Wire size

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Old 04-07-09, 09:42 AM
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Wire size

I need to install a 70 amp 240 volt circuit(3 wire + ground) about 70 feet in length. I was looking at table 310.16 and if I read this correctly I can use #6 wire in conduit if I use one of the wire types specified for 194 degrees F(such as thhn). If I use a wire designated 167 degrees F then I need #4 wire? Am I reading this correctly.
 

Last edited by ednu99; 04-07-09 at 11:25 AM.
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Old 04-07-09, 10:07 AM
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Is this an instant water heater or welder the circuit is for? What is the full load amps of the device. What is the size of the main breaker. What other loads do you have? 70amps is a very large load to add to a residential situation. Wire size is only one of the factors that need to be considered. You say three wire plus ground. Is this a sub panel? Most 240v/70a loads would only require 2 wires plus ground.
 

Last edited by ray2047; 04-07-09 at 10:39 AM.
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Old 04-07-09, 10:18 AM
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The 70 amps I quoted was the breaker size not the load. It's for a pc oven and the full load is 12,400 watts or around 52 amps. The neutral is needed to drive a light and small fan. The service panel is rated 200 amps and has the capacity to support this circuit.
 
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Old 04-07-09, 10:42 AM
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Is the fan and light part of the oven? Is this a detached building? Are there any other circuits to the building? A detached building can have only one circuit. If you already have power to the building you are going to need a sub panel. or is the panel in the building and this is commercial?

Just curious is this an oven specifically made for powder coating or a residential bake oven? What are the manufacturers instructions. If this oven is part of a stove and burners won't be used you may not need as much as you think.
 
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Old 04-07-09, 10:56 AM
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Originally Posted by ray2047 View Post
Is the fan and light part of the oven? YES Is this a detached building? Are there any other circuits to the building? A detached building can have only one circuit. If you already have power to the building you are going to need a sub panel. or is the panel in the building and this is commercial? The service panel is in the building, it's not a subpanel.

Just curious is this an oven specifically made for powder coatingYes or a residential bake oven? No What are the manufacturers instructions. Custom made.If this oven is part of a stove and burners won't be used you may not need as much as you think.There are 4 elements rated at 3100 watts each. It will be drawing the 52 amps I mentioned.
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Old 04-07-09, 11:23 AM
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I would suggest #4 THWN but wait for the pros to answer now that we have all the details.
 
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Old 04-07-09, 11:48 AM
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I think the #6 should be okay. Ampacity of #6 is 65A; 52A load * 125% continuous rating factor = 65A. Installing a 240V light and fan motor might be cheaper than pulling the #6 neutral.
 
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Old 04-07-09, 01:15 PM
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If you're going to put this on a 70 amp breaker you will need to run #4's. As for your neutral you can drop down to #6 and your grounding wire can be #8. If you had this on a 60 amp breaker then you could run #6's, #8 for your neutral and #10 for your grounding wire. The way I read this is your full load is 52 amps. You will need to factor in the continuous load factor which is 125% continuous rating factor= 65a. You should put a 70 amp in this set up.

Jim
 
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Old 04-08-09, 02:43 PM
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If the watts are bumped up to 15000 with the amps at 63 and the load factor bringing it to 78.75 amps, using #4 wire and 1 #6 neutral could a 80 or 90 amp breaker be used as long as the THHN wire was rated at 95 amps (90C)?
Thanks,
Grumple
 
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Old 04-08-09, 03:08 PM
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Originally Posted by Grumple View Post
If the watts are bumped up to 15000 with the amps at 63 and the load factor bringing it to 78.75 amps, using #4 wire and 1 #6 neutral could a 80 or 90 amp breaker be used as long as the THHN wire was rated at 95 amps (90C)?
Thanks,
Grumple
The max. breaker you could use is a 80 amp on the #4 wire. You can't go by using the 90c rating on that wire. This is used only for derating a wire.

JimBeer 4U2
 
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Old 04-08-09, 03:31 PM
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Originally Posted by rukkus11 View Post
The max. breaker you could use is a 80 amp on the #4 wire. You can't go by using the 90c rating on that wire. This is used only for derating a wire.

