Main trips

[Mreboy85]

Ive just moved into a small house with a small out building without power. In the crawl space there was 8/3 12 meters and 20 meters 10/3 outbuilding is 26m. I purchased PVC 1” conduit and ran it to a sub pnl put a 2 pole 20a breaker in main pnl and 15a in sub with plug nippled to pnl. Here’s the question i ran a grinder the other day why does the breaker in the main pnl trip not the sub

 

[Pete999]

Ive just moved into a small house with a small out building without power. In the crawl space there was 8/3 12 meters and 20 meters 10/3 outbuilding is 26m. I purchased PVC 1” conduit and ran it to a sub pnl put a 2 pole 20a breaker in main pnl and 15a in sub with plug nippled to pnl. Here’s the question i ran a grinder the other day why does the breaker in the main pnl trip not the sub

What size breakers are you using in main and sub panels?

 

[Lister1987]

What size breakers are you using in main and sub panels?

20A main panel, 15A sub

 

[DPG]

Not enough selectivity between a 15A and a 20A breaker.

 

[Mreboy85]

so if I was to change the main to a 30 (still within wire size rules) would that give enough selectivity I ask that like I know what it is lol

 

[pc1966]

Seems you are in Canada so maybe some more local folks will know, but here in the UK/EU breakers have two ratings:

• Main current limit (e.g. normal current 'In' = 20A) which is slow & thermal in operation, typically starting at around 135% of In and getting faster with rising overload
• The "instant" magnetic trip which comes in at different ratios depending on the "curve" selected, so B-curve =3-5*In, C-curve = 5-10*In and D-curve = 10-20*In

So for stuff with a high start surge you would go for C or maybe D curve MCBs, but base the 'In' thermal rating on the current-carrying capacity of the cables, sockets, etc.

Do not change the main breaker termal limit until you have established a replacement still provides adequate protection from faults and overload for the circuit(s) it feeds!

Again, I don't know the Canadian regs, but here you would need to determine that the current rating provides protection against overload (for socket outlets) and the MCB trip curve to protect against faults (e.g. short to earth/ground) by disconnecting in under 0.4s (typically it means the instant trip has to be less than the prospective fault current at the end of the wire, from the supply volts and earth loop impedance)

 

[Mreboy85]

Thank you very much for the info Sir
have a fantastic day

 

[pc1966]

Also forgot to ask if the main panel breaker is a GFCI type? That might be a very different reason for it tripping.

Generally if you have two breakers in series you won't get very good selectivity (the ability of the down-stream one to isolate the fault without the up-stream one noticing). Manufacturers often publish selectivity tables showing combinations of their own breakers and what sort of fault level gives to the wanted outcome of only the final one tripping, but if it is a mix between two makes or very different families there is usually no accurate information about what sort of outcome can be expected.

Do you have the details of the two breakers (make & model)?