Main Distribution Panel breaker size

mavrck

Member
Location
ky
Hello,
I'm looking for a code reference on how to verify or size a breaker for a MDP to determine it is correct. I'm looking into adding a sub panel to a existing MDP and I'm not sure how to verify the breaker is adequate.

There is currently a 400 amp main breaker feeding a existing MDP. The MDP currently has 1-175amp breaker, 1-40 amp breaker and I'm looking at adding 1-200amp breaker for a sub-panel. Does this seem doable? If so do you know where there may be a code reference on how to size such?

I'm sure this has been asked before but I wasn't able to find it in the search bar. Thx
 

mavrck

Member
Location
ky
Are you referring to figuring load calculations on each individual breaker? Then adding together to confirm. Sorry I’m new to configuring this
 

mavrck

Member
Location
ky
Sorry I guess I’m confused.

if I check the electrical bill, I’m not sure that will give me the load calculation per panel.

that’s what I think I should be looking for but I may be wrong.This is new territory for me
 

tom baker

First Chief Moderator
Staff member
The amp size of the breakers is not what you want to look at, Do a load calculation in KW, then convert to amps for each breaker or feeder.
If you have not done a load calculation before, there are examples in the back of the NEC, or probably some spread sheets at MikeHolt.com
If its 3 phase then there is a square root of 3 that gets used in the calculation
The add up for the total load
The wire needs to be protected at its rated ampacity (from the breaker) and also be able to carry the calculated load.
The code reference is Article 220, 310 and 240.
Or ask here!
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Really you have to do a load calculation That's the only way. If you don't know this it makes me suspicious that you are asking for DIY electrical advice which the forum rules prohibit.
 

mavrck

Member
Location
ky
This is not for a diy project. This to verify that work is completed safely. Unfortunately it has been 15 years in apprenticeship class since I had to do this so I’m trying to get back up to speed. I have never seen a home project with a 400amp mdp.
 

mavrck

Member
Location
ky
This is actually in a commercial building is why I ask most load calculations I see are for dwellings
 

Strathead

Senior Member
Per the NEC there are two ways to do this. You can perform a load survey for thirty days, or you can do load calculations per NEC 220. Since you are looking for the total service load, you can ask the power company if they have a demand meter installed. If they do then get the data and your 30 day load study is it. If they don't, just go to the load calculation example that is closest to your application and follow it, substituting the information from your building. Or you can hire a company to put a demand meter on the service for thirty days. That will cost around $1500. Your choice.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
If y
This is actually in a commercial building is why I ask most load calculations I see are for dwellings
If you just eyeball what you have 175 amp + 40 amp = 215 amps there would be no problem adding another 200 amp OCPD however the proper method is to calculate the load not adding the existing circuit breaker ratings together. If I saw a 400 amp panel with 200, 175, and 40 amp CB's I wouldn't look twice at it.
 

mavrck

Member
Location
ky
I agree, I think once get my math together and actually do a load calculations it will be much more clear as to how nec views this.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Unfortunately you can open a rat's nest when you start looking at load calculations for this kind of thing, especially if it's been around a long time. If The three circuit breakers you mentioned cover all the loads for the entire facility you are likely okay.

However if this is just one panel of multiple panels you might end up having to work your way all the way back to the service to prove that the service is large enough. That makes renting a meter and determining what the highest usage is kind of attractive if you can't get that data from the power company.
 

kwired

Electron manager
Location
NE Nebraska
Are you referring to figuring load calculations on each individual breaker? Then adding together to confirm. Sorry I’m new to configuring this
That is basically how you determine minimum size for those individual breakers. For feeders and services you possibly get to apply demand factors to certain kinds of loads depending on conditions. Take a look through art 220.

Also just because you have a 175 and a 200 amp breaker doesn't automatically mean they have a draw of 175 and 200, but they might.
 

Strathead

Senior Member
Just an observation. When we all get in the weeds here with things like using fork tongue lugs on a receptacle, advocating not performing load calculations is anathema. Code requires calculations when adding a new load for the record. Not saying I would do one.:devilish:
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Just an observation. When we all get in the weeds here with things like using fork tongue lugs on a receptacle, advocating not performing load calculations is anathema. Code requires calculations when adding a new load for the record. Not saying I would do one.:devilish:
I think most people don't bother for smaller load additions because we all know the load calculations are extremely conservative so there is usually plenty of spare capacity, even if the load calculations say otherwise. But in a legalistic sense, it is a requirement of the code to do so.
 
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