of the replies I made were not accurate. I am starting all over to study fault current and tap conductors more

- Status
- Not open for further replies.