Here's the problem: I have a 400 ampere feeder (Fused at 400 amperew) and I'm taping to feed a 40 ampere load using a 60 ampere saftey switch and installing 40 ampere fuses to protect my 40 ampere load conductors. What size wire should I use from the feeder tap to the 60 ampere disconnect? (Using THHN in a damp location).

Forgot to tell you the tap is 8 feet long.

I would say #8

George you ask some interesting questions.

Assuming a lot here, but if all the requirements of 240.21(B)(1) are met I agree with Ryan 8 AWG copper.

Now one for you, or anyone, what size grounding conductor is needed from the point of the tap to the 60 amp disconnect?

Bob

Yes, but the question remains.

The tap conductors are the conductors feeding the equipment and so the EGC is #8. Table 250.122 requires a #3 EGC for a 400 amp OCPD, but #3 is larger than the conductors feeding the equipment and the EGC is not required to be larger than the conductors feeding the equipment.
Don

IMO a lot of people would size the conductor I am asking about based on the 40 amp fuse which would be 10 AWG copper which will be correct on the load side of that 60 amp disconnect.

I wanted to point out that the EGC on the supply side of the 60 amp disconnect will need to be based on the 400 OCP.

That would normally be a 3 AWG but for this application, will only need to be 8 AWG as the supply conductors are only 8 AWG.

I thought Georges example was perfect for pointing all this out, only a little off topic.

Bob

FWIW I would wire this disconnect up with 6 AWG, the length is short the difference in price and difficulty will not be a big deal.

Besides all I do is order the stuff, I do not pay for it.

JMO, not required, Bob

I agree with Iwire
#6 for the full load amps of the 60/as ( in the future you could change the load without rewire the whole mess )
#10 ground
all wire thwn ( damp location )

As long as the total wire lenth of the tap is less then 10'

dfe

How do you come up with a #10 EGC?

Curt

How did I come up with a #10 EGC?

Jump ahead of myself and made a mistake. Good thing there are sharp people around that catch mistakes like that. I would like to change that to a #2 ( #3 is ok but lots of short lengths of #2 in the shop)

Thanks Curt
P.S. Keep a eye on me I do that now and then.

dfe

Bob posted this question to test everyone here and expected to get some wrong answers.

The equipment ground should be sized from Table 250.122 using the rating of the overcurrent protection which in this case is 400 amps and would require a #3 cu conductor. As Don and Bob pointed out your equipment ground doesn’t need to be larger than your ungrounded conductors so in this case a #8 cu conductor could be used. If you decided to use #6 cu conductors for the tap then you would need to use a #6 EGC.

I hope you and everyone else here are having a great weekend!

Curt

P.S Can we call you by a name other than dfe?

Thank you Curt.

Bob

bob [ Iwire ]


tell me if i get this question right about the tap rules here the max length for the tap is 25 feet aka 8 meter length rules too.

but what happend if the tap do run at 33 feet [ 10 meter ] length then the rules do change too ??

i just want to clear up this question before it get out of the hand here because i did see few tap do run more than 25 feet [7.5 meter] but i was not sure if that is the case the conductors have to be same as ocpd amprage rating or larger like main cables ??

please let me know because i dont have 02 code book with me right now all i have is 96 codebook with me it say on there on art 210-19 and 364-12 but if that did change please do correct me on this one.

merci , marc

As to my orginal post, IMHO my answer agrees with Bob (IWIRE). If we assume 75 degree terminations, which is what we see most of the time, the wire feeding the device, which is the 60 ampere disconnect switch, would have to be #6 valued at 65 amperes. If we look at 240.21(B)(1)b., it says the tap conductor can not be "less than the rating of the device supplied by the tap conductors". Since we are feeding a 60 ampere device (see Definitions), the conductors must be at least 60 ampere capacity. If we were feeding a MLO panel that did not require a main, for example a 100 ampere MLO, the tap conductors would have to be rated at 100 amperes. If we were feeding a 40 ampere inline fuse or a circuit breaker the tap conductors could be rated 40 amperes. As for the equipment ground wire Don has made it clear and I agree that the EGC does not have to be larger than the ungrounded conductor. 250.122(A). These conductor sizes are based on the 10 foot tap rule covered under II Location 240.21

The 40 amp fuse is a device also.

