Mobile Home Service Conductors

[david]

The way I would inspect this job is as follows. If the SE is 2/0 ungrounded conductors and a number 1 grounded conductor, then I would require the same in the feeder circuit along with a number 6 green for the equipment grounding conductor. The feeder can also be sized so as not to e xceed the ampacity of 310.15(B)(7) 1 & 2.

notto be nit picky though i think it is an important distinction it says (1) or (2) not (1) and (2)

 

[david]

Strange rule, strange language, left to different interpretations. I just can’t get past the language of ”entire load”. That’s what I’ll be hanging my hat on.

I understand your hung up on the phrase,
but once again that is not mentioned in (3)

and if the service conductors are sized to handle the entire load any feeder sized equally can handle that same load or any load less than the service

 

[Greentagger]

Thanks , you’ve made a nice case but not convinced. It sez what it sez.

 

[JMCDONALD70]

notto be nit picky though i think it is an important distinction it says (1) or (2) not (1) and (2)

Agree, it should be 1 or 2.

 

[Fred B]

So by the sounds of the debate, it amounts to what my father would often say "Because I said so."
It seems to me the SE would need to be larger not the feeder if it was sized to a given amperage the house is still getting the same demand whether by feeder or SE, in fact the feeder would be in a better position to handle the load as it would be protected unlike the SE.

 

[Fred B]

Just looked at 310.15(B)(7). That looks to me that if other loads are in the first panel, that would increase the total load and would require a larger SE rather than the feeder to the home. It seems to be implied by #1 that the entire load for for a dwelling unit would have to be provided by SE to qualify for 83% rule on the SE, the added loads of AC and Well pump to the service panel then fails that clause. #2 of that section, where it states "...... ,or the feeder conductor supplying the entire load associated with with an individual dwelling unit .......". Seems to allow for the 83% rule to apply to feeders as it indicates it applies to 2 or more seperate feeders to individual units.
Also this entire section is relating to the "dwelling units" when it specifying the 83% rule. Specific Loads when moved to, or on the service panel are not part of the "dwelling unit". I can see if this panel is used to provide power to more than 1 panel in an individual dwelling unit, that then would also fail section #2 criteria.
So if moving loads (water pump, storage building, [AC?]) outside dwelling unit's panel after the fact, it would then cause a violation related to load calculation for the SE, as these loads no longer fall under the 83% rule, but not necessarily that of the feeder of the dwelling unit.
Or am I way off base as to the rationale of this section and the Why?

 

[augie47]

The issue has been debated for years. Goes back to a time when the Table 310.15(B)(7) was in the Code and it gave a blanket wire size without taking into consideration insulation ratings or ambient adjustments. The 83% rule seemingly corrected that but the fact that you reduced the overall load on the feeder by relocating some loads to the service requires increasing the feeder size due to loss of diversity even if required defies logic hence the problem.

 

[Buck Parrish]

Strange rule, strange language, left to different interpretations. I just can’t get past the language of ”entire load”. That’s what I’ll be hanging my hat on.

An example ..." Thousands of EC's would wire large houses with a 200 amp 12 space panel outside. They would put six 2 pole breakers in it. One 90 amp for #2 AL SER to 30/40 space panel in the house,. They would put the dryer and all the branch circuits on the 90 amp breaker.
The stove, AC, WH, WP, some times heat strips and something else on the main out side panel . Any way the 90 amp inside would always be maxed out. It caused some problems. I have personally found the #2 burned at the terminals.
This code use of wording "entire load" made it not prudent to wire a house like that.

This is where the term 'entire load' came from.

I don't have any data to back up my hypothesis

 

[Greentagger]

No you’re not off base . Matter of fact you pointed out something I didn’t catch in original post. With circuit in panel going to well, there’s no way the service conductors carry the entire load associated with dwelling.

 

[augie47]

That said.... how often is the 2/0 feeder rejected.. in this area I don't recall any inspector red tag for that.
Similar to a 400 amp (320) meter base with a 2/0 to each of two 200 amp panels.... see it every day...........
No doubt both situations are not acceptable in some areas but overlooked in many.

