fuses

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surf more

surf more

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stupid --lost again--if someone could just point me in correct direction..i am looking for minimum standard size fuse to protect a conductor 208amps...
300amp fuse???
 
surf more said:
stupid --lost again--if someone could just point me in correct direction..i am looking for minimum standard size fuse to protect a conductor 208amps...
300amp fuse???
Click to expand...

article 240.6 A in the 2008 NEC lists all the standard sizes of fuses and breakers

and article 240.4B is devices rated 800 amps or less

you can use the next higher OCPD if it follows the rules of that article
 
surf more said:
are you multiplying 208amps by 1.25=260and then you would make it 300????feeder
Click to expand...

your thinking about 215.3 to size a feeder you multiply 208 times 1.25 and add that to the noncontinuous

example:
continuous motor load 100 amps
100 x 1.25= 125 amp continuous load

noncontinuous load = 83 amps

125+83= 208

208 amps doesnt go with any standard breaker size in 240.6 so i would protect those conductors with a 225 amp breaker

if its a tap conductor you will have to look at 240.21B and could have different rules that has to be followed

dont forget about conductor ampacity either. if you have a 208 feeder you can use a a 4/0 75 degree wire copper or a 75 degree AL 300 mcm
 
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alright-208-120----the first question was a diagram..find the total load supplying conductor a.
30kw 3 phase con..heater load----30kw into360=83
15kw store lighting 15kw into 120=125
83 +125=208amps
that was the first one and how i came to my answer..


second one--- what is the smallest fuse to protect conductor a in queston #1



just some practice questions


i was thinking maybe 215.3 came into affect?????????reason i could not get correct answer,because next fuse up is 225amp.the answer key says 300amp..so ilooked at 215 andif i multiply by1.25 that gives me 260 and nexthigher is 300amp.am i way off or is that it???
 
surf more said:
alright-208-120----the first question was a diagram..find the total load supplying conductor a.
30kw 3 phase con..heater load----30kw into360=83
15kw store lighting 15kw into 120=125
83 +125=208amps
that was the first one and how i came to my answer..


second one--- what is the smallest fuse to protect conductor a in queston #1



just some practice questions


i was thinking maybe 215.3 came into affect?????????reason i could not get correct answer,because next fuse up is 225amp.the answer key says 300amp..so ilooked at 215 andif i multiply by1.25 that gives me 260 and nexthigher is 300amp.am i way off or is that it???
Click to expand...

is this a service load you are calculating or is it a feeder to a subpanel load?
 
alright-208-120----the first question was a diagram..find the total load supplying conductor a.
30kw 3 phase con..heater load----30kw into360=83
15kw store lighting 15kw into 120=125
83 +125=208amps
that was the first one and how i came to my answer..


second one--- what is the smallest fuse to protect conductor a in queston #1



just some practice questions


i was thinking maybe 215.3 came into affect?????????reason i could not get correct answer,because next fuse up is 225amp.the answer key says 300amp..so ilooked at 215 andif i multiply by1.25 that gives me 260 and nexthigher is 300amp.am i way off or is that it???
 
a diagram of a 4 wire feeder 208-120
phase a b c n
3= 15kw nonlinear lighting load
1 =30kw con...heater load
the question in prev..... post
 
215.3 says

where a feeder supplies continuous loads or any combinations of continuous and noncontinuous loads the rating of the overcurrent device shall not be less than the noncontinous load plus 125% of the continuous load.

that says the breaker cant be smaller than the 208 calculated load but it can be larger

the first part of the article says feeders shall be protected against overcurrent in accordance with the provisions of part 1 of article 240.

that means part 1 of article 240 needs to be in effect for that code to be used. if the OCPD is sized according to the wire size of 310.16 and terminal temp rating of article 11.14 A - C

the conductor i would pick for the 208 amp load would be a 4/0 copper which is good for 230 amps since the next size down would be 200 amp. that feeder can be protected by a 225 amp breaker according to 240.4
 
since you go there there is also a third question.
#3
terminations75c thhn copper conductors
smallest ungrounded conductor in the feeder???

answer key 300kcmil
 
every time i answer these 3 questions,i come up with225amp fuse and 4/0 also but i am boggled about how they come up with 300amp fuse and 300kcmil..i am super curious now!!!!!!!!!!!!
 
surf more said:
since you go there there is also a third question.
#3
terminations75c thhn copper conductors
smallest ungrounded conductor in the feeder???

answer key 300kcmil
Click to expand...

110.14C

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 termination shall be permitted to be used for ampacity adjustment correction or both

you can use the 90c table to adjust the ampacity of the conductor as long as its not over the temperature rating of the termination. 75degree terminations would be 4/0 copper. but the AL 75 degree rating will be 250 mcm
 
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