two family dwelling service

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Stevenfyeager

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United States, Indiana
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electrical contractor
It's been a while since I have installed a multi dwelling service. For a two family apartment overhead service, having two separate 100 amp single phase services, I see in 310.16 I need AL 250 wire, is that correct? (Neutral 3/0) It's an old house made into two apartments and the outside cable jacket is worn and the inspector is asking me to replace it. It presently has 4/0,4/0,2/0 AL exposed cable. Thank you.
 
Dennis Alwon said:
How did you come up with 3/0 for the neutral
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I didnt do the math on this one however I seem to recall a rule/code for downsizing the neutral two wire sizes of the ungrounded conductors for services? and service entry cable comes that way?

On an aside, that jacket is just a cover and not insulation; its deterioration alone wouldnt necessitate replacement.
 
Stevenfyeager said:
It's been a while since I have installed a multi dwelling service. For a two family apartment overhead service, having two separate 100 amp single phase services, I see in 310.16 I need AL 250 wire, is that correct? (Neutral 3/0) It's an old house made into two apartments and the outside cable jacket is worn and the inspector is asking me to replace it. It presently has 4/0,4/0,2/0 AL exposed cable. Thank you.
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Sounds like you will have ONE service, possibly with two sets of service entrance conductors if you use 230.40 exception #1 (service conductors running to each unit). Or, if you have discos at the meters, you will still have ONE service. As mentioned, you get the next size up rule so 4/0 works. Also note since you have one set of SEC feeding more than one disconnect, you get to size by the load, not the sum of the disconnects 230.90 Exception #3. Around here, I would likely just use 4/0 SEU - could go smaller if they are all gas, but the cost difference is so small its not worth the few minutes it takes to think about it ;)

JFletcher said:
I didnt do the math on this one however I seem to recall a rule/code for downsizing the neutral two wire sizes of the ungrounded conductors for services? and service entry cable comes that way?

On an aside, that jacket is just a cover and not insulation; its deterioration alone wouldnt necessitate replacement.
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I think that neutral two size down thing is just a common practice. Technically you need the load calc to give the neutral size, but pretty much everyone accepts the two size down for resi.
 
electrofelon said:
Sounds like you will have ONE service, possibly with two sets of service entrance conductors if you use 230.40 exception #1 (service conductors running to each unit). Or, if you have discos at the meters, you will still have ONE service. As mentioned, you get the next size up rule so 4/0 works. Also note since you have one set of SEC feeding more than one disconnect, you get to size by the load, not the sum of the disconnects 230.90 Exception #3. Around here, I would likely just use 4/0 SEU - could go smaller if they are all gas, but the cost difference is so small its not worth the few minutes it takes to think about it ;)



I think that neutral two size down thing is just a common practice. Technically you need the load calc to give the neutral size, but pretty much everyone accepts the two size down for resi.
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I am with you here. NEC has no rule that says one or two sizes smaller is acceptable on neutral, many times noboby will ask for calculations either though if you don't go any more then one or two sizes smaller.

One service with multiple disconnects - that common service conductor only needs sized per load calculation. If his two apartments only had load calculations of 75 amps each - he could use a 150 amp common supply conductor and is NEC compliant even though there is two 100 amp service disconnecting means supplied.

Load calculations of 75 amps each could possibly still be subjected to demand factors that will lessen the common service/feeder to less then 150 amps also.
 
kwired said:
I am with you here. NEC has no rule that says one or two sizes smaller is acceptable on neutral, many times noboby will ask for calculations either though if you don't go any more then one or two sizes smaller.
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Yeah the two size down thing is so common, that every URD/RHH-2 type conductor assembly I have ever seen has a two size down neuter.
 
kwired said:
sometimes same size as other conductors.

I have used a lot of 4-4-4-4 for irrigation installations.
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The op is talking about a dwelling.

The 2 size thing is exactly what I was expecting you to say.:D The point is there is nothing in the nec that supports that theory hoever, it usually is not an issue. In fact, if the load allows you can size the neutral as small as the grounding electrode conductor.
 
Dennis Alwon said:
The op is talking about a dwelling.

The 2 size thing is exactly what I was expecting you to say.:D The point is there is nothing in the nec that supports that theory hoever, it usually is not an issue. In fact, if the load allows you can size the neutral as small as the grounding electrode conductor.
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I never said anything about NEC having the 2 size thing, it only needs sized to load it will carry but no smaller then GEC.

