Tap Question

[deputygrsmith]

I am having a hard time understainding the relationship of sizing up or down. I have a practice question about the 25ft tap rule. What conductor and OCPD should be sized from a 400a OCPD Tap. 1/3 of that would be 133a and would need a 1/0 conductor? The answer book states that 1/0 requires a 125a OCPD, but I am coming up with 150a? Ampacity of 1/0 is 150a, so couldn't I use 150a OCPD? am I wrong?

 

[jwelectric]

400 divided by 3 equals 133.33
133.33 is the largest over current device that is allowed on this tap

240.21 Location in Circuit
(B) Feeder Taps Conductors shall be permitted to be tapped, without overcurrent protection at the tap, to a feeder as specified in 240.21(B)(1) through (B)(5). The provisions of 240.4(B) shall not be permitted for tap conductors.

there is no 133.33 amp device so drop down to the next lowest and it is 125

 

[Pierre C Belarge]

If you are following the '05 NEC, 240.21(B) says
The provisions of 240.4(B) shall not be permitted for tap conductors.

so you will need to round down from 133 amps, not round up.

 

[deputygrsmith]

tap question

tap question

Thank you for the clarification.

 

[deputygrsmith]

One More Question

One More Question

This is where I am getting confused. The previous book question was about a 10 ft. tap with an 400a OCPD. So, I divided by 10 and came up with 40a, which needs #8. The book states that #8 is rated for 50a, so the OCPD would need to be 45a.

In my earlier post, if the 1/0 is rated for 150, but 1/3 of the 400a OCPD is 133a and I need to size down to 125a OCPD.

If the 10ft tap is 40a, why don't I do the same thing and use a 40A or maybe 35a OCPD. The book states is would be a 45a OCPD and I do not understand between the 2 questions why the first question sized down (from 133a) to 125a and this question sizes up (from 40a) to 45a.

 

[infinity]

If you are following the '05 NEC, 240.21(B) says
The provisions of 240.4(B) shall not be permitted for tap conductors.

so you will need to round down from 133 amps, not round up.

If the tap conductors are rated for 150 amps, how does 240.4(B) apply?

 

[deputygrsmith]

I agree with rounding down, but in the second question, wouldn't 40a be the max OC allowed for 400a using the 10ft tap rule? My Master exam book states the OCPD would be #8 using 45a OCPD. So, I don't understand why that is rounding up. What am I missing or is this possibly a typo? The choices were 45, 50, or 60a.

 

[Pierre C Belarge]

Trevor
I believe the OP came up with 1/0. The amperage is 133 which is not a standard size (240.6), because of 240.21(B), he will have to round down to 125.

If he had 1/0 conductors, he could use them, but by 310.16, he could use 1 AWG - with copper prices today........

 

[don_resqcapt19]

wouldn't 40a be the max OC allowed for 400a using the 10ft tap rule? My Master exam book states the OCPD would be #8 using 45a OCPD. So, I don't understand why that is rounding up. What am I missing or is this possibly a typo? The choices were 45, 50, or 60a.

Frist 240.21(B)(1)(4) says that the ampacity of the tap conductor must be at least 10% of the rating of the feeder OCPD. In this case the feeder OCPD is 400 amps so the minimum ampacity of the tap must be 40 amps. The 60?C ampacity of #8 is 40 amps and if you are required to use this ampacity, then 40 is the maximum permitted OCPD for the load end of the tap conductor. If you are permitted, but the equipment listing to use the 75?C ampacity, then you would also be permitted to use a 50 amp OCPD. With the information provided I don't know why the answer key says 45A.

Don

 

[deputygrsmith]

Book Question: What size THWN cooper conductor and OCPD (75 deg.) are required for a tap using the 10ft. rule with the feeder OCD rated at 400a?

Book Answer: #8 and 45a OCPD.

From what I read, would it be correct to say that the 10ft. rule gives you a minimum OCPD and you will size UP and the 25ft. rule gives you a maximum OCPD and you will size DOWN?

Thanks for your help.

 

[deputygrsmith]

What size OCPD is required for a tap, over 10 ft but not more than 25 ft in length, that originates at a junction box supplied by a feeder circuit with a 500A OCPD?

(a) 100A (c) 150A
(b) 125A (d) 175A

(d), 175A. Tap conductors more than 10 ft in length but less than 25 ft do not require an OCPD at the point of the tap. However, you must install an OCPD at the end of the tap; protect the conductor with conduit, tubing, or metal gutter; and properly size it to carry the total load. The ampacity of the tap conductors must also be at least ⅓ the rating of the OCPD.

Step 1: Calculate the minimum size tap per Sec. 240-21(b)(2)(a).
500A OCPD?⅓ = 166.7A

Step 2: According to Table 310-16, 166.7A requires a No. 2/0 cu. conductor.

Solution: Select OCPD per Secs. 240-3(b) and 240-21(b)(2)(a). A No. 2/0 cu. conductor requires a 175A OCPD.

why am I sizing up here?

My first post was 400a X .33333 = 133a and you told me to size down to 125a OCPD? #1/0 requires 150a protection, so why would this not have the same answer if using the same 25ft. tap rule?