JimBeer 4U2
Jim,
So the circuit as described will work with an 80 amp breaker? Without any changes to wire size?
Thanks,
Grumple
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Old 04-08-09, 03:59 PM
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Originally Posted by Grumple View Post
Jim,
So the circuit as described will work with an 80 amp breaker? Without any changes to wire size?
Thanks,
Grumple
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Yes it will. Remember you only need a #8 for your grounding wire.

Jim

 
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Old 04-08-09, 05:48 PM
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Originally Posted by rukkus11 View Post
Yes it will. Remember you only need a #8 for your grounding wire.

Jim

What table or calculation are you referencing to come up with the 80 amp number? I would just like to understand how you determined this.
 
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Old 04-08-09, 07:04 PM
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Originally Posted by ednu99 View Post
What table or calculation are you referencing to come up with the 80 amp number? I would just like to understand how you determined this.
NEC book-310.16. This is based off the 75c chart. #4 is rated at 85 amps and you are allowed to use this as long as all terminals connections are rated at 75c. Most breakers are 60/75c rated and pretty sure the oven he's wiring up is 75c. Now for some reason if one of his terminals are only rated at 60c then he would have to use the 60c chart. Very hard to explain all of this over this thread and I didn't what to confuse anyone. Hope this may help some towards your question.
Thanks!
Jim

 
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Old 04-09-09, 05:27 AM
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rukkus11,
Ok, I also see that in my Wiring Simplified book. One more question. Is the load factor applied only once for the breaker, or is it applied once for the wire AND once for the breaker? Thats all the questions I have. So if I want to run my pc oven at a max 15000 watts (5-3000 watt elements) plus a 120v light and a small low amp blower, the following will work:

2- #4 THHN wires Black for main loads
1- #6 THHN White for neutral
1- #8 THHN Green for ground

In a 1" conduit

80 amp circuit breaker

I am picking up the wire this morning.
Thanks for all your help,
Grumple
Beer 4U2
 

Last edited by Grumple; 04-09-09 at 05:56 AM.
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Old 04-09-09, 06:32 AM
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Originally Posted by rukkus11 View Post
NEC book-310.16. This is based off the 75c chart. #4 is rated at 85 amps and you are allowed to use this as long as all terminals connections are rated at 75c. Most breakers are 60/75c rated and pretty sure the oven he's wiring up is 75c. Now for some reason if one of his terminals are only rated at 60c then he would have to use the 60c chart. Very hard to explain all of this over this thread and I didn't what to confuse anyone. Hope this may help some towards your question.
Thanks!
Jim

Thanks for the explanation. That was the chart I have been referencing. I was just a little confused by the 80 amp breaker recommendation and not 85.
 
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Old 04-09-09, 08:21 AM
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Ed,
They don't make an 85 amp breaker so I guess that means use an 80 amp as the wire is capable of no more than 85 amps?

Grumple
 
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Old 04-09-09, 10:14 AM
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Originally Posted by Grumple View Post
rukkus11,
Ok, I also see that in my Wiring Simplified book. One more question. Is the load factor applied only once for the breaker, or is it applied once for the wire AND once for the breaker? Thats all the questions I have. So if I want to run my pc oven at a max 15000 watts (5-3000 watt elements) plus a 120v light and a small low amp blower, the following will work:

2- #4 THHN wires Black for main loads
1- #6 THHN White for neutral
1- #8 THHN Green for ground

In a 1" conduit

80 amp circuit breaker

I am picking up the wire this morning.
Thanks for all your help,
Grumple
Beer 4U2
You should be good to go.
Jim
Beer 4U2
 
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Old 04-09-09, 10:53 AM
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Is the oven going to be hardwired or cord and plug connected?
The oven will require a NEC approved disconnecting means.
 
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Old 04-09-09, 11:58 AM
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It will be in the same room as the main disconnect and the breaker for the oven. It is in a workshop. It will be hard wired in conduit.

Grumple
 
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