A one-family dwelling is supplied with a 120/240-volt, single-phase electrical system from the local utility company and has a demand load of 200 amps

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solar man

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A one-family dwelling is supplied with a 120/240-volt, single-phase electrical system from the local utility company and has a demand load of 200 amperes. What is the MINIMUM size THWN/THHN aluminum service-entrance conductors required?

In this question - I understand the following -

200 times .83 % - = 166 Amps

What I don't understand is which column to use between 75 deg and 90 deg.

What is the significance of THWN/THHN? Does this mean use either or?

If so - then I would get 3/0 AWG aluminum with a rating of 175 amps ( 90 deg column)

But not sure why the answer is 4/0 AWG with a rating of 180 amps (75 deg column)
 
I gather this is a test question.

Well first of all, look at the wire types at the top of the table and note which column has types with a "-2" at the end and which doesn't. From this, you can figure out what the question is telling you about the wire temperature rating and therefore which column to use.

In real life, you will most often have 90C rated wire but only 75C terminals, which per 110.14(C) means you still have to use the 75 column as a minimum, although you can use 90C for other calcs like ambient temp derating.
 
solar man said:
A one-family dwelling is supplied with a 120/240-volt, single-phase electrical system from the local utility company and has a demand load of 200 amperes. What is the MINIMUM size THWN/THHN aluminum service-entrance conductors required?

In this question - I understand the following -

200 times .83 % - = 166 Amps
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I would do the opposite if the actual demand is 200a; 1.2/1.25 x 200 is 240/250a.

solar man said:
What I don't understand is which column to use between 75 deg and 90 deg.
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To make it simple, always use 75 deg.

solar man said:
What is the significance of THWN/THHN? Does this mean use either or?
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Most wire now is dual-rated; the W means wet, the second H means hotter.

solar man said:
If so - then I would get 3/0 AWG aluminum with a rating of 175 amps ( 90 deg column)
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Not for a 200a demand!

solar man said:
But not sure why the answer is 4/0 AWG with a rating of 180 amps (75 deg column)
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That has to do with available breaker ratings and load diversity.
 
jaggedben said:
I gather this is a test question.

Well first of all, look at the wire types at the top of the table and note which column has types with a "-2" at the end and which doesn't. From this, you can figure out what the question is telling you about the wire temperature rating and therefore which column to use.

In real life, you will most often have 90C rated wire but only 75C terminals, which per 110.14(C) means you still have to use the 75 column as a minimum, although you can use 90C for other calcs like ambient temp derating.
Click to expand...

Yes this is a test question -

so I am using NEC 2017 - Column 2 has "-2" so then how do I know to use the 75C column?
 
LarryFine said:
I would do the opposite if the actual demand is 200a; 1.2/1.25 x 200 is 240/250a.


To make it simple, always use 75 deg.


Most wire now is dual-rated; the W means wet, the second H means hotter.


Not for a 200a demand!


That has to do with available breaker ratings and load diversity.
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this is a test question - so if W means wet rated, how do we know to use the 75 C column?
 
There are two factors, the minimum conductor ampacity which is 166 amps and the minimum conductor size which the answer is #4/0. The conductors are sized according to their 75°C ampacity becuse the terminals that the #4/0's are connected to are rated for 75°C. Therefore you cannot use the 90° C ampacity of the #3/0 which is below the minimum conductor size of 166 amps.
 
See the table in Annex D, Example D7. This is the table that was found in previous editions of the code, before they put in the 83% rule. They put in the 83% rule, because the table in Example D7, and the tables in the previous editions of the code can only be used where there are no required ampacity adjustments or ampacity corrections.
The table was returned to code text in the 2020 code as Table 310.12. There is a table note that says: "Note: If no adjustment or correction factors are required, this table shall be permitted to be applied." It important to understand that table notes, unlike informational notes, are enforceable code language.
 
solar man said:
this is a test question - so if W means wet rated, how do we know to use the 75 C column?
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Because presumably your service conductors are outside and therefore in a wet location (see 300.9 and/or 300.5(B)). So you must use the wet location rating of the conductors (see 310.10(C)). So the THHN rating is irrelevant, you must use the THWN rating. Which is a 75C rating without the "-2".

