Sub panel and 2-2-4-6 Aluminum cable

[dm9289]

Details 200 amp aerial service a 90 amp breaker feeding garage subpanel approximately 100ft UG in PVC piping. The path is from house basement panel out of building and buried about 80 ft to attached garage outdoor routing is due to many finishes in this split level home. This garage in future may or may not have a hot tub or split AC unit, garage lighting plugs currently fed from 200amp panel 20 amp breaker from original construction. Size is 2 car garage

Was going to use 90amp breaker in main to feed sub panel. Supply house said 2-2-4-6 mobil home cable is appropriate. (What they stocked)

Checked into #6 ground according to 250.122 table this is fine

For neutral I'm a bit confused 220.61 max load neutral to ungrounded conductor determines size.
So i guess you could have an odd situation that all loads would be on "A" phase and you could put 90 amp on Neutral so 2-2-2-6 would be required.

Permitted reduction seems only to be with cooking and dryers

Am I correct in thinking 2-2-2-6 is the necessary cable for this application

 

[infinity]

Why would you assume that the neutral may carry 90 amps? If the loads are balanced on both of the ungrounded legs then the reduced size neutral is not an issue.

 

[dm9289]

Why would you assume that the neutral may carry 90 amps? If the loads are balanced on both of the ungrounded legs then the reduced size neutral is not an issue.

I think it is very unlikely it would carry the 90 amps but i could not find a reference in code allowing me to do this. 220.61(B)1 was the only permitted reductions i know off.

Also thank you for your reply it is greatly appreciated

 

[winnie]

In general large loads (hot tub, EV charger, most split AC units) are 240V L-L. It is very unlikely that any reasonable install in a residence on a 90A feeder would have only L-N loads. 2-2-4-6 is almost certainly fine, as would be 2-2-6-6 Al. But you can only 'really know' if you do a proper load calc.

So do your load calc to figure your ungrounded conductors. Then do a load calc figuring only L-N loads. Half of these get installed on each 'side'. Figure your worst case scenario is only one side 'on'. This worst case is the minimum ampacity required for the neutral. My bet: the minimum ampacity is smaller than the required EGC, and your neutral must be at least as large as the EGC.

But if everything installed is L-N loads, and your load calc comes to 90A using L-N loads only, then the worst case is that the neutral carries 90A and you would need 2-2-2-6.

-Jon

 

[infinity]

If you performed a load calculation for the feeder it would tell you the minimum neutral size.

 

[dm9289]

In general large loads (hot tub, EV charger, most split AC units) are 240V L-L. It is very unlikely that any reasonable install in a residence on a 90A feeder would have only L-N loads. 2-2-4-6 is almost certainly fine, as would be 2-2-6-6 Al. But you can only 'really know' if you do a proper load calc.

-Jon

Thank you for your reply the individual wants a sub panel but is not aware of equipment yet. Mentioned chances of hot tub are slim, chances of mini split for room above garage 50/50. He is a factory coworker and he asked what to install and it gave me a chance to calculate and brush up on code. I think 2-2-4-6 realistically is fine and i think many would not give it a second thought. Im trying to improve my code skills and i just cant find a code ref that makes 2-2-4-6 ok. Im sure its something im missing.

Thank you for your reply you guys help me learn

 

[LarryFine]

I agree that the 2-2-4-6 should be adequate, as all L-L loads can be ignored, but I believe you can also find 2-2-2-4 if you're still concerned. Make sure it's also rated for interior use.

 

[dm9289]

If you performed a load calculation for the feeder it would tell you the minimum neutral size.

This is an odd situation with unknowns so i cant see how i could reduce it. Even though a 240v hot tub or mini split in reality will reduce neutral load they do not appear as allowed reductions, or even 1 breaker with current draw on "B" phase i cannot make this correct code wise in my knowledge

 

[dm9289]

In general large loads (hot tub, EV charger, most split AC units) are 240V L-L. It is very unlikely that any reasonable install in a residence on a 90A feeder would have only L-N loads. 2-2-4-6 is almost certainly fine, as would be 2-2-6-6 Al. But you can only 'really know' if you do a proper load calc.

So do your load calc to figure your ungrounded conductors. Then do a load calc figuring only L-N loads. Half of these get installed on each 'side'. Figure your worst case scenario is only one side 'on'. This worst case is the minimum ampacity required for the neutral. My bet: the minimum ampacity is smaller than the required EGC, and your neutral must be at least as large as the EGC.

But if everything installed is L-N loads, and your load calc comes to 90A using L-N loads only, then the worst case is that the neutral carries 90A and you would need 2-2-2-6.

