sub panel wire size

[wireman3736]

In a dwelling I believe a #2AL feed to a sub panel needs to be protected by a 90 amp breaker max after a 200 amp main breaker panel. I also believe if the main breaker panel is supplied by a #2 and protected by a 100 amp breaker then a sub panel feed from this with #2AL can also be protected by a 100 amp breaker, I cite 215.2 (A) (3). opinions if this is correct, Thanks.

 

[iwire]

I don't see what 215.2(A)(3) has to do with it.

310.16 lists 2 AL at 90 amps and as there is a standard size 90 amp breaker 90 amps is the largest OCP you can use.

 

[suemarkp]

Doesn't that seem kind of stupid? If #2 AL is OK for the whole house, why wouldn't #2 AL be OK for a feeder that could be 0 to 100% as loaded as those #2 service conductors?

How about 215.2(A)(4). Notice this says feeders FOR and not TO. Is a feeder within a dwelling FOR a dwelling?

 

[M. D.]

Why Would table 310.15(B)(6) not be applicable in this case ?
100 amp ocpd for this #2AL feeder in a dwelling.

 

[M. D.]

?

How about 215.2(A)(4). Notice this says feeders FOR and not TO. Is a feeder within a dwelling FOR a dwelling?

What else would it be for??? The car-wash next door?

My 05 book does not have a 215(A)(4).

 

[iwire]

Why Would table 310.15(B)(6) not be applicable in this case ?
100 amp ocpd for this #2AL feeder in a dwelling.

It might but this..

after a 200 amp main breaker panel.

....leads me to believe that feeder supplying the sub panel is not the 'main power feeder' for the dwelling unit.

However I find 310.15(B)(6) to be less than clear....

 

[wireman3736]

I don't see what 215.2(A)(3) has to do with it.

310.16 lists 2 AL at 90 amps and as there is a standard size 90 amp breaker 90 amps is the largest OCP you can use.

215.(A) (3) says they need not be larger then the service conductors,


QUOTE=suemarkp
Doesn't that seem kind of stupid? If #2 AL is OK for the whole house, why wouldn't #2 AL be OK for a feeder that could be 0 to 100% as loaded as those #2 service conductors?
How about 215.2(A)(4). Notice this says feeders FOR and not TO. Is a feeder within a dwelling FOR a dwelling?



I didn't think it was that stupid of a question, I got two answers,
By the way Whats 215.2(A) (4)

 

[bikeindy]

My feeling on this is that they realize most services are oversized for what the acctual load will be. on the other hand if I have a 200 amp service panel and am feeding a sup panel in the house it is probable that I may have close to a full load on that sub panel and those wires are run in the structure and not in conduit from the panel to the meterbase.

 

[M. D.]

My feeling on this is that they realize most services are oversized for what the acctual load will be. on the other hand if I have a 200 amp service panel and am feeding a sup panel in the house it is probable that I may have close to a full load on that sub panel and those wires are run in the structure and not in conduit from the panel to the meterbase.

I don't understand this at all

 

[LarryFine]

Another factor: the 100a-protected #2al main service is not as likely to damage a house as a 100a-protected #2al sub-panel feeder is, due to the difference between the two cables' locations. (My opinion only.)

 

[wireman3736]

Another factor: the 100a-protected #2al main service is not as likely to damage a house as a 100a-protected #2al sub-panel feeder is, due to the difference between the two cables' locations. (My opinion only.)

again though 215.2(A)(3)says they need not be larger then the service conductors,

 

[iwire]

I don't understand this at all

I do, and I agree.

Once we size a service per the NEC for an entire dwelling unit the NEC has a real good idea what the actual load and diversity will be. (The loading will be much less than the calculated load)

In that case using table 310.15(B)(6) makes sense.

When a sub panel is added the NEC has no idea the conditions of loading.

Say a home has a few central AC units added and the electrician decides to add a sub panel to supply all those loads. He keeps the feeder to this sub as cheap as possible.

