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stdave
New Member
Canada
7 Posts |
 Posted - 03/12/2010 : 10:13:55
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I am wondering about rounding with respect to the Canadian Electrical Code. For instance if I do a calculation for a motor conductor ampacity and find that the current for reference to table 2 is 30.09 amps, is it permitted to use a #10 cable rated for 30 amps? Example 2 - what
about approaching table 13 with a motor ampacity of 29.85 amps - must it be a 25 amp fuse? At what point can you round up and put a 30 amp fuse in? I can't seem to find any reference to that in the CEC - I would think that would be some reference to rounding located in section 0 or 2.
I realize that in practice we can make judgment calls - but my question here is just determine the demand of the code in this respect.
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bytemylobster
Avid Member
Canada
414 Posts |
Posted - 03/12/2010 : 11:55:41
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Not a simple question. What kind of motor is it? Continuous or non-continuous duty?
And for fuse size, #28-200 for branch circuit protection or #28-204 for feeder protection.
If it is a motor, you must use Section 28.
I have a schematic with instructions from SAIT but I don't know how to show it once I have scanned it.
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stdave
New Member
Canada
7 Posts |
Posted - 03/12/2010 : 15:37:47
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Thanks for your response - I know section 28 - my question is not so much about motors - but about the rounding in general - If the code even deals with rounding of small amounts. I just noticed some calculation in a book recently where the current calculation, say for a motor, was 17.1 amps * 1.75 = fuse size - then brought to table 13 = 29.93A
The person in the book used a 30A fuse, however code states that fusing (transformers excepted) is to be taken to the next -lowest- rating, not highest - that lead me to believe there is some allowance for rounding.
I just noticed that the example from the book I was quoting is actually -from- the CEC Handbook, Part 1 2006 - page 353 in the table on the description of 28-200 |
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avoidingwork
New Member
6 Posts |
Posted - 03/12/2010 : 19:53:30
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| I can't remember seeing anything about rounding but rule 8-106 and table 13 might be of interest to you. |
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stdave
New Member
Canada
7 Posts |
Posted - 03/12/2010 : 22:49:58
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I googled around and found one comment on this topic, but it looks like it's from the NEC;
"Refer to 200.2(B) to end the rounding mystery. When the ampere calculation exceeds a whole number by 0.5 or more, round up to the next whole number. If the extra is 0.49 or less, round down to the next whole number. For, example, round 29.5A up to 30A, but round 29.45A down to 29A."
I wonder if the CEC handles it the same way.
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farlsincharge
New Member
6 Posts |
Posted - 03/15/2010 : 00:43:22
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There is no blanket statement in the CEC, it varies from section to section. There are lots of factors that come into play here, including load type, demand factors, and conductor sizing to name a few. In some cases we can go up, and in others we have to go down. There are "5%" rules and next common trade size rules. There are lots of places where it says "may not exceed", and then two subrules later it will allow you to.
My codebook is out in my truck right now, I'll try and cite some specifics tomorrow and maybe address some of the examples you have given. |
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Eddy Current
Active Member
Canada
70 Posts |
Posted - 03/15/2010 : 06:12:49
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CEC 28-200(a) A branch circuit supplying a single motor shall be protected by using an overcurrent device of rating not to exceed the values in Table 29 using the rated full load current of the motor.
I've always been told to round down for motors. Table 29's values are high enough |
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stdave
New Member
Canada
7 Posts |
Posted - 03/15/2010 : 13:41:35
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| Eddy, I wholeheartedly agree with you - my question is only related to the CEC Handbook -itself- which I mentioned earlier - where they round -up- and not down while sizing a fuse for a motor in a sample how-to-do-it table. |
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Eddy Current
Active Member
Canada
70 Posts |
Posted - 03/15/2010 : 15:22:24
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Sorry, I don't have the handbook, only code books.
The code on this is pretty clear, It's been that way as far as I can see.(1994)I find it odd that the handbook shows the opposite. We are supposed to round up everything except motors so maybe it's a typo. What year is your handbook? |
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stdave
New Member
Canada
7 Posts |
Posted - 03/15/2010 : 18:42:02
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2006 - but I also have the CSA computer program to test yourself for the C of Q. Here is one question on the exam - "A three phase 2 HP AC fan motor, 208 volts. What is the maximum size of the 3 pole breaker that can be used?" - their correct answer they put at 20A. I'm guessing there is something wrong there too then. I put that calculation at
7.48 amps * 2.5 (Table 29) equals 18.7 amps = 15 amps (Table 13). |
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farlsincharge
New Member
6 Posts |
Posted - 03/15/2010 : 19:11:21
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| The only thing I can figure with that question is that they are assuming a 15a would not allow it to start. The absolute maximum is 400%. rule 28-200 (d)(iii). It comes out to 29.92. By all rights you are correct in your calculation, it's a bogus question and poorly worded if that is indeed its intent. |
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CEC and rounding current
