120 volt only sub-panel (no 240)

[SteveM]

I?ve just replaced my 2 conductor electric range cable (# 8 Al, 50 amp breaker) with a three conductor cable (#8 Al). I moved the old cable over by three studs and dropped it down to my basement and pulled the other end out of the main breaker panel. The basement end is temporarily stapled to a beam about 8 inches from a 100-amp sub-panel.

I would like to use the old 2-conductor cable as a feed for a 50-amp sub-panel mounted next to the 100-amp sub-panel. The neutral wire is not insulated in the old cable so I can?t use that wire as a neutral any longer. I think I have two choices.

[I?m aware that I need to insulate the neutral buss from the grounding buss.]

1. A 240 volt only (no 120 volt) sub-panel, only slightly useful to me.
2. A 120 volt only (no 240 volt) sub-panel, very useful to me.

If I went the 240vac only method, the breaker in the main panel would be a 50-amp, two pole breaker feeding the two insulated conductors, with the un-insulated wire used for grounding (not normally carrying current). Two twenty-amp, 2 pole breakers would be used in the new sub that would have no neutral buss.

If I went the 120vac only method, the breaker in the main panel would be a 50-amp, single pole breaker feeding one insulated conductor. The other insulated conductor would be connected to the neutral/grounding buss, as would the un-insulated wire. In the sub, the neutral buss would be insulated from the grounding buss. I?d like to supply 3 to 6 circuits, 12-gauge 2+g cable (have it) 15 amp breakers and 15 amp outlets and some lighting.

Are one or both methods up to code requirement?

Thanks and sorry for long post.

Steve

...

 

[dirkhats]

Re: 120 volt only sub-panel (no 240)

Make the sub-feed circuit breaker 40 amp (See Table 310-16, 75 degree column for the old cable)and you may be ok.
Better yet, throw the old cable away, use 8-3wg cu and do it right!

[ August 21, 2005, 03:16 AM: Message edited by: dirkhats ]

 

[jeff43222]

Re: 120 volt only sub-panel (no 240)

I'm also curious as to why you need to use the old cable at all. Since the subpanel is going to be located next to the main panel, you won't need too much wire for a feeder. I'd just buy some nice, shiny new #8 Cu THHN for a 40A 240V feeder and connect the two panels with a rigid offset nipple (which almost guarantees the location of the knockouts won't matter). The cost of the offset nipple plus the wire wouldn't be all that high, probably under $10.

Doing it this way will allow your new 3-6 circuits to be properly balanced, rather than running them all off the same leg.

[ August 21, 2005, 05:37 AM: Message edited by: jeff43222 ]

 

[iwire]

Re: 120 volt only sub-panel (no 240)

Originally posted by dirkhats:
Make the sub-feed circuit breaker 40 amp (See Table 310-16, 75 degree column for the old cable)and you may be ok.

Actually if this is any year NM or old SE / BX cable you would have to use the 60 C column.

That being the case 8 AWG aluminum is rated 30 Amps.

I see that you are an EE.

So a few questions.

1)Are you sure it is Al and not tin plated copper?

2)How old would you say the cable is?

3)Are you sure it is 8 AWG, that is to small for the 50 amp OCP that was feeding it.

4)What kind of cable is it?

My suggestion is this, get an electrician involved, run a new cable, don't be a penny pincher when it comes to wiring.

 

[infinity]

Re: 120 volt only sub-panel (no 240)

I'm assuming that your 100 amp subpanel is not where the existing feeder for your range is terminated. It originates at the service and you want to add a new subpanel adjacent to an existing 100 amp sub.

As Bob has said you need to first determine the type of old cable that you actually have. Sounds like tin coated RH copper cable. After you determine that, than you can properly size your OCPD at the service. Either of the scenarios that you've presented would be code compliant providing the OCPD were sized properly.

 

[SteveM]

Re: 120 volt only sub-panel (no 240)

Thanks for the replies. I screwed up using #8 Al; I have to do it all over again with copper. I?ll use the new Al cable for the sub-panel with a 40-amp breaker and run another new cable (#8 copper) for the range. Arrrgh!

Thanks - Steve

 

[jwelectric]

Re: 120 volt only sub-panel (no 240)

Originally posted by SteveM:
Thanks for the replies. I screwed up using #8 Al; I have to do it all over again with copper. I?ll use the new Al cable for the sub-panel with a 40-amp breaker and run another new cable (#8 copper) for the range. Arrrgh!

Thanks - Steve

WHY?

210.19(A)
(3) Household Ranges and Cooking Appliances. Branch-circuit conductors supplying household ranges, wall-mounted ovens, counter-mounted cooking units, and other household cooking appliances shall have an ampacity not less than the rating of the branch circuit and not less than the maximum load to be served. For ranges of 8? kW or more rating, the minimum branch-circuit rating shall be 40 amperes.

 

[iwire]

Re: 120 volt only sub-panel (no 240)

Originally posted by jwelectric:
WHY?

210.19(A)
(3) Household Ranges and Cooking Appliances. Branch-circuit conductors supplying household ranges, wall-mounted ovens, counter-mounted cooking units, and other household cooking appliances shall have an ampacity not less than the rating of the branch circuit and not less than the maximum load to be served. For ranges of 8? kW or more rating, the minimum branch-circuit rating shall be 40 amperes.

Why?

8 AWG AL is rated 30 amps @ 60 C.

If this circuit is NM cable 334.80 requires us to use the 60 C column.

If this circuit is SE cable 338.10(B)(4)(a) requires us to use parts I and II of 334 which includes 334.80s 60 C requirement.

 

[charlie b]

Re: 120 volt only sub-panel (no 240)

I am closing this thread.

With all due respect to the "Retired EE," but speaking as an engineer I must say that engineers are not electricians. Forum rules do not allow us to offer advice or assistance on performing electrical installation work at your own home. Our concern is that you may get only the answer to the question that you ask, and get no answer to the thousand other questions that you should have asked, but did not know that you needed to ask.