 

[Greentagger]

Red tag here, but definitely agreed. Mostly overlooked.

 

[david]

No you’re not off base . Matter of fact you pointed out something I didn’t catch in original post. With circuit in panel going to well, there’s no way the service conductors carry the entire load associated with dwelling.

please explain how the "service" conductors are not sized to carry the entire load

 

[augie47]

maybe the term "feeder" rather than "service"............................

 

[david]

So if moving loads (water pump, storage building, [AC?]) outside dwelling unit's panel after the fact, it would then cause a violation related to load calculation for the SE, as these loads no longer fall under the 83% rule, but not necessarily that of the feeder of the dwelling unit.
Or am I way off base as to the rationale of this section and the Why?

Moving loads around from a distribution panel to a service panel would not increase the calculated load for a service.

Adding loads to the service panel would potentially dictate a need to increase the service conductor size

But that still would not change a statement that no feeder down stream of the service conductors would be required to be sized larger than the service conductors

Edit: we are only talking about ampacity not voltage drop or other correction factors that the service conductors are not subject to

 

[Greentagger]

please explain how the "service" conductors are not sized to carry the entire load

Is the branch circuit to the well part of the dwelling or not?

 

[david]

Is the branch circuit to the well part of the dwelling or not?

its part of the calculated load for the service
the service is sized for the total calculated load and in (3) i am told i can consider the size of the service conductors in (1) to determine the max size per ampacity a feeder has to be regardless of the feeder caring the total load of the dwelling or not

in (3) i am told that any feeder down stream of a total dwelling feeder does not need to be increased for ampacity greater than the upstream feeder suppling the dwelling

 

[Fred B]

The issue has been debated for years. Goes back to a time when the Table 310.15(B)(7) was in the Code and it gave a blanket wire size without taking into consideration insulation ratings or ambient adjustments. The 83% rule seemingly corrected that but the fact that you reduced the overall load on the feeder by relocating some loads to the service requires increasing the feeder size due to loss of diversity even if required defies logic hence the problem.

It seems there are 2 seperate issues at play, 1 SE conductors sizing related to the 83% rule, and 2 feeder sizing related to the 83% rule. The reading almost seemed to make them 2 distinct individually reviewed criteria that only impacts each other in 310.15(B)(7)(3) that feeder doesn't need to be larger than service conductors. It reads as if SE can be sized to 83% if entire load is supplied for dwelling(s) and separately the feeder can be sized to 83% if it is supplying entire load for an individual dwelling unit.

So it seems that if you have 2 dwelling units provided by a single service, each unit is 100A total service will be 200A. Service conductors can be sized to the 200A using the 83% rule as it is providing the entire loads for the dwelling units per 310.15(B)(7)(1). Feeders also can be sized to the 100A individual unit at 83% per 310.15(B)(7)(2).
Or if you have a dwelling at 200A provided by a feeder, but if there are additional loads on the service panel those loads would be added to the dwelling loads of 200A, ie. You have 50A of additional non dwelling loads your SE load would be 250A and that would be the basis of the for total load for SE conductor sizing and it is no longer providing the entire load of an individual dwelling unit, but also non dwelling loads and cannot use the 83% rule for the SE, but the feeder can be sized to the 83% feeder rule.
A correction to my previous statement:
"So if moving loads (water pump, storage building, [AC?]) outside dwelling unit's panel after the fact,....."
It seems at looking at the definition of dwelling unit the well pump and AC unit if associated with the individual dwelling regardless of where the item is supplied from it can still be part of the dwelling unit calculations and 83% rule of SE or feeder conductors, and will not alter SE load if simply moving it from one panel to the other. But the addition of the storage building would.

 

[Flanative]

We install 2-3 mobile home services per week. Our typical setup is 200 amp overhead/underground service with meter main feed thru panel. 4/0 xhhw alum service entrance conductors in the riser. 4/0 xhhw alum underground to the mobile. Even if it is a 150 amp panel in the mobile due to the 200 amp main at the pole. The well and package unit is typically fed from the meter main but occasionally the AC is fed out of the mobile if its a split system.

 

[david]

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