Now you have a panel that will often see max loading with little diversity on hot days.

In this case sizing the conductor per 310.16 would make sense.

 

[bikeindy]

again though 215.2(A)(3)says they need not be larger then the service conductors,

The OP stated that the service panel was 200 Amp main breaker . while I agree that it is only being fed with a #2 AL protected with a 100 Amp breaker this could easily be upgraded to 200 Amps and the you would have to do one of two things replace your feed to the sub or be in violation. I tend to look at future situations when I wire something. just use #1 AL be done with it.

 

[M. D.]

However I find 310.15(B)(6) to be less than clear....

It is muddy to be sure, but they do include "individual dwelling units of one family...." and then ,..

"For application of this section the main power feeder shall be the feeders between the main disconnect and the lighting and appliance branch circuit panelboards."

This is what we have, is it not???

 

[bikeindy]

I do, and I agree.

Once we size a service per the NEC for an entire dwelling unit the NEC has a real good idea what the actual load and diversity will be. (The loading will be much less than the calculated load)

In that case using table 310.15(B)(6) makes sense.

When a sub panel is added the NEC has no idea the conditions of loading.

Say a home has a few central AC units added and the electrician decides to add a sub panel to supply all those loads. He keeps the feeder to this sub as cheap as possible.

Now you have a panel that will often see max loading with little diversity on hot days.

In this case sizing the conductor per 310.16 would make sense.

This is what I was trying to say in an earlier post with less words. Thanks Iwire

 

[bikeindy]

It is muddy to be sure, but they do include "individual dwelling units of one family...." and then ,..

"For application of this section the main power feeder shall be the feeders between the main disconnect and the lighting and appliance branch circuit panelboards."

This is what we have, is it not???

No I don't think so. I think there is a 100 Amp main disconnect feeding a 200 Amp Panel board for lighting and apliance circuits and we are adding a sub panel to that. I don't think it is covered by 310.15 (b)6

 

[iwire]

This is what we have, is it not???

I am not sure.

What I thought when I read the post was a 200 amp 30 circuit panel supplying a bunch of branch circuits and the sub panel.

IMO if this is the case than the feeder to the subpanel is not the "main power feeder" it is something less than the "Main power feeder"

However if for instance the "200 amp panel" is really just a service disconnect outside with no other branch circuits leaving it then in that case the feeder would be a "main power feeder" for a dwelling unit.

It's just my opinion I don't have any code references to give, take it for what it is worth. There are certainly other opinions on this section of code.

 

[M. D.]

Most of the sub ,appliance and lighting panels , I have installed in dwellings of the individual dwelling type , have been for convenience. The circuits have the same diversity of load as if they were fed from the main panel way in basement.

Why would we not assume you loaded the hell out of the 200 amp panel.

 

[bikeindy]

Most of the sub ,appliance and lighting panels , I have installed in dwellings of the individual dwelling type , have been for convenience. The circuits have the same diversity of load as if they were fed from the main panel way in basement.

Why would we not assume you loaded the hell out of the 200 amp panel.

You might and I have seen the results of that on 100 Amp services. it was a good thing that #4 copper was in conduit.

 

[iwire]

Most of the sub ,appliance and lighting panels , I have installed in dwellings of the individual dwelling type , have been for convenience.

The circuits have the same diversity of load as if they were fed from the main panel way in basement.

No they would not be the same.

Did you also feed ranges, dryers, HVAC etc. from these panels?

But your right they might end up the same but you and the NEC can't guess that.

If the sub gets added for a specific purpose as in my HVAC example it could have little diversity.

Why would we not assume you loaded the hell out of the 200 amp panel.

Because we can't.

Article 220 when applied to entire dwelling unit prohibits that.

What does main power feeder mean to you?

I am not busting your chops it is a term they used but did not explain so IMO it is unclear what the heck it means.