Breaker Size


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Old 08-16-08, 05:39 AM
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Breaker Size

What is the correct breaker size for a 115V circuit using #8 wire?
 
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Old 08-16-08, 06:51 AM
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Depending on the equipment connected it could change the allowable breaker size.

Be advised that general purpose circuits are limited to a 20 amp maximum.

What purpose are you considering this circuit for?
 
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Old 08-16-08, 08:37 AM
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The circuit is for my well pump.
 
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Old 08-16-08, 08:43 AM
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Originally Posted by md2lgyk View Post
The circuit is for my well pump.
what does the nameplate on the well pump say? In other words, whats the HP? You dont always size the breaker by the wire size, with motors, you could easily have a much bigger breaker than wire size.
 
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Old 08-16-08, 08:46 AM
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Is your well pump convertible to 240 volts? If so you would downsize the wire size as the ampacity would be cut in half. this could save you money on the materials.

Generally #8 is good for 40 amps.

Wires can also be up sized to allow for voltage drop due to long circuit distances while the breaker sizes stay the same. When this happens the ground wire frequently needs to be upsized as well.
 
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Old 08-16-08, 11:51 AM
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The pump has a 1.5 HP motor.
 
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Old 08-16-08, 12:31 PM
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Originally Posted by md2lgyk View Post
The pump has a 1.5 HP motor.

Okay, a 1.5 HP motor at 115 volts the Full Load Current (FLC) is 20 amps, (table 430.148 NEC) to figure out short circuit and ground fault protection you take 250% x FLC, so 250% x 20 (FLC) = 50 amps, this is your circuit breaker size.

Now to find out the conductor size you take 125% x FLC, so 125% x 20 (FLC) = 25 amps, so at least #10 AWG is required.


So, for your pump, you need a 50 amp breaker, and a minimum wire size #10 AWG. You already ran # 8, so just install a 50 amp breaker and your all set.
 
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Old 08-16-08, 01:33 PM
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Excellent. Thank you for the information.
 
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Old 08-16-08, 01:41 PM
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There one thing I have little condersaton with well pump motor I know you mention 120 v and how far is the pump is ??


And the other thing you may run into this situation is that you may have hard time to find a single pole 50 amp breaker.

Yeah they are around but if your big box store don't stock this check out the electrical supply centre.

Merci,Marc
 
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Old 08-16-08, 06:26 PM
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The pump is set at 640 feet. I guess that's why it needs such a large motor. And I know what you mean about finding a breaker.
 
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Old 08-16-08, 09:02 PM
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Originally Posted by md2lgyk View Post
The pump is set at 640 feet. I guess that's why it needs such a large motor. And I know what you mean about finding a breaker.
Just use a 2 pole breaker, obviously your only going to use 1 leg, but there is nothing wrong with that.
 
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Old 08-16-08, 10:49 PM
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20% voltage drop is unacceptable at that distance. You will need a #2 copper conductor to deliver motor operating voltage at 640 feet assuming your starting with 120 volts at the panel.

I would refer back to PC BOSS asking if the motor can be wired 240 volts. This would reduce motor amperage to 10 amps. Your #8 would then deliver operating motor voltage.
 
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Old 08-16-08, 10:55 PM
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I ran the figures with my caluator and you will need much larger wire size to handle the voltage drop for this distance

You will end up need #0 or 1/0 cable to keep the voltage drop withen 2.5% range

Was this well motor is submersable or surface mounted motor ?? if latter it will pretty easy to swap over 240 volts

With 240v 1.5 HP motor you will need #6 wire size { that based on 2.5% again }

so there is a big diffrence if you think we are kidding on that just get anlog voltmeter and when the pump is running at the well head you will see how much voltage it will drop.

The bigger the voltage drop the harder the motor have to overcome it and it will automatically draw more current to keep up.

Merci,Marc
 
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Old 08-16-08, 11:52 PM
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Motors are rated at 115/230 volts, typically this is about 5% drop from what the calculators use at 240 volts. If you use 5% drop in the calculator you will stay consistent with the wire size charts for well pump motors specified by the manufacturers.


A 115 volt motor is not acceptable at 640 feet both in cost of wire to deliver rated voltage and design.
 
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Old 08-17-08, 03:39 AM
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Now I'm confused. The pump (a submersible) was selected and installed by a large professional plumbing contractor. They also told me what size of wire to get. I have to think they knew what they were doing.
 
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Old 08-17-08, 07:21 AM
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I think I've found the source of some of the confusion. I pulled out the contractor's quote and got the part number of the pump (Goulds 5SG15422C). According to the specifications from their website, that is a 230V pump, not 115V. So I think there's a mistake in the quote.

Assuming it is indeed a 230V pump, I expect the recommended 50A breaker is too big? Goulds' Motor Application and Installation Data says 30A.
 
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Old 08-17-08, 10:58 AM
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Now if that the case if that is confirmed that motor is a 240 volts.,

Then you have to change the breaker rating down to max of 30 amp { normally i used 20 or 25 amp depending on which manufacter it have }

And the wire you have buried which you stated #8 That will work for that distance { it will be pretty close to the max VD limit I came up 4.1% so you are ok with that }

Merci,Marc
 
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Old 08-17-08, 06:11 PM
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Great. Thanks for the info.
 
 

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