100A sub #4 wire??
 

Hi just paid to have a guy add a 100A sub panel to my house. Just checked his work and I could have sworn that it should be higher guage like #3 or #2 but he used #4 isn't this to small an amperage carrying wire?? Sub is stubbed right next to the main so distance is less than 5 feet from new 100A in main to sub box (box to box).

thanks

Walt

Number 4 CU THHN is rated for 95 amps using the 90 degree column. However you are limited to the temperature rating of the terminations also. You will not find a 90 degree termination is a standard panel. You would need to use the lower rated 60 or 75 degree columns. #4 @ 75 = 85 amps or @ 60 = 70 amps.

Your wire is undersized. Even moreso if they used aluminum.

I remember a discussion long ago about this and do not remember the outcome. If one refers to 310.15(B)(6) it states a feeder does not have to be larger than the service entrance conductors feeding the premises. If one reads the section very strictly, it would apply to just the main feeder from a main disco to the panel it serves. The discussion was basically: why would the sub feeders have to be any larger than the service feeders or main power feeder. If I remember correctly the ultimate decision was: it doesn't. I cannot back that up with authority though but I do believe it was so at the time of the discussion.

So, then if we go to table 310.15(B)(6), we will find that a copper service entrance feeder in a residential 3 wire 120/240 system does not have to be any larger than #4.

So, if the OP's service feeder is #4 copper (and obviously that is not known, here at least) would the sub feeder actually have to be any larger than #4 copper?

thanks Jim Port and nap. Funny thing is I have a 500 foot roll of #4 CU that I showed him saying I have this but I think it is too small and he agreed!! So he left for an hour to get couple things and returned with some gray wire service looking stuff so I left him alone thinking he was OK. Came back with the same #4 size wire and guess he thought his #4 was better than mine??

nap the service entry is aluminum 200A not sure but something like 350 CMM or something like that.

So to be safe? do I need #3 or go to #2. I have some #1 but the guy said it wouldn't fit in the circuit breakers. I thought it was better to remove a strand or two to make it fit than to have these headaches now.

I can use the #4 for the ground right??

thanks

Walt

You shouldn't need to change anything. #4 is good for a 100 amp sub panel all day.

So now I'm more confused than ever! Jim Port seems to suggest that #4 is undersized for a 100A sub panel addition but Wirenut1110 says it's OK.
Can I get a ruling on this. Not looking to start any argumernts just want to know if I HAVE to change it or not?

thanks

Walt

To further the issue I do not know what state the OP is living that will affect the code cycle it will be using.

Really if you are on the recent code cycle either 08 or 11 code cycle then Jim is correct but on older code cycle or area where they don't have recent modern code enforcement it may let it slide { this part I will be carefull with it }

Basically #4 copper if this is a service entrance { for 100 amp breaker }yeah that is common but for feeder part then the bet is off }

With the feeder #4 copper is good for 85 amp @ 75°C { THHN type } so you will need to use But with NM or SE or UF cable then it will even drop down more so that will make the diffrence there.

Merci.
Marc

Wouldn't the addition of a sub-panel require an inspection?

Genrally Oui it will required the inspection of subpanel and majtory of the location will do that.

The subpanel it can be done by DIY if the inspection dept allow homeowner to do it otherwise some area only electrician can do this.

So it will be wise to check it out first.

Merci.
Marc

My 100a sub was inspected & required #3 wire
Funny HD has their wire board marked #4 = 100a :no:

would that be funny like; ha ha or funny like; odd, weird, strange?:)


I have seen the same thing. Don't know why I wasted my breath but I told them there were several ratings that were just plain wrong. Nothing changed.

IF you have a 200 Amp service in the house, and install a 100 Amp sub-panel from it, THEN the minimum wire size allowable would be a #3 copper, or #1 Aluminum.

Table 310.15(B)(6) does not apply here, since the sub-panel does not carry the entire load of said dwelling unit.

You will have to use table 310.16 to calculate wire size, using the 75 degree column, due to temperature limitations of your terminations.

Yeah, and that same board says a #8 is limited to 40 amps. I install #8 THHN all day long using 50 Amp overcurrent protection, and Code compliant.

just to be sure what size breaker is it protected by in the main panel?

a "100a" sub panel can not be serviced by #4 cu under current code! anyone who thinks otherwise please post a code reference.

All I can say is, that the 15 localities I work in and the manufacturers of pre-wired transfer switches must be wrong.

310.15(B)(6) says between the main disconnect and the panelboard that supplies, either by branch circuits or feeders, or both, all loads that are part or associated with the dwelling.