I am really getting confused how to calculate all of this correctly. Some questions round up, some round down. I can always get the correct wire size.

[/b]

 

[jwelectric]

What size OCPD is required for a tap, over 10 ft but not more than 25 ft in length, that originates at a junction box supplied by a feeder circuit with a 500A OCPD?

(a) 100A (c) 150A
(b) 125A (d) 175A

Why would you go to 310.16 and look at the conductor. The question is not asking you to size the conductor, it is asking for the overcurrent.

Your math is correct and the maximum size overcurrent is 166.7. there is no overcurrent device listed in 240.6 that matches this so by the rule in 240.21(B) look for the next lowest which is a 150 amp.

 

[don_resqcapt19]

What size OCPD is required for a tap, over 10 ft but not more than 25 ft in length, that originates at a junction box supplied by a feeder circuit with a 500A OCPD?

That is a stupid question. You can't answer the question without knowing the tap conductor size. The code only specifies a minimum size for the tap conductor.
Don

 

[don_resqcapt19]

From what I read, would it be correct to say that the 10ft. rule gives you a minimum OCPD and you will size UP and the 25ft. rule gives you a maximum OCPD and you will size DOWN?

You can never use the next larger size OCPD to protect a tap conductor. It must be protected at or below its ampacity as shown in Table 310.16. The tap rules specifiy a minimum conductor size and the ampacity of the conductor is used to select the OCPD.
Don

 

[infinity]

Your math is correct and the maximum size overcurrent is 166.7.

How can you know this without knowing the size of the tap conductors?

 

[don_resqcapt19]

Book Question: What size THWN cooper conductor and OCPD (75 deg.) are required for a tap using the 10ft. rule with the feeder OCD rated at 400a?

Book Answer: #8 and 45a OCPD.

The minimum conductor size for this tap is a conductor with an ampacity of 40 amps. In this case a #8 THWN was specified with a table ampacity of 50 amps. It is a suitable conductor because it has an ampacity greater than 40 amps. The conductor must be protected at or below its amapcity. Any OCPD with a rating of 50 or less is code compliant for this installation.
Don

 

[deputygrsmith]

Your math is correct and the maximum size overcurrent is 166.7. there is no overcurrent device listed in 240.6 that matches this so by the rule in 240.21(B) look for the next lowest which is a 150 amp.

Ok, you came up with 150a, the last question came from EC Magazine, and they state it would be (d) 175a OCPD, which is sizing up from 166.7a

What is the correct standard. There are differnt answers from different sources.

400a OCPD /10 = 40a, but can be protected by #8 45a OCPD? (sizing up)
400a OCPD X .33333 = 133a, but must be #1/0 125a (size down)
500a OCPD X .33333 = 167a, but must be #2/0 175a (size up)

Some of you said that 50a or less OCPD is code compliant to protect 400a 10ft. OCPD because #8 has an ampacity of 50a. This is sizing up when other posts stated I have to size down.

I am not the only one confused here.

If #1/0 has an 75 deg. ampacity of 150a, Why do I have to size down to 125a OCPD and not be permitted to use 150a OCPD?

Looks like some want to size down once you come up with the ampacity after dividing. This is where I select my wire size based on that ampacity. I.E. 400a OCPD X .33333 = 133a, but must be #1/0

Can someone tell me if what should be used, is the ampacity of the wire you selected (not ampacity from division) and size down to the proper OCPD?

 

[infinity]

If #1/0 has an 75 deg. ampacity of 150a, Why do I have to size down to 125a OCPD and not be permitted to use 150a OCPD?

This sounds code compliant to me. The tap is 1/3 or greater than the ampacity of the feeder OCPD. I am unsure why there is some much confusion here. 400 amps(1/3) = 133.33 amps. The tap conductors can be any size conductors equal to, or larger than those rated for 133.33 amps.

 

[deputygrsmith]

Thanks. I am not having a problem sizing tap conductors. It's the OCPD I am having problems with and getting different answers for.

It makes sense to size down the OCPD to protect the conductor. Once I select the proper wire size, do I take the wire's max ampacity per 310.16 and size the OCPD to either equal or less than it's max ampacity?

ex. 500a(1/3) = 166.7a, size to #2/0, which is rated for 175a at 75deg. Since it is rated for 175a, it matches the standard OCPD of 175a I should be able to use that OCPD correct?

If this is true, then 400a(1/3) = 133a and is a #1/0 which it's max ampacity is 150a and I should be able to use the standard OCPD of 150a? I should not have to use 125a OCPD as the book states, correct?

The book is applying different answers and is not consistent in it's calculations and the inconsistentcy is throwing me off.

Thanks for your help.

 

[infinity]

It makes sense to size down the OCPD to protect the conductor. Once I select the proper wire size, do I take the wire's max ampacity per 310.16 and size the OCPD to either equal or less than it's max ampacity?

Yes, if you so choose.

ex. 500a(1/3) = 166.7a, size to #2/0, which is rated for 175a at 75deg. Since it is rated for 175a, it matches the standard OCPD of 175a I should be able to use that OCPD correct?

Yes. Or you could use a smaller sized standard OCPD.