Sub Panel Assistance Request - USA

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  #1  
Old 09-07-07, 11:49 AM
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Question Sub Panel Assistance Request - USA

I am finishing a 2,200 sq ft basement, subdividing it into 8 rooms with 2 halls. I am have run 12-2 and 12-3 w/ground AWG NM-B for all lights and outlets and will connect them to eight 20 amp breakers in the sub panel. I am not wiring any equipment like heating, pumps, or stoves, as this was included in the original 400 amp main panel. I am only wiring lights and outlets at this time. I would like to leave the option expand in the future.

I have a 100 amp double pole sub panel. I plan to pull power from the main panel from a 125 amp double pole breaker. I plan to use 1.5 inch schedule 40 PVC conduit (the same type conduit originally used for the main panel) to run the power from the main panel to the sub panel. The travel distance is about 10 feet with two 90 degree sweeps. As stated above, I will use 8 spaces in the sub panel and will have several available for expansion.

My plan is to use #1 AWG THHN for the 2 hot lines, #3 AWG THHN for the neutral, and #4 AWG bare for the ground. Is this good or excessive? Also, will I have any concerns with pulling these in the 1.5 inch schedule 40. I have never pulled wire of this nature in conduit of this type, so all pointers will be appreciated. Does NEC or anyone else suggest/dictate where in the main panel box I place the 125 amp double pole? Towards the main disconnect or at the opposite end? Any other concerns or suggestions will be appreciated.
 
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  #2  
Old 09-07-07, 11:53 AM
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Your breaker for the main panel is too large. With a 100 amp sub panel, use a 100 amp (or smaller) breaker.
 
  #3  
Old 09-07-07, 04:27 PM
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> I have a 100 amp double pole sub panel. I plan to pull power from the
> main panel from a 125 amp double pole breaker.

You need to either reduce the breaker size to 100A or purchase a 125A (or greater) panel. The subpanel can be "main lug only" MLO type; it doesn't require a main breaker.

> My plan is to use #1 AWG THHN for the 2 hot lines, #3 AWG THHN for
> the neutral, and #4 AWG bare for the ground. Is this good or excessive?

That's fine if you buy a 125A panel. If you drop to a 100A breaker, you can use #3 for the hots and #8 for the ground. I'm not a big fan of reduced neutrals unless you can show that a significant portion of your load is 240V. Given that your run is 10', savings on wire will be minimal, so I would not use a reduced neutral.

> Also, will I have any concerns with pulling these in the 1.5 inch schedule
> 40. I have never pulled wire of this nature in conduit of this type

Shouldn't be a problem. Buy a small bottle of wire lube (like Ideal Clearglide) and have a helper handy. Pulling wires with two people is many times easier than doing it alone. For only 10', you can probably push them through with no actual "pulling" needed.

> Does NEC or anyone else suggest/dictate where in the main panel box
> I place the 125 amp double pole?

Place it so that you maintain adequate bend radius of five times the diameter on the wires. Don't put it so close to the conduit entrance that you need to kink the wires.

An additional tip: you may need to buy a neutral lug kit for your panel; many of the bus bars do not accept conductors larger than #4 without a lug kit. Check the label on the panel door.
 
  #4  
Old 09-10-07, 09:42 AM
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Smile Sub Panel Assistance Request - USA

Thanks to both for your input and suggestions.

I have performed most of my electrical work in other countries. While the same major rules apply overall, most places are different in the details and treat work in their country as if their rules are the only ones that can possibly be right. The information on the 125 amp breaker to the 100 amp sub panel actually came from an inspector in a southern USA city. He required that from the main to the sub panel there must be a cascade down, thus the 125 amp in the main to the 100 amp in the sub. I did not understand all of the logic for it, and I have never been required to do this before, but I am glad to see that it is not required everywhere here. I am not in the same state as that city/inspector, so I am going with 100 amp breaker in the main and the sub panel, with wire sized to match.

All of the information was very helpful. Again, thank you for your help.
 
  #5  
Old 09-10-07, 10:46 AM
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> The information on the 125 amp breaker to the 100 amp sub panel actually
> came from an inspector in a southern USA city. He required that from the
> main to the sub panel there must be a cascade down, thus the 125 amp in
> the main to the 100 amp in the sub.

This is rubbish. What he suggested was unsafe and does not meet NEC or any other code that I am aware of. The maximum rating on the subpanel is just that: a maximum rating; you can use any size breaker 100A or less, but never greater.
 
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