240.21(B)(1)(1)(b)


IMO even with 60 C terminations 8 AWG is code compliant.

My own choice would be at 6 AWG for this.

[This message has been edited by iwire (edited 05-30-2004).]

Yes the 40 ampere fuse is a device but you are not feeding it directly. You are feeding the 60 ampere switch which has the potential of having a 60 ampere fuses (device) installed in it. If we wee feeding a circuit breaker and terminating our tap on the circuit breaker, we would size oru tap to the circuit breaker. As to the ampere rating of the device terminations, 110.14 would have us using the 60 degree column for equipment 100 ampere or less unless identified for higher temperature rating. If we are using 90° rated conductors the ampacity must be selected from the 60° column when 60° terminations are used.

George as I said I like the idea of sizing the wire to the disconnect but don't you think the use of the word "or" in 240.21(B)(1)(1)(b) allows us to size to the fuse?

This all seems pointless when we talk about 40 amp fuses in a 60 amp disconnect. 6 or 8 AWG is almost no cost difference, but change the load to 410 amps, you would need a 600 amp disconnect switch, the cost of providing 600 amps to the switch instead of 410 amps will be significant.

Is it your interpretation that the tap conductors to a 600 amp fused disconnect would need to be say 2 sets of 350s (620 Amps) instead of one set of 600s (420 Amps)?

I do not agree, IMO the use of "or" in 240.21(B)(1)(1)(b) allows us to size to fuse or breaker ratings.

If the NFPA meant it only to apply to breakers I feel they would have used the word 'breaker' in place of 'overcurrent protective device' in 240.21(B)(1)(1)(b).

JMO and I am interested in other peoples opinions too.

Bob

[This message has been edited by iwire (edited 05-30-2004).]

Marc


I think you are swapping standard for metric measurements.

The lengths of taps that I know of are taps not over 3 m (10 ft) Long and taps not over 7.5 m (25 ft) Long.


If you did see a tap over 25' IMO it is not a tap, it is be a feeder and has to be protected at the supply end not at the load end of the conductors.

There are times outside for transformer secondary conductors you can tap longer than 25' feet. Service conductors come to mind.

Bob

Bob, there is one condition for an indoor tap exceeding 25' for a high bay building you forgot about that one. Of course this tap has the "qualified persons" caveat. Back to our 10' tap rule: If I'm wrong on this one, I would like to know. My understanding is based on information from Jim Pauley of Square D. I think that what he taught that day made sense. So if you are installing a tap that requires a 600 amp switch, and the load is only 410 amps, I guess I'd put in an 11 foot tap :-) Maybe some of our more learned monitors and book writers will express their opinions. Good to spar with you Bob.

Another thought: For our 10' tap rule, we are not required to fuse it in any way, we are only asked to size our tap conductors to minimum 1/10 the size of the overcurrent device on the feeder and to be adaquate size for the load and have the ampacity of the device they terminate at. If this device is a MLO panel, or a safety switch, makes no difference. If it is a CB or a MLO makes no difference. Not required to terminate in a single set of fuses or a CB. When we move to the 25' rule we must terminate in a CB or single set of fuses.

George


Thank you and I feel the same way.

How about looking at this thread and giving your opinion.


https://www.electrical-contractor.net/ubb/Forum2/HTML/001514.html

Bob [ Iwire]:

thanks for getting me straght with me with the tap rules here. i am accommodated [ adapted] with metric mesurement for long time.

and one more question it came up in my mind here about the transformer tap here


quote:
There are times outside for transformer secondary conductors you can tap longer than 25' feet. Service conductors come to mind.

sure bob, but if the tap is super long run from the transformer to the ocpd like 100 feet [33 meter] away from transformer you have to put a ocpd at the transformer or not bear in mind this is a secondary side.

merci, marc

One can install breakers/fuses smaller in ampacity than the rated enclosure they are installed into, as long as the breaker/fuse is listed ('fits') for the enclosure.
So a 40 amp breaker/fuse can be installed in a 60 amp rated enclosure fed by 8 AWG. See Art 240 and Art 408.
What some may want to do (possibly exceeding code requirements) is up to the individual, but code is minimum and an inspector cannot ask for anything more.
Some of us have installed work a certain way as a practice, and when we see a 'code' minimum installation we may cringe, but it is permitted none the less.

Pierre