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Ampacity #198713 02/02/11 01:27 AM
Joined: Feb 2011
Posts: 2
Deakn Offline OP
New Member
im in school right now got a few questions from the CEC.

Minimum size of NMD90 for hot water tank 4500 watts 240v?

Minimum size of Dryer NMD90 for dryer 6000w 240v?

Can you explain to me how to calcuate using the code book? thanks

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Re: Ampacity [Re: Deakn] #198736 02/02/11 06:55 PM
Joined: Jun 2006
Posts: 613
mikesh Offline
Answer 1 = #10 4500/240*1.25 = 23.44 amps. next wire size is 10

second answer same math =31.25 amps so next wire size is #8.

Ok so here is a trick question residential range 12,000 watts at 240 volts.
I'll give you the answer you tell me why.
8000/240 = 33.33 or #8
Hint answer is in section 8

Re: Ampacity [Re: Deakn] #198739 02/02/11 08:29 PM
Joined: Feb 2011
Posts: 2
Deakn Offline OP
New Member
Why do you multiply by 1.25? which rule is that

Re: Ampacity [Re: Deakn] #198746 02/02/11 10:19 PM
Joined: Jul 2007
Posts: 265
wire_twister Offline
In NEC if a load is continuous you must use wire size and overcurrent protection rated for 125% of the nameplate rated ampacity. If I recall a continuous load is one that will be in use for 3 or more hours consecutivly.


Life is tough, Life is tougher when you are stupid
Re: Ampacity [Re: Deakn] #198776 02/03/11 06:18 PM
Joined: Jun 2006
Posts: 613
mikesh Offline


Re: Ampacity [Re: Deakn] #198885 02/07/11 12:26 AM
Joined: Mar 2004
Posts: 947
twh Offline
A continuous load, under 225 amps, is a total of 1 hour in any two hour period, under normal operation. If you normally operate your drier at high heat for more than one hour at a time, it might be continuous. Unless you have a commercial dryer, the element will cycle, and it probably isn't continuous. In my home, the dryer runs for a few hours on the weekend, so the normal operation is off. Since I can select "any two hour period", I'll pick a weekday. It clearly doesn't say "every" two hour period.

Re: Ampacity [Re: Deakn] #199940 03/14/11 09:37 PM
Joined: Mar 2011
Posts: 28
mersadrad Offline
Let me answer

First 12 kw at 6000w
over 12 kw @ 40%
---why did you say 8000/240 am I wrong...I don't have code home but I will deffinetly check tomorow

Other loads over 1500 w are taken at 25%:
Dryer 4 kw@25% 1000 w

“If you don't know where you are going, any road will get you there”.
Re: Ampacity [Re: Deakn] #199984 03/16/11 01:44 PM
Joined: Jun 2006
Posts: 613
mikesh Offline
A 12 KW range in domestic use is considered to have a 8000 watt demand see 8-300. This is why we use 8-3 instead of 6-3 to feed a 12 kw range
8kw=33.33 amps on a 40 amp breaker
12 kw = 50 amps and in a commercial application X 1.25 or 62.5 amps on an 70 amp breaker
The range itself is different too.
A domestic range while capeable of using the full 12 kw with all 4 elements and both oven elements on high it does so for ver brief periods. Domestic ranges are not approved for use in a restaurant even though they often try to use them since they are 1/4 the cost.

The commercial range is rated for use the whole day, every day.


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