I did say many inspectors will let you get away with two sizes smaller without asking about neutral load calculation, 99% of the time two sizes smaller will probably meet and even exceed calculations for 120/240 single phase applications.

I'd probably run 2 AWG aluminum for service neutral for most any 120/240 volt 200 amp single phase service/feeder if inspector wouldn't question it - such applications very seldom even see enough unbalance neutral load to be too much for 2 AWG, but if I run 2/0 with 4/0 ungrounded conductors, I won't be questioned at all.
 
kwired said:
I never said anything about NEC having the 2 size thing, it only needs sized to load it will carry but no smaller then GEC.

I did say many inspectors will let you get away with two sizes smaller without asking about neutral load calculation, 99% of the time two sizes smaller will probably meet and even exceed calculations for 120/240 single phase applications.

I'd probably run 2 AWG aluminum for service neutral for most any 120/240 volt 200 amp single phase service/feeder if inspector wouldn't question it - such applications very seldom even see enough unbalance neutral load to be too much for 2 AWG, but if I run 2/0 with 4/0 ungrounded conductors, I won't be questioned at all.
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I'd dare say it impossible w/o intentionally selecting most/all the 1p breakers on a single leg and loading them to a test condition that would never exist in the real world. Take a 200A 42 space panel wired with 4/0-4/0-2/0. 8 2p breakers leaves 26 spaces left, 13 per leg. Even if one leg somehow got all the receptacle loads, you'd pretty much need to plug in 8 1500W space heaters, all your 120V kitchen appliances, a vacuum cleaner, hair dryer, clothes and curling irons, washing machine, sump pumps, etc. etc. and run them simultaneously to have a shot of overloading the 2/0 noodle. and if any of those loads wound up on the other leg, of course the current thru the neutral would be lower.
 
JFletcher said:
I'd dare say it impossible w/o intentionally selecting most/all the 1p breakers on a single leg and loading them to a test condition that would never exist in the real world. Take a 200A 42 space panel wired with 4/0-4/0-2/0. 8 2p breakers leaves 26 spaces left, 13 per leg. Even if one leg somehow got all the receptacle loads, you'd pretty much need to plug in 8 1500W space heaters, all your 120V kitchen appliances, a vacuum cleaner, hair dryer, clothes and curling irons, washing machine, sump pumps, etc. etc. and run them simultaneously to have a shot of overloading the 2/0 noodle. and if any of those loads wound up on the other leg, of course the current thru the neutral would be lower.
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:thumbsup::thumbsup:

I doubt I ever clamped a 120/240 single phase service/feeder neutral at a single family dwelling that was drawing over 40-50 amps.

Dwellings that do need higher capacity supply is typically because of 240 volt loads they have such as electric heating or many tons of air conditioning
 
JFletcher said:
I'd dare say it impossible w/o intentionally selecting most/all the 1p breakers on a single leg and loading them to a test condition that would never exist in the real world. Take a 200A 42 space panel wired with 4/0-4/0-2/0. 8 2p breakers leaves 26 spaces left, 13 per leg. Even if one leg somehow got all the receptacle loads, you'd pretty much need to plug in 8 1500W space heaters, all your 120V kitchen appliances, a vacuum cleaner, hair dryer, clothes and curling irons, washing machine, sump pumps, etc. etc. and run them simultaneously to have a shot of overloading the 2/0 noodle. and if any of those loads wound up on the other leg, of course the current thru the neutral would be lower.
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Which is prolly true; however, if one is asked what size does a 200A resi noodle need to be?

The answer is:

A. Per 220.61
B. 2 sizes smaller than the hots
C. Whatever I had over from the last job.
D. I just ask the supply house dude.

Only one answer is always gonna be correct:D.
 
jumper said:
Which is prolly true; however, if one is asked what size does a 200A resi noodle need to be?

The answer is:

A. Per 220.61
B. 2 sizes smaller than the hots
C. Whatever I had over from the last job.
D. I just ask the supply house dude.

Only one answer is always gonna be correct:D.
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Um, F? :D

Obviously A tho I do wonder how/why cable mfg came up with the 'two sizes down' rule of thumb for neutrals in SE cable assemblies.
 
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