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I'm looking at getting a bussman fusebox for making my standalone harness. I see that some of the fuseboxes use micro relays. What would these micro relays be used for? When would I use a full size relay? Since I can order a fusebox with or without the mini relays, I don't want to order a box that has them if I won't be using them for anything.
Sounds like that would go by amp load. What range of loads do micro-relays come in? Use those for anything where a full size one is not required, it would seem to me
I have that same panel and I use two relays. One is for my fuel pump and the other I believe is for my electric fans. I have my electric fans hooked up to a switch right now so its not in use. I used Delphi 8866 relays
Amps are mostly what you will be concerned with for relays. The micro relays are typically around 30-40A capable, but you can buy some larger rated relays. I would be hesitant to use those above 30A or if you plan to max out the relay, you may want to consider one of the larger bodied relays (mini relay) with larger legs, they should be tougher.
Replacement relays are quite cheap, so unless you intend to use all or most of them and the price difference isn't huge then no reason to get it with all of the relays included. It just depends what accessories you will be hooking up and how many, if it is worthwhile.
Amps are mostly what you will be concerned with for relays. The micro relays are typically around 30-40A capable, but you can buy some larger rated relays. I would be hesitant to use those above 30A or if you plan to max out the relay, you may want to consider one of the larger bodied relays (mini relay) with larger legs, they should be tougher.
Replacement relays are quite cheap, so unless you intend to use all or most of them and the price difference isn't huge then no reason to get it with all of the relays included. It just depends what accessories you will be hooking up and how many, if it is worthwhile.
On the money. Most of the Micro relays and their contacts match or outperform their older style bigger bodied relays. As silentbravo said around 30-40 amps. Don't let the size fool you. if you feel you need more than 30A, then consider spreading the load with additional relays (and their fuses). For example don't run your fans or fuel pump or big radio, power windows, etc off the ignition relay. Have separate relays for them with their coils slaved off the ignition relay. Fuel pump relays coil may come might be tied to the engine management, depending on the setup.
That fusebox can probably only handle 30A per terminal and 80A total anyway, so spread the relay load and lots of fuses. BTW 30A is the max, I'd design for less than that per relay.
OK, thanks guys. Sounds like if I order a bussmann fusebox with the mini relays I'll be able to handle everything I need. I'm not familiar with the amp draw of most features. Will the mini relays be sufficient to control an AC compressor?
Thanks guys. I'm planning for future expansion of this fuse block as I'm probably going to add the items I had on the car when I first got it for nostalgia (dixie horn, cb radio) and perhaps add some running lights.
Regardless of the physical size of the device(fuse, switch, relay, etc), they all have a rating in amperes. So, all you need to check is the amp rating of the device. They have been tested for a particular amperage. So don't sweat it based on the size of the electrical device. They are rated for it. You just need to make sure they are rated for the particular voltage as well......forgot about that. However, there are times that you need to install a relay with higher amp ratings because of the stronger contacts in them.....i.e. fan relays. Contacts in there don't like to last with high current stuff......all the time. Fans have a high inrush current because of startup......any motor for that matter.
I ended up ordering the dual bussed Bussman fuse holder. It holds up to 10 relays and 40 fuse holders. I did this because I plan to separate out more of my wires than most people, who tend to group them together in single wires. The downside is that the relays are all micro relays. I'll be running dual electric fans, but the fans came with their own relays, so I suppose I'll mount those separate next to the fuse box. I've also ordered several weatherpack connectors so I can make the entire fusebox removable for servicing issues in the future.
Am I thinking about the relays correctly?
Each system with its own relay:
AC system
fuel pump
dual fans (3 relays)
1 relay activated by an ignition switch to turn on the bussed side of the fusebox for ign-run-start powered wires.
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What size relays to use? 
