BillPratt58
Member
- Location
- sioux city,ia
What size fuses need to be installed in a service disconnect that feeds 2 240V 100A rated main lug only panels? I think they would be 100A fuses but others are telling me 200A.
Sizing of fuses in service disconnect
- Thread starter BillPratt58
- Start date
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electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
- Occupation
- Electrician
Correct. See 408.36.
WarrMann
Senior Member
- Location
- Atlanta, GA
I think you would need more info to determine what size fuses are appropriate.
What size conductors are used on the line side of the disconnect?
Load size conductors don't matter so much as long as they can support your 100a panel rating.
How does the disconnect branch out to the 100a panels? Does it meet the 10ft tap rule?
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petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- Semi-retired engineer
you have to protect each MLO panel with 100A OCPD (or smaller). Depending on the load calculations this could be a single 100A (or smaller) OCPD, or muitiple OCPD, but neither panel can be protected by an OCPD rated greater than 100A.
WarrMann
Senior Member
- Location
- Atlanta, GA
Why? I just did a 400 amp service split into two 225amp mlo panels. I ran 500 into a wireway, than tapped down to 3/0 per 240.21(b)1. All went well at inspection.
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petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- Semi-retired engineer
the panelboard bus has to be protected at or below its rating. what is protecting it if there is not a 225A or lesser rated OCPD in front of it?
Coppersmith
Senior Member
- Location
- Tampa, FL, USA
- Occupation
- Electrical Contractor
225 amp panels can handle a maximum of 225 amps. If you are protecting them with a 400 amp breaker, it will not trip before they burn up. The fact that it passed inspection only means the inspector did not catch the violation.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
MLO equals violation, not sure why you passed inspection.
WarrMann
Senior Member
- Location
- Atlanta, GA
- Location
- New Jersey
- Occupation
- Journeyman Electrician
WarrMann
Senior Member
- Location
- Atlanta, GA
Why is it a violation? 240.21(B)(1), 1b states - "not less than the rating of the EQUIPMENT containing an overcurrent device(s) supplied by the tap conductors..."
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WarrMann
Senior Member
- Location
- Atlanta, GA
Ocpd required for the panel board is the 400 amp disconnect next to it.
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- Location
- New Jersey
- Occupation
- Journeyman Electrician
How does the 400 amp OCPD protect the 225 amp panels according to 408.36?
Coppersmith
Senior Member
- Location
- Tampa, FL, USA
- Occupation
- Electrical Contractor
240.21(B)(1) Taps Not over 3 m (10 ft) Long. If the length of the
tap conductors does not exceed 3 m (10 ft) and the tap
conductors comply with all of the following:
(1) The ampacity of the tap conductors is
a. Not less than the combined calculated loads on the
circuits supplied by the tap conductors, and
b. Not less than the rating of the equipment containing
an overcurrent device(s) supplied by the tap conductors
or not less than the rating of the overcurrent
protective device at the termination of the tap conductors.
tap conductors does not exceed 3 m (10 ft) and the tap
conductors comply with all of the following:
(1) The ampacity of the tap conductors is
a. Not less than the combined calculated loads on the
circuits supplied by the tap conductors, and
b. Not less than the rating of the equipment containing
an overcurrent device(s) supplied by the tap conductors
or not less than the rating of the overcurrent
protective device at the termination of the tap conductors.
Last edited:
WarrMann
Senior Member
- Location
- Atlanta, GA
Look, I can't keep arguing this. I see your point in regards to 408.36 and respect your opinion.
But fact is i was taught how to build this service a decade ago, and have used it many times. Never had a problem. But i wouldn't brag about the inspectors around here either.
I didnt mean to hijack this thread from OP. I have no hesitations on joining a different thread
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- Location
- New Jersey
- Occupation
- Journeyman Electrician
IMO it's fine to keep it in this thread as your installation is germane to the OP. The fact is that your installation contains two violations, one being the OCPD required to protect the panel is not the correct size (225 amps) and also the need for an OCPD at the end of the tap conductors. If you want to use MLO panels then you'll need 400 amp conductors to feed them and 400 amp bus.
WarrMann
Senior Member
- Location
- Atlanta, GA
Ok. Tap rule states RATING of EQUIPMENT containing an over current device(S). My panel is the equipment that is rated, and i have overcurrent devices. My 400 amp is a single point of disconnect, so no more than 6 disconnects.
You've got me a little on the 225 mlo when i use 3/0, and on a 400 amp disco. But that's just what came from the supply house when i ordered a 200 MLO.
As i said, i understand your concern in regards to 408, and i respect your opinion. You give good advice on this forum. But for this one, you can tell me im wrong till your blue in the face. Ill keep doing it till they take the (S) out of the 10ft tap rule
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- Location
- New Jersey
- Occupation
- Journeyman Electrician
That's fine with me you can install it anyway that you like, myself and others are just pointing out the violations so that others who are reading this thread can see why it's incorrect.
WarrMann
Senior Member
- Location
- Atlanta, GA
That's why i stopped working for the man.
Out of curiosity...have you ever seen it done my way and been turned down?
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