Residential Service Entrance Sizing

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elect007

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I have a question if the following is correct:
1. 80A CB can protect 71 to 80A optional calculated service load wired with #6Cu or #3Al.
2. 90A CB can protect 81 to 90A optional calculated service load wired with #4Cu or #2Al.
3. 100A CB can protect 91 to 100A optional calculated service load wired with #3Cu or #1Al.
 
Re: Residential Service Entrance Sizing

Chances are, they're throwing you a trick question. The right answer for question 1, 2 and 3 is 100 amps, fed with a #2 AL, or #4 CU.

The minimum size for a residential service is 100 amps, per 230.79(C).

The conductors are permitted to be sized per Table 310.15(B)(6).

Hope that helps. :D
 
Re: Residential Service Entrance Sizing

Thank you GS for your reply, I was questioning the sizes in response to a situation in a dwelling unit in a multifamily building: The service is fed from a 100A CB by the meter through a 3 or 4AWG? AL cable to a MLO 125A Panel. They want to add a washer and dryer circuit in the apartment. The STD load calc number is 120A load. The optional load calc is 80amp. If its a 4AWG AL it will not work, but if its a 3AWG AL it should be OK?
 
Re: Residential Service Entrance Sizing

I'd be amazed if the actual connected load in an apartment is 80 amps. How do you come to this number?

What is the rating of the circuit breaker supplying the existing feeder? What size are the conductors, for sure?

If your connected load before the addition is 80 amps, your load after adding a washer and dryer would be over 107 amps. You'd need at least a 110 amp, #1 AL feeder to do the job. If it is going to be 80 amps after the addition, and you have a #3 AL feeder, you're 5 amps short.

But I seriously doubt you're close to 80 amps, unless we're talking about a serious apartment. :D
 
Re: Residential Service Entrance Sizing

The CB supplying the panel is 100A, on the conductors I am not positive, will have to double check in next few days, there was no marking on wires, probably on the cable jacket, but it is in the wall. I got the optional load calc with the dryer load to be 80A: 870SF, range-8200VA, Dryer-5000VA, Dishwsr-1032VA, GarbDispl-800VA, Watr Heater-4500VA, Four Elect heaters-4000VA.
With that 100ACB protecting the feeder it should be #1AL to be compliant, but it looked smaller. Where in the NEC does it allow smaller than 100A service for mutifamily dwelling?
 
Re: Residential Service Entrance Sizing

With that 100ACB protecting the feeder it should be #1AL to be compliant, but it looked smaller.
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If the feeder matches the description of Table 310.15(B)(6), it can be #2 for 100 amps. It has to be single phase, 120/240, and the main feeder for the apartment.

Where in the NEC does it allow smaller than 100A service for mutifamily dwelling?
Click to expand...
Looking closer at the text, 60 amps is the minimum requirement for a multifamily dwelling unit. :D

First off, we should be using 225.39(C). It's a feeder, not a service. :D

So, looking at 225.39(C), it specifies for a "one family dwelling", which by Article 100 is a "building that consists solely of one dwelling unit." That doesn't fit your apartment. So we move onto 225.39(D), which specifies 60 amps minimum for all others.

Wait a minute--who's teaching who here? Until I looked closer, I would have thought an apartment had to be 100 amps. ;)
 
Re: Residential Service Entrance Sizing

georgestolz

Check out the scope of 225.
225.1 Scope.
This article covers requirements for outside branch circuits and feeders run on or between buildings, structures, or poles on the premises; and electric equipment and wiring for the supply of utilization equipment that is located on or attached to the outside of buildings, structures, or poles.

I don?t think that 225 would apply here. Take a look at 215 instead.

ARTICLE 215 Feeders
215.2 Minimum Rating and Size.
(A) Feeders Not More Than 600 Volts.
(3) Individual Dwelling Unit or Mobile Home Conductors. Feeder conductors for individual dwelling units or mobile homes need not be larger than service conductors. Paragraph 310.15(B)(6) shall be permitted to be used for conductor size.

Notice that there is no mention to a single family dwelling here only to an Individual dwelling unit. Could this be a single unit in an apartment complex?
:)
 
Re: Residential Service Entrance Sizing

Good (general) catch, JW. I thought of the "outside" bit on the drive to work this morning.

215.2(A)(3) doesn't do much for us, as the service conductors are generally way bigger than the feeder for the apartment anyway. :D

215.2(A)(1) is all that really applies to this situation. So, the feeder rating only needs to be sized according to that code. ;)

So, if an apartment comes up to 45 amps, I reckon a 50 amp feeder is ok, hum? :eek:
 
Re: Residential Service Entrance Sizing

Originally posted by jwelectric:
Notice that there is no mention to a single family dwelling here only to an Individual dwelling unit. Could this be a single unit in an apartment complex? :)
Click to expand...
I'd say that's right. ;)
 
Re: Residential Service Entrance Sizing

Using 220-31(A) Optional Calc for additional Loads, the apartment load with added washer and
dryer = 72 AMP. The feeder wire from the service 100ACB is #2AL 60 deg C. (ampacity=75Amps). Therefore service is large enough to add washr & dryer. Now only one question: How did the feeder get approved by the LJA on #2 AL being protected by 100ACB? A #2AL at 60degC ampacity is 75Amp, so it should be protected by next size up= 80Amp CB.
 
Re: Residential Service Entrance Sizing

Originally posted by elect007:
Using 220-31(A) Optional Calc for additional Loads, the apartment load with added washer and
dryer = 72 AMP. The feeder wire from the service 100ACB is #2AL 60 deg C. (ampacity=75Amps). Therefore service is large enough to add washr & dryer. Now only one question: How did the feeder get approved by the LJA on #2 AL being protected by 100ACB? A #2AL at 60degC ampacity is 75Amp, so it should be protected by next size up= 80Amp CB.
Click to expand...
Table 310.15(B)(6) Conductor Types and Sizes for 120/240-Volt, 3-Wire, Single-Phase Dwelling Services and Feeders. Conductor
 
Re: Residential Service Entrance Sizing

You see, it's a residence, and as a result, it's automatically "demanded" by that table. Right off the top, you get one or two conductor sizes leeway, because of the erratic, diverse nature of residential loads. ;)
 
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