(I don't love the 'presumably' there, but for service conductors it's almost always going to apply in real life. For a feeder I think they would have to make it less ambiguous.)
 
solar man said:
A one-family dwelling is supplied with a 120/240-volt, single-phase electrical system from the local utility company and has a demand load of 200 amperes. What is the MINIMUM size THWN/THHN aluminum service-entrance conductors required?
Click to expand...
If that is exact wording of the question it is poorly written question.

What is demand load of 200 amps? Could be a short time peak demand that generally isn't seen very often. That can and does happen to a service or feeder that is sized per art 220 but the art 220 load calculation itself is likely well below 200.

Art 220 service or feeder calculation of 200 amps is just that, 200 amps. All demand factors including continuous load adders are included in the figure. No need to add another 25% to this figure. Also for a single family dwelling we have 310.15(B)(7) (since you mentioned 2017 NEC) which lets us use alternate sizing of 83% of normal conductor ampacity - which gives us 2/0 copper or 4/0 aluminum being what is acceptable for a 200 amp service/feeder for a single family dwelling.

If the actual load calculation is 200 amp many likely would consider going even higher with their actual installation - most likely would install a 400 amp supply so there is room for future additions. There is possibility of going 225, 250, 300, 350 but other than 225 which doesn't give you much additional capacity the others are not so commonly used and will cost about the same as going with 400 anyway.
 
solar man said:
this is a test question - so if W means wet rated, how do we know to use the 75 C column?
Click to expand...
All equipment you will find over 100 amp rating and is under 1000 volts will have 75C terminals. Go back to equipment under 100 amps and over about 40-50 years old it may have 60C terminals. Over 100A was still 75C terminals back then. NEC mentions this in 110.14.
 
kwired said:
If that is exact wording of the question it is poorly written question.

What is demand load of 200 amps? Could be a short time peak demand that generally isn't seen very often. That can and does happen to a service or feeder that is sized per art 220 but the art 220 load calculation itself is likely well below 200.

Art 220 service or feeder calculation of 200 amps is just that, 200 amps. All demand factors including continuous load adders are included in the figure. No need to add another 25% to this figure. Also for a single family dwelling we have 310.15(B)(7) (since you mentioned 2017 NEC) which lets us use alternate sizing of 83% of normal conductor ampacity - which gives us 2/0 copper or 4/0 aluminum being what is acceptable for a 200 amp service/feeder for a single family dwelling.

If the actual load calculation is 200 amp many likely would consider going even higher with their actual installation - most likely would install a 400 amp supply so there is room for future additions. There is possibility of going 225, 250, 300, 350 but other than 225 which doesn't give you much additional capacity the others are not so commonly used and will cost about the same as going with 400 anyway.
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Demand load is a term commonly applied to the results of the Article 220 load calculation.
 
don_resqcapt19 said:
Demand load is a term commonly applied to the results of the Article 220 load calculation.
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IIRC NEC doesn't use that terminology though it's just called load calculations or calculated load though those probably aren't specifically defined in art 100 either. The word demand might appear associated with specific loads or situations but not necessarily for a net calculated load for a feeder or service calculation.

I just hate it when NEC related testing questions won't use NEC terminology when asking the question. Leaves the door open for misunderstanding IMO and makes you wonder about the qualifications of those that wrote the test.

The word demand tends to come up more often when dealing with a POCO?
 
kwired said:
IIRC NEC doesn't use that terminology though it's just called load calculations or calculated load though those probably aren't specifically defined in art 100 either. The word demand might appear associated with specific loads or situations but not necessarily for a net calculated load for a feeder or service calculation.

I just hate it when NEC related testing questions won't use NEC terminology when asking the question. Leaves the door open for misunderstanding IMO and makes you wonder about the qualifications of those that wrote the test.

The word demand tends to come up more often when dealing with a POCO?
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I agree, poorly worded question. I immediately was like WTF is "demand load" when I read the question
 
electrofelon said:
I agree, poorly worded question. I immediately was like WTF is "demand load" when I read the question
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NEC isn't simple to learn. Is somewhat easier if you learn terminology and style of the NEC. Those taking licensing exams are still somewhat new. Last thing they need is misleading information because the exam writer doesn't use NEC terminology and NEC style in the questions. Yet we get it time and time again here where someone post a thread asking about a test question and the question itself is poorly written.

Thing is the exam writers probably get very little feed back as those taking the exam likely trust they simply don't fully understand and then go asking questions about it later, maybe to a site like this but nobody ever tells the exam writer his question needs some tweaking.
 
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