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# Replace 90 with 80 or 100?

#1
12-20-04, 07:42 PM
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Join Date: Nov 2004
Location: CA
Posts: 2,041
Replace 90 with 80 or 100?

This is the "plumber who knows a little bit of electric" speaking, so please be gentle. Question came up about finding replacement for a 90 amp double pole main breaker. Can't find a 90. Parts place says it is OK to bump that up to 100, but not OK to undersize the main at 80. What's the straight story?
Thanks,

Jim

#2
12-20-04, 08:33 PM
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Join Date: Jan 2004
Location: Oregon
Posts: 1,219
Parts place is feeding you a line.

In general you cannot simply increase the size of a circuit breaker. To do so means a risk of overloading the conductors that the breaker protects.

But you also cannot simply decrease the size of the breaker protecting the circuit, since you can then overload the _breaker_.

It is probably simpler to keep looking for the 90A breaker.

To determine if you can increase the size of the breaker, you need to figure out the ampacity of the feeder conductors to your panel. You need to figure out the gauge, insulation type, and composition of the conductors, both going into the meter and going from the meter to the panel. If the ampacity of the conductors and the maximum rating of the panel is 100A, then a 100A breaker will be fine; but you need to check first.

To determine if you can decrease the size of the breaker, you need to do a demand calculation for your house. Article 220 of the NEC describes how to do this; if you search this forum you will also find discussions about doing this sort of calculation. If the calculation shows that 80A is sufficient, then an 80A breaker would be fine, but again you need to check first.

What circumstance makes you want to replace the main breaker?

-Jon

#3
12-21-04, 10:25 AM
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My queston would be why is there a 90 amp main??? This is a completely non-standard size for a service.
IF this is a true main breaker and not a sub-feed breaker I suspect it should have been 100 to begin with. I have never even seen less than a 100 amp service with breakers. Anything smaller would have been fuses.
The 90 likely replaced an original 100. But like Winnie asked, what causes you to need to replace it?

I truley feel a demand calculation is not at all necessary to replace the breaker. If the service conductors, meter & panel are rated for a 100 amp service a 100 should be used.

BIG red flag though! A DIYer really should not be messing with a main breaker. You need to contact the power company to cut the power at the pole or service drop so this job can be done dead, so you don't become dead!

#4
12-21-04, 12:54 PM
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Join Date: Jan 2002
Location: Chicago
Posts: 1,915
I've never seen a 90A breaker.... that's an add size.

You can always just pull the meter to replace the breaker.. but most utilities don't like that very much, especially if you don't tell them.

#5
12-21-04, 02:26 PM
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Originally Posted by trinitro
You can always just pull the meter to replace the breaker.. but most utilities don't like that very much, especially if you don't tell them.
True, but this is NOT something I recommend a DIY'er do either. If you get a cranky enough utility you might have to cough up some hefty fines if done on the sly. No to mention the fact it can be very dangerous if everything is not in sound condition or ther is a load on the line.
The utility will pull the meter if they allow homeowners to do this work. Some might require an inspection even for a repair.

#6
12-21-04, 06:46 PM
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Join Date: Nov 2004
Location: CA
Posts: 2,041
Thanks, guys. I was trying to help out the "maintenance guy" who apparently knows even less about 'tricity than me. i think we should both keep our fingers out of this one!

Again, Thanks!

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