service conductors

[e2me]

OK I have a code question that i cant figure out why the answer is what the book says so I hope you all can help me understand.

A commercial building has a connected load of 395 amps after demands are taken If the service conductors are paralleled in one conduit what is the minimum size conductor permitted.

I say 4/0 As 4/0 is rated 260 amps in 90% ( I'm using this column because I'm doing derating) so X.80 (because of the 6 conductors in one conduit as per 31015b2) X.80=208 and 208x2 since parallel=416 so 416 covers my 400amp service and i felt correct

however the book says 250mcm and uses 255 in the 75% column then derates it x.80 to get 204 then X to get 408 which again will cover the 400 but it says the minimum is there some reason I cant use the 90% column when dong derating for service conductors

 

[Pierre C Belarge]

What conductor type is used in the Handbook?

 

[e2me]

What conductor type is used in the Handbook?

does not say question was as quoted just want min size permitted

 

[iwire]

You can only use the 90 C column for derating if the conductor is rated 90 C.

For example THHN/THWN is rated 90 C dry, but only 75 C Wet as in underground locations.

To further confuse the issue THHN/THWN-2 is rated 90 C wet or dry.

You really have to know the insulation type before you can determine the ampacity.

 

[e2me]

You can only use the 90 C column for derating if the conductor is rated 90 C.

For example THHN/THWN is rated 90 C dry, but only 75 C Wet as in underground locations.

To further confuse the issue THHN/THWN-2 is rated 90 C wet or dry.

You really have to know the insulation type before you can determine the ampacity.

ok i understand that but since the question does not state the wire type and i use thhn then would my answer be correct? as i can use the 90 column for service connductor or any other conductors to do my derating
Yes?

 

[iwire]

ok i understand that but since the question does not state the wire type and i use thhn then would my answer be correct? as

Probably not.

THHN is for use in dry locations, few service conductors are in dry locations.

Your THHN is likely dual listed as THWN but will need to be figured at 75 C.

4/0 CU @ 75 C rated 230 amps * 2 = 460 * .8 = 368 amps

That is 'enough' for a 400 amp breaker but not enough for a load of 395 amps.

On the other hand

250 CU @ 75 C rated 255 amps * 2 = 510 * .8 = 408 amps

Which is enough for the 395 amps of connected load.

Now if the wire type has -2 after the designation than you could use the 90 C column in wet locations.

 

[e2me]

Probably not.

THHN is for use in dry locations, few service conductors are in dry locations.

Your THHN is likely dual listed as THWN but will need to be figured at 75 C.

4/0 CU @ 75 C rated 230 amps * 2 = 460 * .8 = 368 amps

That is 'enough' for a 400 amp breaker but not enough for a load of 395 amps.

On the other hand

250 CU @ 75 C rated 255 amps * 2 = 510 * .8 = 408 amps

Which is enough for the 395 amps of connected load.

Now if the wire type has -2 after the designation than you could use the 90 C column in wet locations.

ok just so im clear if the question was as stated
no wire type given just what is min size can use then if i use thhw or use2 or whatever in 90 column then i can use the smaller wire and my answer was correct
yes as i can always use 90 degree if deratinfg as long as it doesnt excede the 75 column usage when done correct
thanks for your time I test next week and want to be very clear on this matter

 

[George Stolz]

I agree with Bob's math.

I haven't had a test that did not provide the wire type, so I wouldn't worry about it. If in a tight spot like that, I'd do the question both ways, chances are one of the answers would not be present in a multiple choice test.

Which book are you using? Can you write out the question exactly as it is presented, so we can rule out the possibility of a trick question? For instance, it could just be a test to see if you know 310.4, depending on how the question is worded.

 

[e2me]

I agree with Bob's math.

I haven't had a test that did not provide the wire type, so I wouldn't worry about it. If in a tight spot like that, I'd do the question both ways, chances are one of the answers would not be present in a multiple choice test.

Which book are you using? Can you write out the question exactly as it is presented, so we can rule out the possibility of a trick question? For instance, it could just be a test to see if you know 310.4, depending on how the question is worded.

the question is this word per word

A commercial building has a connected load of 395 amps after demands are taken If the service conductors are paralleled in one conduit what is the minimum size conductor permitted.
:-?

 

[RonWhee]

did you forget 125% for cont. load? the 80% would reduce 250 mcm too 204 amps per run does this help? always use 75c column on wire 1ga. or larger lugging is rated 75c read art.110.14 1 b

 

[iwire]

did you forget 125% for cont. load?

I dd not forget, the opening post states

A commercial building has a connected load of 395 amps after demands are taken

If this has been calculated per Article 220 the continuous load is already part of the figure.

always use 75c column on wire 1ga. or larger lugging is rated 75c read art.110.14 1 b

No, we are not limited to 75C by 110.14 for the purpose of derating.

We can use the 90 C column on 12 AWG THHN for the purposes of derating.

 

[RonWhee]

I said larger then 1 ga. how do you feed 395 amps w/12ga.


stay in Maintenance

 

[iwire]

I said larger then 1 ga. how do you feed 395 amps w/12ga.


stay in Maintenance

Stay in Maintenance? :grin:

Well as proud as I am of my maintenance abilities I also know the NEC.

14, 10, 1 AWG, 750 Kcmil, it does not matter, we can use the 90 C column with THHN, XHHW etc for the purposes of derating.

 

[e2me]

did you forget 125% for cont. load? the 80% would reduce 250 mcm too 204 amps per run does this help? always use 75c column on wire 1ga. or larger lugging is rated 75c read art.110.14 1 b

So if a question states the conected load is the 125% not already figured into that load or is it added after and if after how to i know that the whole load is cont and what part can i derate if this is so

 

[infinity]

So if a question states the conected load is the 125% not already figured into that load or is it added after and if after how to i know that the whole load is cont and what part can i derate if this is so

Load calculations used to size services already contain the additional 125% for continous loads.

 

[George Stolz]

the question is this word per word

A commercial building has a connected load of 395 amps after demands are taken If the service conductors are paralleled in one conduit what is the minimum size conductor permitted.
:-?

Considering they haven't given the number of parallel runs, the size of the pipe, the insulation type, etc, then I would be inclined to answer "1/0". I think this is a trick question, and the 395 was to make it trickier.

What answers were provided, this was multiple choice?

If that's not the case, it's a bum question, IMO. Not enough information.