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Old 02-13-2012, 07:20 PM   #1
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proper calculation for circuit size


I am about to run wire for 8kw double oven, 120/240 4 wires, 80 feet one way. instruction says 'protection circuit recommended 40A'. However, i want to double check with calculation. not sure which method to use or none at all.

method A
8000 / 240 = 33.33
then
33.33 x 1.25 = 41.66
go for 50 amp breaker???

50 amp breaker x .8 = 40 amp limit.

method B
10% over 8kw
hence
8000 x .1 = 8800
8800 / 240 = 36.67
go for 40 amp breaker???

method C, no calculation needed, variance already built in???

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Old 02-13-2012, 07:47 PM   #2
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proper calculation for circuit size


A range is NOT a continuous load. 40 amps is correct.

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Old 02-13-2012, 07:53 PM   #3
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proper calculation for circuit size


Moi aussi 2X as above not a contionous load at all.

Just wired 8-3 avce 40 amp breaker and be done with it.

Just be aware of junction box location so you will have to check the manufacter specs on location due it will varies a bit so be on safe side check it out.

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Old 02-13-2012, 08:11 PM   #4
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proper calculation for circuit size


It figures. just had a brief look at the definition of continuous load. vague description but thanks anyway.
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Old 02-13-2012, 10:43 PM   #5
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proper calculation for circuit size


Note that ovens cycle on and off to maintain the desired temperature. It is highly unlikely that both ovens would have cycled on and remained on together for a period of more than 3 hours, which is the defnintion of a continuous (as opposed to intermittent) load.
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Old 02-13-2012, 10:48 PM   #6
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proper calculation for circuit size


Being that far of a run I might suggest upgrading the wire size to a 6/3 wg instead of an 8/3 wg ,keeping the same 40a breaker
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Old 02-13-2012, 10:50 PM   #7
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proper calculation for circuit size


Not to hijack his thread, but if you know an appliance will run continuously like an air conditioner in south Texas, should it be considered a continuous load? My two 7.5 ton units run 18 hours per day in our 110 degree heat. They never cycle until we close the door and turn off the lights.

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Old 02-13-2012, 11:01 PM   #8
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proper calculation for circuit size


I dont know if i would consider that a continuous load or a cry for help myself
Have you had them serviced lately to figure out why theyre running so much?
Maybe its time for a replacement/upsizing
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Old 02-13-2012, 11:24 PM   #9
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proper calculation for circuit size


Quote:
Originally Posted by n175h View Post
Not to hijack his thread, but if you know an appliance will run continuously like an air conditioner in south Texas, should it be considered a continuous load? My two 7.5 ton units run 18 hours per day in our 110 degree heat. They never cycle until we close the door and turn off the lights.

David
David.,

I can understand the nature of southern Texas with heat wave it is simauir to our most southen France on tempture wise it is not too far off from there.

But did the HVAC contractor did look at the unit to make sure it is sized properly if sized properly it will cycle from time to time if it was undersized may have to upgrade to twin 10 tonnes units but there is a clevation with it is you will have to check the supply to see if they can handle addtional load for the A/C units.

But I can understand some unit that can run almost contionuns they will design the circuit properly to handle that.

The last 7.5 tonne unit I have see one Trane unit it did have dual compressours so it will adjust the compressour as need to for addtional load to keep it up on hevey demand.

Merci,
Marc
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Old 02-14-2012, 08:17 AM   #10
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proper calculation for circuit size


The 8 gauge copper wire will be fine for the 80' (160' round trip) run in terms of wire resistance (.64 ohms per 1000'). The voltage drop at maximum draw (34 amps) will be about 3-1/2 volts or about 1-1/2 percent of 240 volts.

When sizing the conductors for voltage drop purposes you do not multiply amperes by 1.25 for continuous loads. Then given the conductor size for amperage and, if applicable, continuous load and the conductor size for voltage drop, you choose the larger of the two.

Air conditioners are considered continuous loads. The installation instructions should give a recommended breaker rating. Sometimes there is a maximum allowable breaker rating as well.
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Old 02-14-2012, 11:46 AM   #11
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proper calculation for circuit size


Here is a link for a number of electrical calculators.
http://www.electrician2.com/calculators/elcal.html

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Old 02-14-2012, 11:52 AM   #12
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proper calculation for circuit size


Quote:
Originally Posted by AllanJ View Post
The 8 gauge copper wire will be fine for the 80' (160' round trip) run in terms of wire resistance (.64 ohms per 1000'). The voltage drop at maximum draw (34 amps) will be about 3-1/2 volts or about 1-1/2 percent of 240 volts.

When sizing the conductors for voltage drop purposes you do not multiply amperes by 1.25 for continuous loads. Then given the conductor size for amperage and, if applicable, continuous load and the conductor size for voltage drop, you choose the larger of the two.

Air conditioners are considered continuous loads. The installation instructions should give a recommended breaker rating. Sometimes there is a maximum allowable breaker rating as well.
Im more worried about actual heat generated in the wire over that 80' when youve got a turkey sitting in there for 8 hrs getting ready for thanksgiving/christmas dinner than I am about the voltage drop,although they both have the same basic cause resistance
And the 6/3 wg does cost that much more for peace of mind
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Old 02-14-2012, 12:24 PM   #13
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proper calculation for circuit size


Quote:
Originally Posted by plummen View Post
Im more worried about actual heat generated in the wire over that 80' when youve got a turkey sitting in there for 8 hrs getting ready for thanksgiving/christmas dinner than I am about the voltage drop,although they both have the same basic cause resistance
And the 6/3 wg does cost that much more for peace of mind
+1 on this post!
During my 36 years working for a large electrical utility, I've seen a number of disasters from customer wiring without proper knowledge. If your calculations are in doubt or borderline I always recommend installing the next larger wire size. When cooking a holiday dinner many ranges are operated at or near maximum amperage for many hours which can generate a lot heat in the conductors & connections.

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Old 02-14-2012, 05:18 PM   #14
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proper calculation for circuit size


For a given wire size and a given number of amperes the number of BTU or calories of heat generated per foot of wire is the same regardless of the length of the wire.
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Old 02-14-2012, 05:50 PM   #15
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proper calculation for circuit size


You dont think there would be more resistance /heat built up in that wire over 80' than there would be in a piece of 6/3 going the same distance with the same load on it?
Either way I guess its not a big deal,we're just beating another dead horse here

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