-Jon

Yes this appears to be an exercise in reality vs code. Im trying to get better and your input helps

 

[dm9289]

I agree that the 2-2-4-6 should be adequate, as all L-L loads can be ignored, but I believe you can also find 2-2-2-4 if you're still concerned. Make sure it's also rated for interior use.

Can you point me to where it states this in code i could omly find permitted reductions in 220.61(B)
Thank you for your input im trying to get better at this and feedback helps

 

[david luchini]

Can you point me to where it states this in code i could omly find permitted reductions in 220.61(B)
Thank you for your input im trying to get better at this and feedback helps

220.61(A)

 

[dm9289]

220.61(A)

Thats where i have been looking seems like only cooking units and dryers. I do know this should not be a problem, long as something is on both phases. I cant find anything on load balancing that would make it OK. The condition where i could see a safety issue is if someone purposely put all the load on one phase.

 

[david luchini]

Thats where i have been looking seems like only cooking units and dryers.

220.61(A) doesn't say anything about cooking units and dryers.

 

[infinity]

Thats where i have been looking seems like only cooking units and dryers. I do know this should not be a problem, long as something is on both phases. I cant find anything on load balancing that would make it OK. The condition where i could see a safety issue is if someone purposely put all the load on one phase.

Let's face it no one is going to put all of the load on one phase. And if they do then the problem is with their installation not yours.

 

[dm9289]

Let's face it no one is going to put all of the load on one phase. And if they do then the problem is with their installation not yours.

Agreed

 

[dm9289]

220.61(A) doesn't say anything about cooking units and dryers.

Sorry about that, to me although very unlikely the max unbalanced load could be 90 amps all loads on one phase.

 

[wwhitney]

Sorry about that, to me although very unlikely the max unbalanced load could be 90 amps all loads on one phase.

I read 220.61(A) as referring to the actual distribution of the L-N loads among the various ungrounded conductors. So that's what matters. If you provide a 65A neutral conductor, then each ungrounded conductor is limited to 65A of calculated load of L-N loads.

But if you are installing this panel for future use, with no loads at present, there's no limitation at this time, beyond the requirement that if you do provide a neutral, it needs to be at least as large as the minimum allowed EGC. If you wanted, you could install a panel with no neutral conductor, for 240V only loads.

So if you install an empty panel supplied with 90A ungrounded conductors and a 65A neutral, it is incumbent upon whoever is installing the loads to recognize that and make sure that (a) the calculated load on each ungrounded conductor does not exceed 90A and (b) the calculated load from L-N loads on each ungrounded conductor is limited to 65A.

Cheers, Wayne

 

[winnie]

Sorry about that, to me although very unlikely the max unbalanced load could be 90 amps all loads on one phase.

If you truly do not know what will be connected to this feeder, then you are correct. Perhaps the home owner will install a set of 120V bitcoin rigs, and then only operate half of them at a time.

Absent any other instructions, (for example the 70% demand reduction for ranges or electric dryers), then you need to do the article 220 calculation for the loads connected to the neutral. 220.61(B)2 gives a demand reduction factor for the neutral when the feeder is larger than 200A, but that demand factor doesn't apply.

The only reduction that you can apply on a 90A feeder to a garage is eliminating any loads not actually connected to the neutral. All you really have is the experience of the members here who say that it is very unlikely that any residential use of this 90A circuit won't have some significant L-L loads.

-Jon

 

[dm9289]

I read 220.61(A) as referring to the actual distribution of the L-N loads among the various ungrounded conductors. So that's what matters. If you provide a 65A neutral conductor, then each ungrounded conductor is limited to 65A of calculated load of L-N loads.

But if you are installing this panel for future use, with no loads at present, there's no limitation at this time, beyond the requirement that if you do provide a neutral, it needs to be at least as large as the minimum allowed EGC. If you wanted, you could install a panel with no neutral conductor, for 240V only loads.

So if you install an empty panel supplied with 90A ungrounded conductors and a 65A neutral, it is incumbent upon whoever is installing the loads to recognize that and make sure that (a) the calculated load on each ungrounded conductor does not exceed 90A and (b) the calculated load from L-N loads on each ungrounded conductor is limited to 65A.

Cheers, Wayne

I agree with you in actual but in doing examples in Annex D D1a D2a D2b it seems the calculate like all 120v circuits can be on one phase. With Ranges and dryers at 70% for neutral deduction. In D2b they added the AC units 10080va and used the number in the neutral contribution

 

[infinity]

Also it's not only L-L loads, equal 120 volt loads on each ungrounded conductor will cancel the neutral load. At 120 volts, 75 amps on each ungrounded conductor will yield a 0 amp neutral load